Damping Factor in Amplifier

[brianti]

That it has broken of the circuit more influence the factor of damping in an amplifier, differencial pair stage, VAS, Drives stage or Output stage.

Thank's
Sidnei

[fab]

hi brianti

All parts of amp have an influence on the damping factor and it depends on the amp topology (voltage feedback, current feedback, no overall feedback, ...). In a voltage feedback amp for example , the ouput impedance of the amp in open loop is reduced by the amount of feedback applied. One way to get a high damping factor is to have the output stage equivalent impedance low along with high open loop gain of the amp (for a voltage feedback amp of course). All stages gain have their influence on the overall open loop gain value.

Fab

[fab]

Also, you can use an output stage error correction circuit like the one discussed in this post
http://www.diyaudio.com/forums/showt...999#post381999
or the whole thread on "Hawksford"

to increase the damping factor ( I suppose that it is the goal of every designers to have a high DF in an audio amp).

Fab

[ChocoHolic]

Hi Brianti!
Don't shoot me, ...BUT I do not have the goal to achieve a high damping factor.
As long as it is around 50 or higher, I tend to put my efforts to get
this damping factor constant all over the audiable frequency range.

Furtheron, I prefer stable frequency compensation for any load much more than tuned damping factor ....

What does damping factor mean? This value finally describes
the output impedance of your amp. This impedance is in series with the
speaker voice coil resistance and by this it has impact on the damping.
OK. But as long as the output impedance of the amp is much lower than the speakers impedance, the damping will be dominated by the speakers properties. Speaker damping will be the same at
an damping factor of 50 or an damping factor of 1000.
Especially if we look at the damping of the woofer many people are very concerned about the last mOhm output impedance of the amp, and the wires and put endless money to that..., while not noticing the 200mOhms of the choke in the passive cross over ...

Curiously waiting for fire from every side....
Markus

[brianti]

Thank's

Sidnei

[traderbam]

Damping factor is an old term.
My rule of thumb is that an amp's output resistance (real part of it's output Z) should be <0.2 ohms over the audio band.
The more "real" it is the less likelyhood of undesirable reactive interaction with the speaker.

I believe damping factor is defined to be speaker resistance divided by amplifier output resistance. So an output R of 0.2 provides a damping factor of 40 for an 8-ohm resistance speaker.

You'll find that the resistive output R of an amp is not, in itself, one of the more critical factors to sound quality. For example, you can try adding half an ohm or so resistor in series with a speaker and listen to the difference. It will be very subtle unless the system is exceptionally good in other respects.

[dkemppai]

Originally posted by traderbam
Damping factor is an old term.

I believe damping factor is defined to be speaker resistance divided by amplifier output resistance. So an output R of 0.2 provides a damping factor of 40 for an 8-ohm resistance speaker.

You'll find that the resistive output R of an amp is not, in itself, one of the more critical factors to sound quality. For example, you can try adding half an ohm or so resistor in series with a speaker and listen to the difference. It will be very subtle unless the system is exceptionally good in other respects.

Just curious, how do you measure an amplifiers output resistance? You can't just look at circuit resistances, because they are electrically removed (at least for small to moderate power levels).

My curiosity comes from thinking of an amplifier running a closed feedback loop. Such an amp would appear to have a resistance of zero. However, zero is an impossible number to attain. In reality, output resistance may very well be some small value. And, is it actually a resistance (i.e. linear)?

I'm currently wondering if the output resistance is some frequency and output current dependent value, based loosely on the open loop gain of the amplifier or gain margin between the open loop gain and the closed loop gain combined with the load current being drawn.

Thoughts on this?

-Dan

[amb]

Originally posted by dkemppai
Just curious, how do you measure an amplifiers output resistance? You can't just look at circuit resistances, because they are electrically removed (at least for small to moderate power levels).

Given that the output impedance of the amplifier is not zero, such impedance and the load impedance forms a voltage divider. By measuring the voltage drop created by this voltage divider you can calculate the output impedance.

Here is a practical example. First, run your amp without a dummy load, and not into any speakers. Feed a sine wave of a certain frequency (say, 1KHz) into the input of the amp, and measure the output voltage of the amp, adjusting the input level as necessary to achieve a particular reference output level (for example, 1Vrms). Let's call this voltage V1. Don't set this voltage much higher than this to prevent overheating the amp or burning out the dummy load.

Then, connect an 8 ohm dummy load resistor (rated at least 5W or more, and use one that whose resistance is stable over a wide temperature range) to the output of the amp. Measure the voltage at the output again. Let's call this voltage V2.

V2 should be slightly less than V1 due to the voltage divider effect.

We can express the relationship as follows:

V1/V2 = (R1 + R2) / R2

where R1 is the output impedance of the amplifier and R2 is the dummy load resistance. Solving for R1 gives:

R1 = (R2 * (V1 / V2)) - R2

So, as an example, if your V1 was 1.000Vrms and V2 was 0.980Vrms, then the output impedance would be 0.16 ohms at the test frequency.

The damping factor, would then be 8 / 0.16 = 50

Note that to get accurate measurement you need to measure the exact resistance of the dummy load resistor rather than just use the nominal "8 ohm". Also, you should use a highly accurate AC voltmeter to do the measurement, especially if the output impedance of the amp under test is very low. You can repeat this test using sine waves at different frequencies to establish a output impedance vs. frequency curve. Most amps exhibit a slight rise in output impedance at higher frequencies. Your AC voltmeter should have good HF response for that.

You may find that using a 4 ohm dummy load would yield a voltage drop that is easier to measure.

-Ti

[dkemppai]

Ah,

I've reverse calculated things like that many many times, I don't know why I just didn't do it here! I guess I never thought of just hooking up my fluke to my amp. I always use the scope.

Anyway, I tried the amp that I've designed, and have found the output impedance to be about .026 Ohms. I did the measurements at 1Khz, repeated at three different power levels(1/10th watt, 1watt, and 10Watts), with results repeating. (Results repeated within the measurment cababilities of my Fluke189). Even though I suptracted lead resistance from the meter, and added lead resistance of the test load, the values are approaching the limits of instruments, and that makes me nervous about trusting, or "publishing" the results.

Anyway, I guess that makes the damping factor something over 300. When I get some more time, I'll test thing over the audio band.


I would like to test the amp over it's whole -3db bandwidth, but don't trust my fluke over the entire range of 1/2Hz to 600Khz.
Since this is more of an acidemic exercise, I really am interested in knowing the HF impedance. Like I said before, I suspect it has some response based on open loop gain, but am not sure how to test the whole thing.

Any ideas on how to get an accurate measurement at those frequency ranges above, say 20Khz??? At this point, I've run out of tools in my toolbox.

Thanks!
Dan


P.S. I would have posted again sooner, but my ISP started blocking my DIY audio mail as spam (aparently DIY audio has a bad reverse DNS lookup value), and did not recieve notification of the reply.

[anatech]

Markus,
No flame. You are so right and it drives me nuts when people forget about the crossover and connection resistance. Tunnel vision.

Dan,
You will find that the HF response on your meter will change with pressure on the case. You can see this effect using a calibrator like a Fluke 5500A or better. It is very hard to adjust a meter correctly on the high frequency end. The newer Flukes use software calibration, but the effect is still there. A bench meter such as the HP 34401A is more accurate and doesn't suffer from this effect. You may be able to use a precision resistor in series and measure the voltage drop with your 'scope as a check. Still a small change.
-Chris