We usually call that downward synamic range to make it very clear.
One should not consider that a generalization. When used with speakers designed for higher output impedance amplifiers, there can be plenty of detail.
dave
If an amplifier appears to have different impedances when measured as output or input at the output then there is either something seriously wrong with the amplifier or the test method. Not knowing the details I can't say more, but a resistor is a resistor - whether a real piece of carbon or simulated via lots of active devices and feedback. Significant non-linearity could cause the effect, but an amplifier with a seriously non-linear output impedance would measure badly into a resistive load anyway.
Assuming competent design, output impedance does tell you what you need to know.
Assuming competent design, output impedance does tell you what you need to know.
As stated in the post "95%..." have serious problems. This is based on having over 1000 different amplifiers across my bench therefore with most amps having serious problems using a load resistor tells little about the amplifiers ability to absorb back EMF- the gist of damping and so we agree.
Never heard of downward dynamic range until today. For about 100 years the distinction was quite clear until recently (last 20 years) when SNR was said to be the DR and that these two values (popularly assumed) were the same. Thanks for the tip on the new name of the old measurement.
Never heard of downward dynamic range until today. For about 100 years the distinction was quite clear until recently (last 20 years) when SNR was said to be the DR and that these two values (popularly assumed) were the same. Thanks for the tip on the new name of the old measurement.
Sumaudioguy, could you explain the kind of measurement you performed with the dummy resistor loads (what is the problem and where do you measure it) and what is the typical oscillation frequency you see?
Thanks in advance.
Thanks in advance.
For many years ran an audio repair shop servicing both consumer and professional gear which was obviously broken and in for repair. In that time hardly a brand I did not see including most models of the popular brands. So while an amp was repaired and being checked out on the bench it ran a resistor it is true. But the amp was also given the task of driving the bench speakers, a full range one way not unlike the Auratone studio AM reference speaker. The amplifier under test driving the speaker could be supplied a music signal or a signal from the function generator on the bench. Further, several different loads such as a .05µF capacitor (or other small value less than .1µF) and/or and 8 ohm resistor could be added to the full range. Driving the speaker by itself with a test signal half the amps showed severe ringing and/or oscillation. I think the test speaker was not the best load as it has a fair amount of network to flatten out the response of the heavy duty 4". For the other 45% adding a (only the right one) capacitor (as above) and/or reducing the load to 4 ohms on the test signal showed the problem. Once the load was adjusted to cause the amp to misbehave a music signal was applied. In every case the amp misbehaved with a music signal also with the "tuned" load. Oscillation frequencies on a few amps were as low as 100kHz with typical being in the 800kHz to 2MHz region. Places on the oscilloscope trace where the line got "fat" or very "furry" looking like a small bead on a string almost always on the negative going slope or at the front or end of a transient signal using the music signal.
I still do some repair today (35 years on) and most of the amps still can easily be made to oscillate or show severe uncontrolled ringing using a transient signal. Nothing has changed that way as far as I can tell. I modify amps all the time to improve stability. The last one modified was a Soundcraftsman that oscillated on one channel all the time no matter the load or input signal. Go figure- it was built that way and deemed acceptable as it also passed signal. A 7pF cap was changed to a 50pF and the oscillation went away. A Crown under warranty came in shortly after that and had problems also so it got fixed and is no longer blowing up. It had 3 warranty repairs before it came here and got fixed to reliable.
Amplifiers are supposed to behave like filters with gain. Unfortunately most amplifiers do not seem to have a stable filter/gain transfer function under loads like a speaker. This is discussed a lot in op amp designing. Damping becomes part of that and listener sound experience becomes a bigger part of that. It is my strong belief that a good amplifier cannot be made to oscillate under any operational condition or show much ringing at all exhibiting a very constant damping versus frequency characteristic. A small percentage of the amps crossing my bench appear to be exceptionally stable and listening to those particular amps has always been a surprising excursion into clarity and musicality. One of the very very good ones was a 5 watt per channel close and play style all in one record player tuner with AUX inputs- Symphonic was the brand. Another was a Kenwood 6200. There were so few. Most amps fail because they break into oscillation badly which causes the bias to soar to extreme values leading to power transistor failure. Less of the same leads to poor damping and bad sound.
As a note, one of the better amps came in for repair last week and it had 4 blown emitter resistors and that was it. The owner later admitted to shorting the speaker wires while the amp was on and blowing the line fuse. That was all that was wrong with that amplifier. A stable design.
Here, http://www.diyaudio.com/forums/chip...ltralow-distortion-chipamp-6.html#post1668797 is an amp that is in oscillation on the clip recovery during the negative going wave. See the very furry waveform? Here is a well behaved amp- http://www.diyaudio.com/forums/chip-amps/163385-so-just-how-good-can-chip-amp-9.html#post2148761
Lots of luck! Hope I did not blow on to long. Sorry to those who got bored with the post
I still do some repair today (35 years on) and most of the amps still can easily be made to oscillate or show severe uncontrolled ringing using a transient signal. Nothing has changed that way as far as I can tell. I modify amps all the time to improve stability. The last one modified was a Soundcraftsman that oscillated on one channel all the time no matter the load or input signal. Go figure- it was built that way and deemed acceptable as it also passed signal. A 7pF cap was changed to a 50pF and the oscillation went away. A Crown under warranty came in shortly after that and had problems also so it got fixed and is no longer blowing up. It had 3 warranty repairs before it came here and got fixed to reliable.
Amplifiers are supposed to behave like filters with gain. Unfortunately most amplifiers do not seem to have a stable filter/gain transfer function under loads like a speaker. This is discussed a lot in op amp designing. Damping becomes part of that and listener sound experience becomes a bigger part of that. It is my strong belief that a good amplifier cannot be made to oscillate under any operational condition or show much ringing at all exhibiting a very constant damping versus frequency characteristic. A small percentage of the amps crossing my bench appear to be exceptionally stable and listening to those particular amps has always been a surprising excursion into clarity and musicality. One of the very very good ones was a 5 watt per channel close and play style all in one record player tuner with AUX inputs- Symphonic was the brand. Another was a Kenwood 6200. There were so few. Most amps fail because they break into oscillation badly which causes the bias to soar to extreme values leading to power transistor failure. Less of the same leads to poor damping and bad sound.
As a note, one of the better amps came in for repair last week and it had 4 blown emitter resistors and that was it. The owner later admitted to shorting the speaker wires while the amp was on and blowing the line fuse. That was all that was wrong with that amplifier. A stable design.
Here, http://www.diyaudio.com/forums/chip...ltralow-distortion-chipamp-6.html#post1668797 is an amp that is in oscillation on the clip recovery during the negative going wave. See the very furry waveform? Here is a well behaved amp- http://www.diyaudio.com/forums/chip-amps/163385-so-just-how-good-can-chip-amp-9.html#post2148761
Lots of luck! Hope I did not blow on to long. Sorry to those who got bored with the post
Hello,
Too many words! Too many words cause confusion.
It is all about amplifier output impedance + cords and stuff divided by speaker impedance. The driver does not have constant impedance. The speaker impedance has a peak. How high the peak is Total Q. mechanical dampening in the speaker box or electrical dampening at the amplifier tame the impedance peak. Yes the impedance peak is also a resonance peak it is all about frequency. Also yes under or over dampening does affect the quality of the sound.
BTW consider efficiency. One electrical watt in and 95 db out is pretty high but only a few % efficient, nowhere near not even the same order of magnitude of acoustic power out).One third of the power returned to the amplifier as back EMF, not on this planet.
An 100 horse power 3 phase motor and a 8 foot diameter plug fan coasting to a stop and turned back on, things break conductors melt and there is a flash of light now that is back EMF.
Some confusion of my own.
DT
All just for fun!
Some amplifiers have marginal HF stability around the feedback loop. Adding capacitance to the output may tip these over the edge. Others may have a marginally stable emitter-follower output - again capacitance can create oscillation, usually at a much higher frequency than global loop instability. I suppose it is possible for some amps to have a negative output impedance, so these could oscillate if the output looks like a tuned circuit. All these are signs of weak or misguided design, and inadequate testing. Part of the problem may be that audio designers don't always know enough about RF.
Regarding back EMF, very little power comes back from the speaker. Most of the forward power gets dissipated in voice coil losses, acoustic damping etc. The little power which does come back is coming from a source with relatively high impedance (8ohms or more at the bass resonance) so unless the amp has a very low damping factor almost all of the 'reverse power' is actually dissipated in the speaker anyway! There is certainly nowhere near enough to heat up the amp. Amp heating is caused by normal DC and AC - signal and unwanted oscillation. My personal view is that much of what is written about back EMF from speakers is complete nonsense, and merely shows a lack of understanding about what is meant by impedance and energy storage.
Regarding back EMF, very little power comes back from the speaker. Most of the forward power gets dissipated in voice coil losses, acoustic damping etc. The little power which does come back is coming from a source with relatively high impedance (8ohms or more at the bass resonance) so unless the amp has a very low damping factor almost all of the 'reverse power' is actually dissipated in the speaker anyway! There is certainly nowhere near enough to heat up the amp. Amp heating is caused by normal DC and AC - signal and unwanted oscillation. My personal view is that much of what is written about back EMF from speakers is complete nonsense, and merely shows a lack of understanding about what is meant by impedance and energy storage.
Reflected power... The lowest point on a given drivers impedance curve is usually near 1.22 times the Re DC resistance value for the voice coil. As DualTriode says...94dBa is but 1% efficient so where does the other 99% of the power go? I said 1/3 of the power is sent back in the form of back EMF. I should have added bad load, not zero phase, and back EMF. In the 1% case 82% of the in phase (real) power goes to voice coil heating. A percent or two goes into other mechanical losses. The power driving the reactance (imaginary out of zero phase) ends up being reflected back to the amp. Simply mismatch wire to load. Reactances do not absorb energy which ends up mostly going back to the amp. Reactive power can easily be greater than the real power as far as the amplifier is concerned. Oh yes- there is that 1% that leaked through to make acoustic output. DF96 has correctly stated the cause of most oscillation- emitter followers and common emitter output stages acting as single port oscillators because there drive is not well controlled and loop instabilities- nicely written.
Thanks for correcting.🙂
Thanks for correcting.🙂
Reactive power reflected back, or net impedance (including energy storage)? Two equivalent ways of thinking/doing the sums. As a general rule, you use the former for RF or high Q resonances and the latter for audio or low-Q resonances. Using either method correctly will give you the right answer. Using the most appropriate method will give you a quicker answer. Trying to use both at the same time brings the risk of double-counting.
In order to reproduce music tolerably well, the resonances in a speaker must have much lower Q than the resonances in the instruments producing the music. This means that the quasi-static impedance approximation is adequate for the task. There may be some issues about current draw during starting transients, but most amps can cope OK nowadays as people are aware of this.
Most speakers are designed to be driven from a voltage source. So if we approximate a voltage source, with sufficient current capability, we should be OK. Anything else is mumbo-jumbo. I suspect that mumbo-jumbo arises because people don't understand the basics as well as they think, so they have to invent esoteric reasons why amps sound bad. Then their fans pick this up and repeat it, in all innocence. If people designed amps which are stable, with good current capability, low intrinsic crossover distortion, highish slew rate and lowish distortion, then added stable global NFB we would all be happy. Current trends still seem to avoid this: SS often uses massive NFB to overcome serious intrinsic distortion, while some of the valve crowd try to avoid NFB altogether and believe that distortion sounds nice.
In order to reproduce music tolerably well, the resonances in a speaker must have much lower Q than the resonances in the instruments producing the music. This means that the quasi-static impedance approximation is adequate for the task. There may be some issues about current draw during starting transients, but most amps can cope OK nowadays as people are aware of this.
Most speakers are designed to be driven from a voltage source. So if we approximate a voltage source, with sufficient current capability, we should be OK. Anything else is mumbo-jumbo. I suspect that mumbo-jumbo arises because people don't understand the basics as well as they think, so they have to invent esoteric reasons why amps sound bad. Then their fans pick this up and repeat it, in all innocence. If people designed amps which are stable, with good current capability, low intrinsic crossover distortion, highish slew rate and lowish distortion, then added stable global NFB we would all be happy. Current trends still seem to avoid this: SS often uses massive NFB to overcome serious intrinsic distortion, while some of the valve crowd try to avoid NFB altogether and believe that distortion sounds nice.
Anyone trying with great horn? when the loudspeaker gets much louder with it, and voice coil still hot like before. Then, where these extra accoustic power coming from?
I agree that voice coil dissipating power, but not that much. Most loudspeaker vibration only move the air and not producing sounds.
Loudspeaker will act like big microphone also springs and resonator(inbox), and more. Thats why it need a damper to dissipate looping of undissipated energy, or it will dissipated at voice coil.
I agree that voice coil dissipating power, but not that much. Most loudspeaker vibration only move the air and not producing sounds.
Loudspeaker will act like big microphone also springs and resonator(inbox), and more. Thats why it need a damper to dissipate looping of undissipated energy, or it will dissipated at voice coil.
Sumaudio, that was a great read! One question though:
Sorry if this question was off-topic maybe. But this topic is about another view of damping factor... so also about the (mis)match between amplifer, cable and loudspeakers.
I would think it to be better to have an active amplifier per loudspeaker unit, what use is a large damping factor when you use inductors of hundreds of milliohms in series with the woofer?
Was this cap the capacitor in the NFB loop to the input diff. pair?
Sorry if this question was off-topic maybe. But this topic is about another view of damping factor... so also about the (mis)match between amplifer, cable and loudspeakers.
I would think it to be better to have an active amplifier per loudspeaker unit, what use is a large damping factor when you use inductors of hundreds of milliohms in series with the woofer?
Last edited:
Commonly called the "Miller" cap was changed. This is a great place to stabilize an amp because it only affects the slew rate and stability while not affecting the delay time through the amp or bandwidth.
You are right- one amp one driver, yummy. This allows use of smaller amps also. A handy feature. The worst passive crossover at low level is probably better than the best one at speaker level being "in the way" of the amp controlling the speaker.
Increasing the Miller cap across the VAS will often increase stability, but will normally reduce the slew rate and open-loop bandwidth by about the same ratio. Reducing the open-loop bandwidth will also reduce the closed-loop bandwidth unless this is already restricted by global NFB. It will also raise HF distortion as there is less loop gain.
A change from 7pF to 50pF will seriously 'slug' an amplifier, and seems rather extreme unless the 7pF is actually wrong. You might get away with this for a PA or guitar amp. Even allowing for some stray capacity too, you might still have made some parameters worse by a factor of five.
I guess a sluggish amplifier is better than an oscillating one!
A change from 7pF to 50pF will seriously 'slug' an amplifier, and seems rather extreme unless the 7pF is actually wrong. You might get away with this for a PA or guitar amp. Even allowing for some stray capacity too, you might still have made some parameters worse by a factor of five.
I guess a sluggish amplifier is better than an oscillating one!
When yours calculate the damping factor, very much keeping in mind the power supply of the audio amplifier. The power supply has a decisive role in the value of damping factor. And other features of the amplifier causes a rather small change the value of this factor.
The topic below I said some very important things about the damping factor:
http://www.diyaudio.com/forums/solid-state/172617-class-ab-amp-made-me-3.html
I recommend yours read it.
Regards
The topic below I said some very important things about the damping factor:
http://www.diyaudio.com/forums/solid-state/172617-class-ab-amp-made-me-3.html
I recommend yours read it.
Regards
Not true. The power supply has only a small effect on damping factor, unless the amplifier is very strange. Damping factor is largely set by the output topology and the amount of global feedback, together with any resistive losses in the output network, if present.
50pF still left that amp with more than 50kHz full power bandwidth so would not say it was "slugged." Changes in open loop full power bandwidth with miller cap changes yes, changes in small signal frequency response no. Miller cap in a last high gain current to voltage converter stage only affects the time domain and does not affect the frequency domain. It does not even change the delay time through the amplifier.
Should we start a thread on slew rate? Believe there is not one as have looked.
If anyone looked at the post http://www.diyaudio.com/forums/chip-amps/163385-so-just-how-good-can-chip-amp-9.html#post2149038 that high gain amp has crazy low output Z. Grounding technique has a gross effect when the numbers get small. Bad grounding and the power supply or a host of other factors can play a big role if there is even one error in layout.
Should we start a thread on slew rate? Believe there is not one as have looked.
If anyone looked at the post http://www.diyaudio.com/forums/chip-amps/163385-so-just-how-good-can-chip-amp-9.html#post2149038 that high gain amp has crazy low output Z. Grounding technique has a gross effect when the numbers get small. Bad grounding and the power supply or a host of other factors can play a big role if there is even one error in layout.
But it will change the amount of negative feedback (and the freq where it starts to drop), which will change the THD figure at "higheer freqs ( in some amps above 100hz)
My take on DF. Tap your woofer, with the amp on then off. Hear the difference. When the amp is on there is a lot less sound. The amp damps the natural vibration of the driver which would interfere with your music. How much damping do you need? Tap the woofer with different resistors atached instead of your amp and see what value stops making a difference.
DF96, you want to tell me that: if you feed a power amplifier with source with X power and the same amplifier with a 2X power source, you get the same damping factor? You are wrong.
And it is true that the damping factor is influenced by the output topology and the amount of global feedback, but very little if the amp is designed properly.
- Status
- Not open for further replies.