| lumanauw |
Can anyone explain to me what is damping factor? I read in handbook that it has to do with output impedance/internal impedance. But how can internal impedance of a power amplifier be measured (or be calculated?).
At first I think it has to do with how many output transistors we use. More output transistors gives more damping factor. But I'm quite surprised when I look at a certain professional studio amp (that has damping factor >1000), it has a very few output transistor. So my assumption is wrong.
I tried to design an ordinary amp with 3 stages (differential, VAS, current amp), but it's goal is to pursue damping factor >1000. What is the key to pursue high damping factor in 3 stages power amp? Please someone share the knowledge. |
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| PaulHilgeman |
You really dont need one that high.
Anything greater than about 200 is fine, and it has been proven to be inaudible above that. I dont remember the sites that go over this, but they are out there. |
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| Steve Eddy |
| quote: | Originally posted by lumanauw
Can anyone explain to me what is damping factor? I read in handbook that it has to do with output impedance/internal impedance. But how can internal impedance of a power amplifier be measured (or be calculated?). |
Damping factor is simply the ratio of the nominal load impedance divided by the amplifier's output impedance.
So if you're looking at a nominal 8 ohm load and the output impedance of the amplifier is 0.1 ohm, the damping factor is 8/0.1 or 80. For a damping factor of 1,000, the amplifier's output impedance would have to be 0.008 ohms.
| quote: | | At first I think it has to do with how many output transistors we use. More output transistors gives more damping factor. But I'm quite surprised when I look at a certain professional studio amp (that has damping factor >1000), it has a very few output transistor. So my assumption is wrong. |
No, you're on the right track. You can reduce output impedance by paralleling a whole bunch of output transistors. You can also reduce output impedance by way of negative feedback. Or both. So while the amp with the high damping factor may have just a few output transistors, it would likely be incorporating a high amount of negative feedback.
| quote: | | I tried to design an ordinary amp with 3 stages (differential, VAS, current amp), but it's goal is to pursue damping factor >1000. What is the key to pursue high damping factor in 3 stages power amp? Please someone share the knowledge. |
Is the goal of a damping factor greater than 1,000 purely for the sake of doing it? If not, then I wouldn't bother.
se |
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| kiwi_abroad |
What's the point of a stupendious damping factor, implying very low output impedance, when your speaker cables will add around 0.05 to 0.1 ohms.:bfold:
Oh, and don't forget the DC resistance of 4-6 ohms of your woofer, which is the 'real' resistance of the voice coil.
Adrian B. |
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| Steve Eddy |
| quote: | Originally posted by kiwi_abroad
What's the point of a stupendious damping factor, implying very low output impedance, when your speaker cables will add around 0.05 to 0.1 ohms.:bfold: |
Yeah. A bit pointless for your amplifier's output impedance to be an order of magnitude lower than the resistance of the speaker cable.
| quote: | | Oh, and don't forget the DC resistance of 4-6 ohms of your woofer, which is the 'real' resistance of the voice coil. |
But it's not the voice coil's DC resistance that really matters. It's the loudspeaker's impedance which for typical dynamic type loudspeakes will vary considerably. So if the amplifier's output impedance is high enough, it can result in some rather significant variations in the loudspeaker's frequency response.
Here's an example of how wild it can get using a SET tube amp driving Stereophile's dummy loudspeaker load which is 2uF in parallel with 8 ohms:
Though the more fundamental context of "damping factor" is about the amplifier's "control" over the loudspeaker at resonance. Dick Pierce *wince* wrote a nice article about this in Speaker Builder some years back.
There's copy of it over on diyspeakers.net. Just look under Articles.
se |
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| uvodee |
I overheard a teenager salesperson at Frys explaining to a middle aged couple that damping (he called it dampening) factor is from the utmost importance.
it can only be achieved in expensive stuff he said, it is the difference in travelspeed between high freq and low freq inside the system and must therefor be the same hence one uses damping factor to mark it. To bring the high freq speed down to the low freq speed is a complex issue and is very expensive!!!!!
I saw him selling a Sansui (if my memory serves me wright) with some spkrs ( i think Polk) and other stuff, probably around 3k ...... within a few minutes....
Jean-Pierre |
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| phase_accurate |
When it comes to taming a driver's fundamental resonance, Rdc of the driver is by far the largest resistance that comes into play.
Regards
Charles |
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| millwood |
| quote: | Originally posted by uvodee
damping (he called it dampening) factor |
I actually think dampening factor is the correct term rather than damping factor, if you think what this "factor" is trying to measure.
| quote: | Originally posted by uvodee
I saw him selling a Sansui |
I thought they had gone out of home audio. |
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| MarcelvdG |
A loudspeaker box designer can take the DC resistance of the voice coil and cross-over inductors into account, such that the quality factor or factors of the low-frequency resonance(s) have the right values and a nice response results when the box is purely voltage driven.
Any resistance from the amplifier is not taken into account by the box designer and increases the quality factors above the optimal values.
An equally valid way to look at it is to assume the loudspeaker to be designed to have a more or less flat response from the voltage between its input terminals to the sound pressure, and to calculate the response deviations in loudspeaker terminal voltage due to frequency-dependent voltage division between the loudspeaker on one side and the amplifier and cable on the other side.
Anyway, the difference in response between a damping factor of 100 and a damping factor of 1000 is negligible, unless there are very deep troughs in the impedance curve of the loudspeaker. |
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| ashok |
In connection with some discussion on this topic , someone mentioned a link to a test report on damping factors. I can't remember where I read this ( on the Net of course).
It said that at damping factors above 15 or so it becomes very hard if not impossible to hear any improvements. I would also guess that they assume that the real world impedance of the speaker system is reasonably uniform over its bandwidth.
( If we talk of the damping factor with reference to bass response only , the factor 15 makes more sense).
For systems with a very large variation in impedance ( over frequency ) a reasonably high damping factor will ensure that the ( electrical ) frequency response at the speaker terminals is really flat. With a low damping factor you will get a pretty variable response as can be seen in the wavy graph in the earlier post.
So the two factors are kind of tied to each other.
Can you imagine what must be happening at the speaker terminals with SE amps and similar circuits !
Most of the time we say something is good because we like it that way. That's good because it leaves the field wide open for DIY projects. We will never get to the end !
And all the time we are having fun !
Cheers.:drink:
ashok. |
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| DrG |
What about current drive to a speaker? An ideal current-drive amp would have an infinite output impedance, and therefore a damping factor of zero.
I have no experience in this regard but remember reading somewhere that good results are to be had with the exception of controlling woofer resonance. My questions are:
a) Does anyone have experience with/knowledge of current-drive of speakers?
b) If this can indeed be successfully done, how is it possible with a zero (or near-zero) damping factor? |
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| ashok |
With a current source the electronic unit would not be able to control (damp) the resonance. In this case you will probably have to depend on the mechanical damping of the driver itself. This would mean that all drivers would not be suitable for current drive. You would need specially designed drivers with very high mechanical damping . Currently ,the electrical damping of speakers seems to be higher than the mechanical damping.
Cheers.
I once asked some speaker designers at Hitachi about why they did not use current drive when it was better than voltage drive. They seemed to be floored and obviously did not follow up on it .
( That was about 20 years ago )! Great guys in any case ! |
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| Steve Eddy |
| quote: | Originally posted by phase_accurate
When it comes to taming a driver's fundamental resonance, Rdc of the driver is by far the largest resistance that comes into play. |
Well, certain tube amplifiers notwithstanding. :)
se |
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| Steve Eddy |
| quote: | Originally posted by millwood
I actually think dampening factor is the correct term rather than damping factor, if you think what this "factor" is trying to measure. |
No, "damping" is the correct term as it relates to controlling the loudspeaker at resonance.
se |
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| PRR |
> how can internal impedance of a power amplifier be measured
Feed a constant signal, at less than maximum output. Load the output with no-load and with rated load. Measure the voltages you get.
For an easy measurement, try a very simple tube-triode amp. With no-load, crank the output up to 3 volts. Then apply an 8Ω load; the output will drop to about 2 volts. Be sure it is not distorting significantly in either case. Now a little math will tell you that the output Z is about 4 ohms, because 8/(4+8)= 2/3. If the Zout is 4 and the load is 8, the Damping Factor (I never saw DampENing Factor) is 8/4= 2.
With a modern "good" sand-state amplifier, you will read something like 3 volts no-load and 2.997 volts at 8Ω. Output Z is about ((3.000-2.997)/2.997)*8= 0.008Ω, DF=1,000. (At these absurd levels, the measurement may be limited by meter accuracy rather than the amp's actual performance.)
Some amps do not like no-load. Also it is probably not a real-world condition when driving a speaker. 50Ω and 8Ω (or same ratio for other design loads) is normally safe, though the exact math is harder.
Output Z is often measured around 1 volt, but usually varies with signal level. And output Z always varies with frequency.
Why do we care? If the speaker was always 8Ω, we might not care. But real speakers are 8Ω at 300Hz and very-high Ω at bass resonance. If the peak at resonance were infinity, then with DF=2 the voltage across the speaker terminals is 20*LOG(3/2)= 3.5dB higher at bass resonance than at 300Hz. If the bass resonance impedance were 50Ω, the rise is 2.85dB.
It is possible to make a speaker acoustic response "flat" from a hi-Z source. Most home radios of the 1930s-1950s were tuned this way. The undamped electrical rise at bass resonance was balanced against an open-back cabinet's and undersized transformer's bass droop. Some of these sound very good. But when you want a little better, it gets very difficult to design.
Most modern speakers are tuned to be flat with a constant voltage input despite speaker impedance variations. If the voltage is not perfectly flat: With a damping factor of 10, the worst error is 1dB; for DF=40 the worst is 0.25dB. Since speakers are not precision devices, not accurate better than 1dB or 0.25dB, DF more than 10 or 40 is usually plenty. 100, 200 is not a lot better (0.1dB, 0.05dB) but generally "easy" with high-feedback transistor amps.
Anyway, 10 feet (3M) of speaker wire will give DF about 40 even with an infinite-DF amplifier. And speaker designers generally assume a few-tenths ohms of wire resistance (not that it makes any large difference to their calculations).
> the amplifier's "control" over the loudspeaker at resonance.
That's the common explanation, and a lot better than that "difference in travelspeed" garbage the Fry's boy was selling.
But the "control" is limited by the 6Ω resistance of the voice coil. And the designer has "controlled" the resistance, magnet, mass, stiffness, and size to give a "slightly uncontrolled" response. If the speaker were perfectly damped, Q=0, the response would droop 6dB/oct below 200Hz. We need to let it resonate with Q of 0.5 to 2.0 (0.8 to 1.1 in most hi-fi designs), and the designer balances resistance against all the other parameters to get in that range. The difference between resistance of 6.1Ω (DF=80) and 6.01Ω (DF=800) is about zero for damping, about 0.1dB for response. Room-effects make much more difference than this.
> I think it has to do with how many output transistors we use.
Not really. The output impedance of a transistor (or a parallel array of transistors) is proportional to current. More transistors is the same as one transistor at the same total current. In theory Zout may be 0.03Ω at 1 Amp (DF=240), but 0.3Ω at 0.1 Amp (DF=24) and 0.6Ω at a typical AB idle current of 0.05 Amps (DF=12). When you add the need for bias stability resistors, output Z is usually 0.1Ω to 0.5Ω for about any transistor amp with any reasonable number of output devices and any usable thermal stability. And that assumes low-Z drive to the emitter followers, which is often not the case. A naked emitter-follower speaker amp has DF in the area of 20.
> 3 stages (differential, VAS, current amp)
In general you need 4 stages to get just a "good" speaker amp. If the "current amp" is a darlington or similar, you have enough current gain for a "good" amp. It is a little marginal for a "great high-feedback" amp or for DF over 1,000.
The naked output Z is probably higher than 0.2Ω because of the hi-Z VAS and finite Beta in output devices. To get DF over 1,000, you will need 1,000:1 feedback factor. Closed-loop voltage gain of speaker amplifiers is usually about 20. You need an open-loop voltage gain of about 20*1,000= 20,000. This is very hard to get in one gain stage. And it is hard to get gain in the diff-stage without reducing gain in the VAS. And if you take gain in two stages, you will have a 2-pole open-loop response which is unstable at high feedback factor. If you can compensate it to look like a 1-pole response, and want DF=1,000 all the way to 20KHz, the gain-bandwidth has to be 20MHz at 20:1 gain, 400MHz if you keep it 1-pole all the way to unity gain. You can not get big output transistors that are flat above 20MHz; good rugged ones may get limp before 1MHz. So demanding hi-DF usually leads to unhappy compromises in stability and feedback margin.
I must point out that the path to high DF is what led to many nasty-sounding transistor amps. The high DF applied only below 1,000 Hz, because feedback was falling (and distortion rising) above 1KHz. So you get ultra-clean bass with a cloud of fuzzy treble. This does not "have to" happen, and I'm sure your "certain professional studio amp" is better-balanced than the old stuff. The DF=1,000 probably "just happened" after all else was optimized for good full-range feedback. But starting from scratch, you may be following a 25 year path from the big bad stuff of 1970 to modern amps. |
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| Steve Eddy |
| quote: | Originally posted by ashok
I once asked some speaker designers at Hitachi about why they did not use current drive when it was better than voltage drive. |
I still don't see why current drive would be fundamentally any better than voltage drive. What's the real advantage here?
se |
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| DrG |
| I don't honestly know, Steve. But looking at the amazing performance gains possible with current-feedback op-amps I have to wonder... |
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| usekgb |
| I have to say, I really love the Crown amplifiers that I use in PA and studio use. Their DF is very high, and they are also high current amplifiers. One thing I love about the MT/MA series amplifiers is their ability to control high power (1200W) subwoofers at loads down to 2 ohms. They control the speakers very well and you always get good, tight controlled bass using Crowns. BTW, the Crown MT/MA amps have a Damping Factor of 1000 from 10Hz to 400Hz, which is where it is needed most for subwoofer use. If that's not enough for you, the Crown Studio Reference series amps have a Damping Factor of 20,000 from 10Hz to 400Hz. Talk about speaker control! These amps sound great on higher frequency as well. They don't have any "fuzziness" in the mids and highs that was referred to in a previous post. One last thing, the correct term acording to Crown is, "Damping Factor." This is right from their data sheets. |
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| Steve Eddy |
| quote: | Originally posted by DrG
I don't honestly know, Steve. But looking at the amazing performance gains possible with current-feedback op-amps I have to wonder... |
Yeah, I was thinking more in terms of the speaker itself, that it would somehow perform better if fed from a current source.
se |
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| DrG |
I was also thinking about the speaker, in a roundabout mental analogy with CFB op-amps...
But on a very basic level: magnetodynamic interaction between the voice coil and magnet is determined by the induced flux in the coil, which in turn is proportional to the current flowing through it...
Intuitively it would seem that current drive is the logical way to go... |
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| Steve Eddy |
| quote: | Originally posted by DrG
I was also thinking about the speaker, in a roundabout mental analogy with CFB op-amps...
But on a very basic level: magnetodynamic interaction between the voice coil and magnet is determined by the induced flux in the coil, which in turn is proportional to the current flowing through it... |
Which of course is a function of the speaker's impedance and the voltage across it.
| quote: | | Intuitively it would seem that current drive is the logical way to go... |
I suppose. But a speaker is also an electromechanical resonant system and whether driven by a voltage source or a current source, it basically boils down to whether the damping of its resonance is electrical or mechaincal. And I'm not sure that I see mechanical damping as being any better than electrical damping.
se |
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| PRR |
> a roundabout mental analogy with CFB op-amps...
Wrong side of the roundabout. Current Feedback is about the input stage, not the output stage that the speaker sees.
> magnetodynamic interaction between the voice coil and magnet is determined by the induced flux in the coil, which in turn is proportional to the current flowing through it...
Force is proportional to current. But air is very thin stuff. The coil's force goes mostly into mass, not acoustic energy.
Velocity is proportional to voltage, and for cone speakers the acoustic output is mostly a function of velocity, not force. Voltage is a perfectly valid input to a cone speaker, and usually better than constant-current because....
> I'm not sure that I see mechanical damping as being any better than electrical damping.
It is worse. Electrical losses are unavoidable; mechanical losses can usually be made extrememely low. It is more efficient to use those electrical losses for damping, than to add mechanical losses and then feed both the electrical and mechanical losses on your way to a sound field. |
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| DrG |
The chicken or the egg...
Well, look at it this way: voltage drive of speakers has been done a million ways, just as *all* op-amps were VFB until (relatively) recently. Along came the CFB op-amps with many performance improvements, notably in GBW.
So there must exist a possibility that current-drive of a loudspeaker could hold advantages over voltage drive, not so? Which I think would be worthwhile exploring. |
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| DrG |
| quote: | | Wrong side of the roundabout. Current Feedback is about the input stage, not the output stage that the speaker sees. |
Ok. I see your point. How about doing both though? Use the speaker as R1 of a NI feedback loop, with a small R2 of say 0R22. I've actually seen this done somewhere...
Would this not provide current-drive as well as far greater speaker control by including it in the NFB loop? What are the disadvantages, PRR? |
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| Christer |
I think the terms VFB and CFB as used for modern op amps
can cause a lot of confusion, depending on what literature
one has read. I had a lot of difficulties understanding the
CFB concept. I tried to relate it to the four types of
feedback schemes discussed in my old electronics bible,
Schilling & Belove: Electronic Circuits - Discrete and
Integrated. First (?) edition, 1968. It made me just more
confused. Eventually, some app notes from TI helped me
on the track, and these plus a lot of thinking about it
finally made me understand CFB. However, it also
made me realize that the terms voltage feedback and
current feedback as used today do not at all mean what
the same terms mean in Schilling & Belove, that uses
the terms to specify if we sense the output voltage or the
output current. The modern terms VFB and CFB are called
voltage error and current error in Schilling & Belove.
We can then combine these into four different feedback
schemes, we can have either voltage or current feedback
and combine this with either voltage or current error. A
richer terminology IMHO.
That is, VFB and CFB as used today refers to the input, as
PRR says, but at least according to some literature it used
to refer to the output.
Please tell me if I am still confused about this. This apparent
clash of terminology has caused me a lot, lot of headache, but
I think I have eventually understood it. |
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| PRR |
> the terms VFB and CFB as used for modern op amps can cause a lot of confusion,
In modern usage: Voltage feedback has hi-Z inputs; current feedback has low-Z inputs (normally just on the inverting input).
The low-Z input also allows open-loop gain to be set by selecting feedback impedance. That's the miracle: we can adjust compensation without a little pile of pFd caps or any extra parts. Indeed usually no computation is needed: the feedback resistor is some fixed value, often 500Ω or 1KΩ, for any gain. The other feedback resistor to ground is adjusted to set the closed-loop gain; at the same time it changes the open-loop gain so the amp has a constant feedback factor and constant closed-loop GBW.
In a voltage feedback amp, the "to ground" resistance is the fixed resistance of the input device junction. To change compensation you have to change that resistance (usually not possible in a chip) or change an external capacitor (not always possible, and if it is then it is a pain). The "simplicity" of fixed-compensation comes with a price: it is fixed at worst-case compensation and performance.
Yes, a 1968 book might describe things different. Transistor op-amps were novel and they were still finding their way around. They settled on approximations of tube op-amps, discovered the 709 and 101 and 741, and stuck in that rut for 20 years (with considerable improvement on the originals). Then when people started thinking outside that box, they forgot (or never knew) the old terminology and wrote it up anew. Some terms got recycled under new meanings.
> Use the speaker as R1 of a NI feedback loop
Been done many times. It was old in 1955: see the old Fisher quad-6L6 console with "Variable Damping". Interesting that, even in integrated amp/speaker units where it might be practical, it is very rare. It works fine to give good damping and then EQ-out any response errors you can afford to over-power. You can't "control the cone better" because of the large coil resistance and the need to let it resonate to lessen the need for box-size and amp-power in the bass. |
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| Steven |
| quote: | Originally posted by PRR
> how can internal impedance of a power amplifier be measured
Feed a constant signal, at less than maximum output. Load the output with no-load and with rated load. Measure the voltages you get.
...
With a modern "good" sand-state amplifier, you will read something like 3 volts no-load and 2.997 volts at 8Ω. Output Z is about ((3.000-2.997)/2.997)*8= 0.008Ω, DF=1,000. (At these absurd levels, the measurement may be limited by meter accuracy rather than the amp's actual performance.)
...
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Another way to measure a high DF without problems with meter accuracy is the following:
Do not apply an input signal to the amplifier under test; the output should be zero (except for some noise perhaps). Now take a second amplifier (could be the other channel of a stereo amplifier) and connect the output of that second amplifier via the usual load resistance (e.g. 8Ω ) to the output of the first amplifier. Now apply a signal to the second amplifier so that the second amplifier will force a current into the output of the first amplifier. The DF is now simply the ratio of the output voltage of the second amplifier to the output voltage of the first amplifier. For example, the second amplifier is driven to 10V output (easy to measure) and the output of the first amplifier shows 10mV (also easy to measure). The DF is now 10V/10mV=1000. The measurement does not depend anymore on the difference between two almost equal values.
It is also interesting to have an amplifier with a negative output impedance. The heavier the load, the more output voltage you get. This has been done in the past (maybe still) to tune bass reflex enclosures, if the port is not properly calculated to match the resonance frequency of the loudspeaker.
Steven |
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| Christer |
PRR,
Yes, I knew about the CFB as used today, since I have finally
understood it I think. I appreciate you explanation/hypothesis
on the different terminology. BTW, I still think that book quite
good, despite it's age. The basics hasn't changed that much,
and it is quite a good book. One has to complement it with
additional more up-to-date ínformation, though. |
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| lumanauw |
Thanks PRR and STEVEN (Netherlands) for giving the way to measure damping factor. (PRR, again thank you. I still waiting for your answers in "Very low voltage preamp", but I don't know where to find you). I would like to ask about Steven's method. From his method, the first amp's output is treated like ground for the second amp. And this first amp output is then measured again to absolute ground. Is this right?
From PRR's answers, I think damping factor is also have to do with the power supply. The more bulk and stiff we make, offcourse the dip will be smaller for the same load. So, if the dip is smaller, the damping factor will be greater. Is this analogy right?
Maybe somebody say that it is useless to pursue damping factor >1000. This is the same case to make the frequency response from 1hz to 100khz, maybe somebody out there say that it is useless too. But how come people get more attratracted by these useless figures? From things that we cannot hear?
I'm just a newbies in audio electronics. I just want to "feel" what is the difference if I make the same basic amp, one with normal damping factor and another with damping factor>1000.
Again from PRR's answer, we can make high damping factor by making high feedback. That is to make as big as possible open loop gain, then closed the gain in about 20-40db. From my imagination, in 3 stages power amp, I can make that by eliminating almost all the emitor resistance (since the gain of a single transistor is like collector resistance/emitor resistance, so if I eliminate all emitor resistance, all the gain will be infinite)
Is there any other method or trick (do-able) to pursue high damping factor? |
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| ashok |
It was mentioned earlier in the thread that the speaker cable
( amp to speaker ) and connections and crossover components will be the limiting factor for the total Zout , as far as the driver is concerned ( Z out of amp +inductor dc resistance +connecting cable resistance + connection point resistance ).
The ideal situation is a short across the speaker terminals. The damping actually is done by the speaker's own coil and magnetic field. Not by the amp. The amp only provides a low impedance return path for the speaker. The external resistances - mainly cable and inductor and connectors will be significant compared to the output impedance of the amp( 0.008 ohms?).
The closest one can come to the ideal is to have the power amp right next to the driver ( on the rear wall of the cabinet ?) and a direct connection to the amp without protection relay and an active crossover.
I like the dc protection on some amps with a SCR crowbar across the power supply. So if there is dc on the speaker the fuse blows.
No devices across the speaker or in series with it.
So in reality we will probably never see a damping factor ( from the speakers point of view) of around 80 or less. I suspect this must be down to 20 or worse in most cases !
Directly driven drivers may fare a bit better.
Cheers. |
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| phase_accurate |
It is always astonishing how people believe in the magic that a high DF should be capable of doing to speaker control.
And there is ab solutely nothing gained (apart from an amp with LESS load stability) to move from an already extremely high DF of 1000 to one of 20000.
Lets do some math:
Load is 8 Ohms, we connect a cable with a total resistance of 20 mOhms (for cable AND connectors, which is a damnblodygood value).
The DF of 1000 would now be reduced to 444 and the DF of 20000 is reduced to 769 !
I don't disagree with the fact that some of these amps have indeed good LF control. But these are usually very generously dimensioned amps and I think it is the SUM of all this aspects that makes for good woofer control.
You can't have more control than the driver's TSPs allow unless you are going the NFB way or you are using an amp with negative output resistance (and don't forget to take VC heating into account when doing the latter !).
As long as your amp's output impedance is real and positive the difference in damping between an amp with a DF of 10 and a DF of 1000 is just a few % !!!
Regards
Charles |
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| Steven |
| quote: | Originally posted by lumanauw
... I would like to ask about Steven's method. From his method, the first amp's output is treated like ground for the second amp. And this first amp output is then measured again to absolute ground. Is this right?
|
Correct.
Steven |
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| MarcelvdG |
Regarding the question of voltage versus current driving a single dynamic loudspeaker:
In the vicinity of the loudspeaker resonance frequency, the voltage across the loudspeaker terminals is largely motional voltage, that is, the back EMF of the moving voice coil. Therefore, there is a pretty direct relation between terminal voltage and cone velocity for frequencies close to the resonance frequency.
For frequencies far from the resonance frequency, the voltage is mainly determined by the product of the current and the impedance of the voice coil. Voice coil resistance changes significantly when the coil heats up and its self inductance can also vary when it moves back and forth, because the surrounding iron gets more or less influence depending on the coil position.
So, near resonance, there is a close relation between the voltage and the cone movements, and it makes sense to voltage drive the loudspeaker. Far from resonance, it makes more sense to use current drive. If you use voltage drive at a frequency far from resonance, the voltage is first converted into a distorted and compressed current by the non-linear voice coil impedance, and then the current is converted into a force driving the cone.
What all of this adds up to, is that with respect to distortion and compression in the loudspeaker, it may not be such a bad idea to equalise the low-frequency part of the loudspeaker's impedance characteristic with one or two parallel LRC tanks and then current drive the whole thing.
If the loudspeaker is designed to have a more or less flat response at high frequencies with voltage drive, but its impedance rises due to voice coil inductance, then a first-order low-pass filter in front of the amplifier will be required to prevent an increasing response at high frequencies. |
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| phase_accurate |
| quote: | | What all of this adds up to, is that with respect to distortion and compression in the loudspeaker, it may not be such a bad idea to equalise the low-frequency part of the loudspeaker's impedance characteristic with one or two parallel LRC tanks and then current drive the whole thing. |
MFB is one topolgy that benefits strongly from current-drive. There would be increased stability of the loop simply due to the elimination of the voice coil inductance's effect.
OTOH it should be possible to use a mixed form of current- and voltage- drive as a compromise. Qtc would be higher, but still well defined so that it could be EQed out.
The other possibility is to make it frequency dependant: Voltage-drive around fs and current drive for any other frequencies.
Regards
Charles |
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| MarcelvdG |
| With an LRC series circuit in parallel with the loudspeaker and a current source driving the whole thing, the loudspeaker sees a relatively low driving impedance near resonance and a high impedance everywhere else. |
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| MarcelvdG |
| I now see that one of my previous posts is a bit confusing. When I wrote "parallel LRC tank", I actually meant an LRC series circuit connected in parallel with the loudspeaker. |
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| phase_accurate |
| quote: | | With an LRC series circuit in parallel with the loudspeaker and a current source driving the whole thing, the loudspeaker sees a relatively low driving impedance near resonance and a high impedance everywhere else. |
This would work but it's neither elegant nor very efficient.
Regards
Charles |
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| PRR |
> Another way to measure a high DF .... take a second amplifier
There's no "correct" way, and this avoids several issues. It does require another amplifier. My main concern is that some amplifiers' output Z varies with level. For cool-running Class AB transistor, Zout is usually highest at zero voltage, so this gives a low number for DF. If you are writing the specs for the ad, you want a higher number to impress buyers. If you actually want to damp the speaker, you want a low Zout over the whole swing. If the speaker is in resonance, some variation in Zout with swing doesn't matter much; but it does increase distortion.
My way has plenty of objections too. But it needs only standard test setup (oscillator, dummy load, scope, and good AC voltmenter) plus maybe a 50Ω 1.4W resistor. It tests over typical signal swings (you can try several amplitudes to see if thy all give similar answers). It also has a conceptual simplicity, easy to understand.
FWIW: in simulation I use both techniques: unloaded/loaded and external source.
> I think damping factor is also have to do with the power supply. The more bulk and stiff we make, offcourse the dip will be smaller for the same load.
No. Or, not with any significant amount of feedback, and assuming the amplifier (including the power supply) is not overloading and distortiong the signal. You can put a 5534 chip on two 9V batteries and demonstrate a damping factor over 1,000 in 8Ω. You will need a very good meter, because the peak output of a 5534 into 8Ω is less than 0.3 volts, possibly o.1 volts if the 9V batteries are not very fresh. But it will damp an 8Ω load. Power supplies are for power, not control.
> Again from PRR's answer, we can make high damping factor by making high feedback.
I had assumed that was obvious to anybody designing an amplifier. You can always reduce Zout with more feedback.
Without feedback: a pentode's Zout is always higher than a good-power load impedance. A triode Zout is usually about hald the Z of a good-power load, though as an extreme you can make it 1/10th. A transistor collector is always very-high impedance compared to a good-power load. A transistor emitter impedance is usually low compared to a good-power load.
The most popular output transistor affair is the emitter follower. What is the impedance of an emitter? Remember Shockley? The dynamic resistance of the emitter at various currents is:
30Ω at 1 milliAmp
3Ω at 10 milliAmp
0.3Ω at 100 milliAmp
0.03Ω at 1 Amp
0.003Ω at 10 Amp
This is true for ANY Silicon bipolar transistor.
Note that a simple Class A emitter follower working at 1 Amp has a damping factor in 8Ω of over 200, without any added feedback (and ignoring some very practical details of bias).
However that runs hot. We more often run Class AB, bias about 50mA, Zout about 0.6Ω, DF is about 10 for small signals changing to about 200 for large signal peaks. That is sure to distort. Also we have ignored thermal stability bias resistors. You can use bias resistors to also stabilize output impedance, but Zout stabilizes around 0.1Ω-0.5Ω, DF around 20.
> the gain of a single transistor is like collector resistance/emitor resistance, so if I eliminate all emitor resistance, all the gain will be infinite
No. And never trust any thought that says gain can be infinite.
There is always emitter resistance. If you don't have an actual resistor, you have Shockley's junction relation above: the emitter always has a dynamic impedance.
The collector has an impedance too. It is usually 300 to 3000 times larger than the emitter impedance.
And you have a LOAD impedance. In power amps, we never think about the collector impedance because the load impedance is much lower than collector impedance.
Go back to a Class A amplifier, biased at 1 Amp, but make it a common emitter (collector follower) instead of the usual emitter follower. The emitter impedance is 0.03Ω plus zero external resistance. The collector impedance is maybe 30Ω. The load impedance is 8Ω. So the collector load is 30 in parallel with 8. 30||8= 6.3Ω. The emitter is 0.03Ω. 6.3/0.03= 210 voltage gain.
But: look at the input impedance of this transistor. It is Beta times emitter impedance. Say 50*0.03= 1.5Ω! How is your previous stage going to drive 1.5Ω? It can be done, but you won't get any voltage gain.
It -may- be possible to get DF=1000 into 8Ω with three stages. I think you have to sacrifice a lot of other important audio goals to get there.
> I still waiting for your answers in "Very low voltage preamp"
It seems to me you know what you want, and I have nothing to add. |
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| PRR |
> with respect to distortion and compression in the loudspeaker
Distortion will be unchanged. Voltage, current, same thing just factored by impedance.
You should never be running speakers into significant thermal compression in non-commercial work. All the parameters shift, performance droops. Get bigger speakers.
In commercial work, some now consider it "necessary" to run in thermal compression.
With constant voltage drive, as the speaker gets hot, it takes less power. Burn-out is delayed.
With constant current drive, as the speaker gets hot, it takes more power. It goes into thermal runaway (except in practice, with practical amplifiers, it will first clip like hell).
> one or two parallel LRC tanks
An expensive and slightly wasteful way to do what constant voltage drive does naturally.
You can drive at any impedance, and then EQ-out the response. It won't even take more power, if you just EQ to the constant voltage response. You will get in trouble if you run in thermal compression.
The synergy between a dynamic small cone loudspeaker and a constant voltage amplifier is a very practical and beautiful thing. People have tried every other thing, but for 85+ years low-Z drive has held its ground (except in millions of cheap pentode radios). Any other plan requires the amp to be mated to the speaker, adds a lot of complication, and almost never gives better results. |
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| phase_accurate |
According to a paper I own the voltage-transfer function (motion vs voltage) has an expression of (B*l)^2 in the denominator where you don't have this for current drive.
This would give less distortion for current drive.
Regards
Charles |
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| usekgb |
| quote: | Originally posted by phase_accurate
It is always astonishing how people believe in the magic that a high DF should be capable of doing to speaker control.
And there is ab solutely nothing gained (apart from an amp with LESS load stability) to move from an already extremely high DF of 1000 to one of 20000.
Lets do some math:
Load is 8 Ohms, we connect a cable with a total resistance of 20 mOhms (for cable AND connectors, which is a damnblodygood value).
The DF of 1000 would now be reduced to 444 and the DF of 20000 is reduced to 769 !
I don't disagree with the fact that some of these amps have indeed good LF control. But these are usually very generously dimensioned amps and I think it is the SUM of all this aspects that makes for good woofer control.
You can't have more control than the driver's TSPs allow unless you are going the NFB way or you are using an amp with negative output resistance (and don't forget to take VC heating into account when doing the latter !).
As long as your amp's output impedance is real and positive the difference in damping between an amp with a DF of 10 and a DF of 1000 is just a few % !!!
Regards
Charles |
OK Charles,
I didn't mean to sound like I was suggesting that these amps sound good because of their damping factor. I was just bringing up the Crown's as an example of an amplifier with VERY high damping factor. There are many other things that go in to these amps that make them sound good. They have many paralleled output devices, overbuilt power supplies, excellent heat dissapation, etc..... I'm sorry for the misunderstanding.
Cheers,
Zach |
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| phase_accurate |
No need to apoligize !! I remember NP writing about his X series amps, mentioning that the more powerful ones ones give more impression of control than the smaller ones even though one of the smaller ones has a higher damping factor.
Regards
Charles |
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| dimitri |
To PRR
> with respect to distortion and compression in the loudspeaker
> Distortion will be unchanged. Voltage, current, same thing just factored by impedance.
Linkwitz wrote:”I have observed that the distortion products in the acoustic output spectrum of a driver are highly correlated with the distortion components of its voice coil current waveform, when driven from a voltage source. Using feedback it should be possible to linearize the voice coil current and thereby reduce distortion to some degree”.
I also find less IMD at least for mid bass and bass drivers at the frequencies higher than 1 kHz. You can see how Z (inductance) varies with cone displacement. With voltage drive we get the correspondent modulation of current, thus higher IMD as comparing with current drive.
To Marcel
>What all of this adds up to, is that with respect to distortion and compression in the loudspeaker, it may not be such a bad idea to equalise the low-frequency part of the loudspeaker's impedance characteristic with one or two parallel LRC tanks and then current drive the whole thing.
Hi, Marcel
The better idea is to use current drive and adjustable DSP filter before the amp to equalize frequency responce, and the best is to use tube amp with output impedance of several Ohm. ;) Then we get not much boost at lf resonance but the IMD improvement at mids. |
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| lumanauw |
From the replies I read, I think I begin to understand what is the point of damping factor. My conclusion is that damping factor is how an amp can control the movement of the speakers. How independent it is. The more damping factor, the power amp is more independent to however the speaker reacts. This amp will only follow the input signal, and the speakers reaction is not important to how the signal will be generated. If this is true, then the simple formula of Zload/Zinternal is not 100% correct. If that formula is correct, you can include cable resistance, connector resistance, etc, that will lower the damping factor.
But if the essence of damping factor is my conclusion above, the damping factor is more independent to any external resistance. It is just how independent the power amplifier to the speakers, no matter how long the cables you use (in rational length). You don't need to put only 20cm cable to reduce DF loss.
Again, if my conclusion is correct, any power amp that have smaller gain (like 10x) will have bigger DF than power amp that have bigger gain (like100x). It is because smaller gain amp will have more control over it's output than the amp with bigger gain, since the sensitivity of a differential transistor is the same value, no matter what closed gain figure we use. |
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| phase_accurate |
| quote: | | The better idea is to use current drive and adjustable DSP filter before the amp to equalize frequency responce, and the best is to use tube amp with output impedance of several Ohm. |
It could be had simpler and cheaper.
Regards
Charles |
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| MarcelvdG |
Quote from PRR:
"Distortion will be unchanged. Voltage, current, same thing just factored by impedance."
This would only be true if the loudspeaker had a perfectly linear relationship between voltage and current, which it has not, for the reasons explained in one of my previous posts and further clarified by Dimitri. |
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| dimitri |
>If this is true, then the simple formula of Zload/Zinternal is not 100% correct.
The formula is correct. Damping factor is measured at the _amplifier output clamps_ Zload/Zout. Loudspeaker sees Zout+Zcable on his clamps. There is no good to do DF less then 50-100, as it is hard to make Zcable less than 0.1-0.2 Ohm.
DF was invented as the next marketing figure when THD comes to it limit (0.001-0.003%) and manufacturers should continue to compete with each other.
The main problem of DF is that it is measured at only one frequency and only one output current value. If the frequency response of DF, individual harmonic content of DF, how DF varies with output current is taken into account you will got a lot of additional info.
>any power amp that have smaller gain (like 10x) will have bigger DF than power amp that have bigger gain (like100x)
If you consider feedback amp, its output resistance and gain will decrease with increase in loop gain. Your statement is true for given amp with feedback, but _not_ the basis to compare different amps |
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| dimitri |
| There is no good to do DF _larger_ then 50-100, sorry |
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| Steven |
| quote: | Originally posted by PRR
> Another way to measure a high DF .... take a second amplifier
There's no "correct" way, and this avoids several issues. It does require another amplifier. My main concern is that some amplifiers' output Z varies with level. For cool-running Class AB transistor, Zout is usually highest at zero voltage, so this gives a low number for DF. If you are writing the specs for the ad, you want a higher number to impress buyers. If you actually want to damp the speaker, you want a low Zout over the whole swing. If the speaker is in resonance, some variation in Zout with swing doesn't matter much; but it does increase distortion.
My way has plenty of objections too. But it needs only standard test setup (oscillator, dummy load, scope, and good AC voltmenter) plus maybe a 50Ω 1.4W resistor. It tests over typical signal swings (you can try several amplitudes to see if thy all give similar answers). It also has a conceptual simplicity, easy to understand.
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Hi PRR,
Although there is no need to argue about which way is best to measure DF, I would like to point out that the current injection method I described is not 'more friendly' to amplifiers than the method of measuring output voltage in loaded and unloaded condition.
It is true that the output voltage of the amplifier under test is fixed at ground level and will not move, but this is not important. What is important is that current is forced to go in and out of the amplifier. In that way it travels along all load conditions, as if it was driven full swing.
A proper class AB push pull amplifier (with emitter follower outputs) that has about 25mV over its emitter resistors will have more or less the same output impedance (and DF) for no signal as for large signals. If the amplifier is over-biased, then it will have a higher DF at zero voltage, just like it will have a lower DF at zero output if under-biased. But the charm of pushing current into and out of the amplifier by a second amplifier (that can be any lousy amplifer; quality is not important here, as long as it is stable) is that the amplifier under test is deliberately forced away of its idle condition. The current ratio of the top and bottom half of the push pull stage is modulated. So, although the output is stable w.r.t. voltage it is modulated in the same way as if it was driving a load resistor itself.
Steven |
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| PRR |
> although the output is stable w.r.t. voltage it is modulated in the same way as if it was driving a load resistor itself.
Yes, I wrote without enough coffee in me.
The current swings. That's usually the biggest problem.
Voltage swing: on modern designs that's not an issue. But I grew up on designs where the stage before the emitter follower was resistor loaded. The gain was very different when the output was +20V or -20V.
I suppose nobody builds them like that any more.
> about 25mV over its emitter resistors
Last night I showed that this is half-right. I am not convinced of my logic and models so I have to ponder some more. But here is what I am thinking:
In a push-pull emitter follower, distortion and Zout flatness (two sides of the same coin) have two minimums. One is "about" 28mV across the emitter resistors. But that does not say what the resistor value or idle current is. And this turns out to be a very narrow minimum: a few mV less and distortion gets gross, a few more mV higher and it gets bad again. With Silicon drifting at 2mV/°C and lots of heat flowing around a power amp, I suspect it is exceptionally difficult to keep an amp on that 28mV minimum. It is only about +/-5mV wide. And any driver impedance shifts the optimum significantly (maybe that is why there is no general agreement, and 15mV is often a good value; it allows for some driver impedance).
The other "minimum" is HIGH current. It actually degrades a hair when you get into Class A: the two sides fight each other. But in the range far above the usual 28mV but below Class A, there is a broad stable minimum.
And the free variable so far is the resistor value. You can get 28mV at any current, by changing the resistor. But lower IS better for distortion in any case, IF bias is either 10-30mV or over 100mV. The low value is very critical and difficult to hold in real life. The high value is nice and stable.
I'm thinking the practical optimum is to set the bias current as high as you can stand, then set the resistors to drop 100-200mV at that current. This is for "hi-fi", not for lowest-dissipation, and will generally run warmer than any mass-market AB design, though cooler than a true Class A design.
BTW: both minimums are obscured by mis-match between NPN and PNP. There are no truly complementary pairs when looking at this scale. To see what was really happening I had to hack-up a SPICE model of a PNP with values from an NPN, to get a "perfect" matched pair. |
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| Steven |
| quote: | Originally posted by PRR
[B>... The other "minimum" is HIGH current. It actually degrades a hair when you get into Class A: the two sides fight each other. But in the range far above the usual 28mV but below Class A, there is a broad stable minimum.
[/B] |
PRR,
I agree that 20-30mV across Re is a narrow optimum that is difficult to keep stable in practice. Turning up the idle current at least moves the point where Gm is halved away. So for small signals Zout is quite stable. But, I'm a little surprised that your simulations shows that it degrades when you get into class A. I would assume that as long as you are in class A without one of the transistors starving for current the output impedance would be mainly determined by Re//Re, which is very stable.
Actually, I don't understand what you mean with 'below Class A'; as long as the amplifier is idling, we are in the class A region. :confused:
Steven
PS I'm currently busy with simulations for the 'Spreading the heat in Class A' case. I changed the circuit such that I don't have jumps on the collector of Q1 anymore that invalidates the Class A benefits. I will post the answer this weekend in that thread. |
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| PRR |
> as long as the amplifier is idling, we are in the class A region.
Point taken about idling (though like the tree in the forest, can we talk of Class when there is no signal?)
I mean: idling at a very high current that allows full-power Class A operation.
Let me check some more things. Unless my pants catch fire, I probably won't post over the weekend (56K modem, new definition of "poverty"). |
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| DrG |
I've been away a bit... I don't know if I agree with the following, PRR:| quote: | | Velocity is proportional to voltage, and for cone speakers the acoustic output is mostly a function of velocity, not force. |
The relationship between applied voltage and flowing current is not linear in a speaker. This being so, it is *only* the current in the coil which generates flux and thence force. The voltage which caused the current is irrelevant.
Acoustic output is not a function of cone velocity per se. It is a function of cone displacement which is proportional to net force, which is determined by coil current, which results in a net acceleration a=F/m. At no time does the oscillating diaphragm possess velocity in the constant sense. Unless you mean angular velocity...
And impedance characteristics varying as they do wrt frequency, I repeat my perception that current drive of a speaker appears more natural/logical... |
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| millwood |
| quote: | Originally posted by DrG
current drive of a speaker appears more natural/logical... |
agreed. But why is that there is few current drive amps out on the market?
I remember that doing similar things (using a current sample resistor in serial to the speaker coil as feedback) in tube days but it seems to have died out (I tried it and other than tighter bass it did nothing audiable).
I guess is that the differernce between a current vs. voltage drive amp is minimal, at best. |
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| DrG |
| quote: | | agreed. But why is that there is few current drive amps out on the market? |
I don't know...| quote: | | I guess is that the differernce between a current vs. voltage drive amp is minimal, at best. |
Perhaps. But I'm not sure and I'd like to hear from the brains trust out there what the concensus is. Anybody tried it?
I saw a little amp some years back incorporating the speaker into the feedback loop, with a small 0R22 shunt resistor. What are the drawbacks of such an arrangement? |
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| millwood |
| quote: | Originally posted by DrG
I saw a little amp some years back incorporating the speaker into the feedback loop, with a small 0R22 shunt resistor. What are the drawbacks of such an arrangement? |
it is exactly what I was talking about from my tube days. I don't know of any drawbacks but I do know that it doesn't do a whole lot of good either.
So the question is "why bother?". |
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| DrG |
| quote: | | So the question is "why bother?". |
The answer is "why not!" Seriously though, my personal approach to everything audio is to try to be as unconventional as possible. I figure the chances of creating something really special are greater when choosing the road less travelled by. And admittedly there is also an ego thing... I did it MY way etc. Each to his own.
Thanks for the link Tube_Dude |
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| jcarr |
>I saw an amp incorporating the speaker into the feedback loop.<
The remote feedback system was invented by Shin Nakagawa, and was subsequently used in consumer audio amplifiers from the likes of Kenwood and Toshiba.
>What are the drawbacks of such an arrangement?<
The feedback loop will now include substantially more reactive elements than if it had been kept within the amplifier only, which can lead to a variety of stability and recovery issues. The phase compensation for a remote feedback amplifier will need to be fairly heavy-handed to keep the amplifier under control.
Good approach for a self-powered subwoofer, but if you want a good-sounding full-range power amplifier, I think that there are better ways than the remote feedback approach.
>my personal approach to everything audio is to try to be as unconventional as possible.<
Unless it makes you fee better about yourself, I don't think that it means all that much to be unconventional or conventional. Rather, I suggest that you do whatever is appropriate for the task at hand. If there is a specific engineering or performance goal that you are aiming for, and if you cook up an original and unusual method to achieve that goal, fine.
But if not, I don't see the point in being different just to be different.
regards, jonathan carr |
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| DrG |
| quote: | | I don't see the point in being different just to be different. |
You needn't. Each to his own, like I said. And you're right, I do feel better about myself when I achieve something via an alternate route. "It works well and I did it my way" does more for my ego that "It works well because I copied someone else's established method." But that's just me.
Jonathan, I admit a problem-oriented approach using the best circuit element for each situation is probably best from an engineering approach. But I'm not a commercial audio engineer. |
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| lumanauw |
I just wondering. Since PRR and Steven has told us the way to measure/calculate damping factor, is there any PRACTICAL way to measure other properties of:
-slew rate
-THD
-S/N ratio
With ordinary hobbyist equipment (voltmeter, scope, dummy load)?
There is some measurement device out there, like NeutrikA2, but it is very very expensive, and maybe only factory can buy it. |
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| dimitri |
| you should add nice sound card and analyzer software |
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| PRR |
Slew rate: there are several ways.
Feed a sine and increase the frequency and voltage until it becomes more of a triangle. The slope of the triangle is the slew rate. Or for a go/no-go test: feed 1KHz at just below clipping, switch to 20KHz, see if it is more sine or triangle.
In real life, if it is pretty-sine at full output up to 6KHz, it will pass music very nicely. Full power sine at 20KHz would burn your ears and tweeters, it really does not happen in anything you want to listen to.
NOTE: full power at high frequency will burn-out some amplifiers in a few seconds. Don't come crying to me.
THD: good THD measurements are very fussy. You can get a useful "Total Garbage" measurement by comparing input to output. You need to set the two gains so the signal cancels. If the amplifier is non-inverting, you need an inverter somewhere. Good 2-channel scopes can do this internally. Trim the gain for minimum 1KHz wobble, crank up the gain, and look at the residual. At high and low frequency you probably can't null the signal without an adjustable phase-shift network.
S/N: define your specification (there must be a zillion ways to specify S/N) and use an AC milliVolt meter.
The sound card test programs do simplify S/N and THD, though you will probably have to build pads to lower the input and output levels below 2V, maybe more like 0.2V for some cards. Always run a "straight wire" test so you know how good/bad your sound card is. Some are pretty good, some are about useless for solid-state work. You can not tell by specs: my "32-bit" onboard sound is missing the top bit and noise starts at the 15th bit, so the usable range is more like 14 bits or 84dB S/N and the test program I'm using always shows high THD. But I have done some very fine measurements even on older cards. |
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| Circlotron |
| I don't really know a whole lot about this kind of stuff but I get the gut feeling that if your amplifier had an output impedance of ZERO i.e. the damping factor by definition was INFINITE, the *actual* damping of the speaker would be 1/Qts or whatever 1/Qt? it may be when in the box That sounds a bit ordinary, doesn't it? :dead: |
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| phase_accurate |
| quote: | | I don't really know a whole lot about this kind of stuff but I get the gut feeling that if your amplifier had an output impedance of ZERO i.e. the damping factor by definition was INFINITE, the *actual* damping of the speaker would be 1/Qts or whatever 1/Qt? it may be when in the box That sounds a bit ordinary, doesn't it? |
Congrats for this conclusion ! ;)
It was already mentioned many times but not many people seem to listen.
IMO a DF of 100 is sufficient. I think it is more important that an amp's output impedance is independant of frequency (not fully achievable), output power and load impedance.
Regards
Charles |
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| mrfeedback |
Originally posted by phase_accurate
Congrats for this conclusion ! ;)
It was already mentioned many times but not many people seem to listen.
IMO a DF of 100 is sufficient. I think it is more important that an amp's output impedance is independant of frequency (not fully achievable), output power and load impedance.
I reckon that it is not mandatory for an extreme or constant damping factor across the full audio frequency range, and can actually be a hinderance.
Good damping factor is required at low frequencies and 100 or 200 is plenty.
High frequencies do not really require damping at all because the crossover networks in a typical decent quality loudspeaker provide local electrical damping of the mid and hf drivers.
Reduced NFB at higher frequencies can imply increased distortion and lowered damping factor, but this does not have to be sonically damaging, and in practice is a benefit.
I am familiar with an amplifier that does exactly this and it sounds great, so good in fact that a bunch of these are going into a seriously high end international mastering studio.
The figures are as follows.........
Harmonic Distortion into 8 Ohms
1W 100 Hz 0.01%
1W 1 KHz 0.01%
1W 10 KHz 0.05%
10W 100 Hz 0.01%
10W 1 KHz 0.01%
10W 10 KHz 0.05%
100W 100 Hz 0.015%
100W 1 KHz 0.02%
100W 10 KHz 0.20%
Intermodulation Distortion
50 Hz and 400 Hz 0.01%
50 Hz and 4 KHz 0.03%
Damping figures are not given, but I understand that they are fairly high (300 maybe) at low frequencies, and reducing at higher frequencies.
These figures don't look spectacular, but I think not worse than say Nelsons Aleph stuff, and better on paper than a heap of other stuff.
What the figures don't describe is the clearness, relaxedness and fluidness in the mid and higher frequencies, and the higher frequency distortion is not audibly detracting/distracting in any way.
This technique also implies better stability and less reactive load dependence, and these qualities indeed allow sonics detail and blackness eons better than all SS NFB amplifiers I have previously heard.
High NFB at high frequencies implies very high frequency open loop bandwidth if the amplifier is to remain stable into reactive loads.
High NFB at high frequencies can also cause SS sonic harshness and forcefulness.
High NFB at high frequencies is not electrically required, so enjoy the dynamic sonic benefits of lower NFB in the mids and highs I say.
Eric. |
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| phase_accurate |
| quote: | | High frequencies do not really require damping at all because the crossover networks in a typical decent quality loudspeaker provide local electrical damping of the mid and hf drivers. |
I can generally agree BUT if DF gets too low (i.e. output impedance too high) then one ends up with a hevily load-dependant frequency response.
| quote: | | High NFB at high frequencies can also cause SS sonic harshness and forcefulness. |
I agree that very high NFB factors, used as a cure-all, are less than optimal. But my opinion is that the NFB factor should be kept constant up to above the midrange at the cost of high NFB at lower frequencies. This would result in an amp that doesn't show increasing THD with rising input frequency (the latter being the case for many SS amps).
Regards
Charles |
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| lumanauw |
If this damping factor issue can be done, why not. If it will be limited by the speakers, it's something else, but doesn't have to stop us pursuing it. I'm sure there will be difference, inspite of the speaker limiting.
I'm very intersted in this. Any schematic for hint how this can be done?| quote: | | Reduced NFB at higher frequencies can imply increased distortion and lowered damping factor, but this does not have to be sonically damaging, and in practice is a benefit. |
If it is OK with this forum, what is the exact brand and type of amp is beeing talked about| quote: | | I am familiar with an amplifier that does exactly this and it sounds great, so good in fact that a bunch of these are going into a seriously high end international mastering studio. |
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| mrfeedback |
Originally posted by phase_accurate
I can generally agree BUT if DF gets too low (i.e. output impedance too high) then one ends up with a heavily load-dependant frequency response.
Agreed, but in my experience relatively minor FR variations are much less sonically damaging than NFB/load induced ringing and higher order harmonics production etc.
I agree that very high NFB factors, used as a cure-all, are less than optimal. But my opinion is that the NFB factor should be kept constant up to above the midrange at the cost of high NFB at lower frequencies. This would result in an amp that doesn't show increasing THD with rising input frequency (the latter being the case for many SS amps).
Agreed, keep the NFB sufficiently high through bass and mids to keep damping appropriate and distortion low, and allow drooping NFB at above mids even though distortion is increased.
Lower order THD (produced/allowed by low NFB ratio) is sonically much more tolerable than even small amounts of higher order harmonics.
Some harmonics production above say 4 kHz is not particularly offensive mainly because higher order products are out of audio band and rendered inadible provided that lower order IM products are not generated also.
Lowered NFB can also imply good IM and TIM distortion performance provided that the open loop bandwidth is relatively high.
To my ear IMD and TIMD are FAR more damaging (and fatigueing) than lower orders of THD.
In practice, good damping/distortion performance at low/mid frequencies, and 'free-er' sounding high mids/highs adds up to very nicely acceptable sonic performance with typical speakers and very high performance large studio monitors also.
With ESL, the capacitive loading can cause a slight drooping response towards the highs, but there are no transient nasties produced as can be the case with high NFB amplifiers.
A droop of 1 or 2 dB at 20kHz may not be perfectly correct, but
sonically works out very nicely.
Eric. |
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| lumanauw |
| MRFEEDBACK, I'm interested in your comments. Could you share with us the schematic you think as good one? |
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| Nexus |
Hello All,
I have a question about simulating the damping factor in spice.
Is the shortest route to do this put a sine wave current source in the output instead of the load, and place a 1k resitor on the input of the amplifier (no drive signal)?
Then choose 1A for the sine wave, and the measured voltage on the output is the output resistance?
Best regards:
Nexus.
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| Steven |
I would say 'yes', Nexus.
Steven |
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| thoriated |
The issue I see with 'current drive' to speakers is that it's sonic effects may be effectively similar (not necessarily the same) as low damping factor. Plus, it may not give optimal results with speakers that were designed for use with low source impedance (high damping factor) amplifiers.
Another effect I think is of some importance wrt damping factor is that if the amp drives a multiway speaker with passive crossovers, a lower damping factor amp may allow more back emf from each driver to make it to the other (but so would higher impedance speaker cables unless biwired).
Probably damping factors over 200 at the speaker terminals become less important in the real world given practical speaker gauge wires and run lengths which will add series resistance, etc. to this.
I agree with one poster about the detrimental sonic effects of achieving high but nonconstant with frequency damping factors with large amounts of negative voltage feedback. However, my DC coupled OTL design achieves its high damping factor with only moderate negative voltage feedback (26db above 1hz) but uses positive current feedback to achieve damping factors over 500 to 10 khz (when trimmed out). One thing about positive current feedback is that when summed in the same loop as I do with the negative voltage feedback, it actually will reduce, if anything the total inverting feedback signal when it is boosting damping factor into any real world load. |
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| thoriated |
| Erratum. I meant to post: "Probably damping factors over 200 at the Amplifier terminals... " above |
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| Ultima Thule |
Just noticed someone earlier asked how to find out the slew rate, I would like to suggest to use square wave at the input at a quite low frequency of 100 Hz for instance(, less risk to burn the amplifier).
Should be noticed that the input shunt capacitor forming a lowpass filter must first be removed before the test (at least one of the legg ;) since it doesnt have anything to do with the amplifier bandwith and decrease the slewrate at the input before the signal reach the first amplifier stage.
Connect an oscilloscope at the output and notice the speed of the rising or falling flanks.
Slewrate is often given in V/uS, a fair amplifier has at least 20V/uS I think.
I dont want to specify any type of standard how to check the slewrate but it could be good to check for both small signals let say 1 volt output and full swing.
When talking about the feedback and distortion the human ear is much more sensitive to TIMD than THD, even <0,1% THD for all frequencies and power levels is very good. |
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