Damping Factor >1000

[Steve Eddy]

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.

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

[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.

[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.

[DrG]

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?

[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.

[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.

[Steven]

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.)
...

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

[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.

[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?

[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.