How do I test an amp for Damping Factor? - diyAudio
 How do I test an amp for Damping Factor?
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 13th August 2014, 06:27 PM #1 diyAudio Member     Join Date: Aug 2011 Location: NC How do I test an amp for Damping Factor? Yeah, I get that damping factor is useless and it's really a matter of output impedance. If I understand correctly, output impedance of an amplifier varies with frequency? So that would mean that it would affect the frequency response of the amp right? I'm wondering how you would do an impedance sweep of an amplifier's output. Does anyone know the standard test that determines an amp's DF at a given frequency? Or do these companies just make up numbers?
 13th August 2014, 06:44 PM #2 diyAudio Moderator     Join Date: Sep 2007 Well if you look it up you'll see damping factor is the ratio of the load impedance to the amplifiers output impedance. So a rough and ready way to estimate... if you set the amp to give (say) 10 volts rms output (with no speakers attached) as measured on a DVM (use something like 400Hz which any DVM should cope with), and then load the amp with a known resistance as a load (say an 8 ohm or 10 ohm resistor of suitable rating) you will see the 10 volts drop a little as measured at the speaker terminals. If we assume the amp internally is "perfect" as a voltage source then you can work out the damping factor. For example if you measured 10 volts and connecting a 10 ohm gave 9.8 volts then the internal resistance within the amp is dropping 0.2 volts. The perfect amp is delivering current I of 10/10 (I=V/R) which is 1 amp. The internal resistance is R=V/I which is 0.2/1 = 0.2 ohm. So the ratio 0.2 to 10 is 50 which would be the damping factor. That is a bit simplistic and there are other ways to measure damping factor more accurately and over a wider frequency range but if you only have a DVM it will get you in the right ballpark Try the same test at the end of the speaker leads too such that they are in circuit as well and their resistance is included. Now that amplifier with a claimed 500 damping factor isn't so much different to one of only 30 or 40.
 13th August 2014, 08:34 PM #3 diyAudio Member   Join Date: Feb 2003 Location: .. "tug-of-war" with a resistor load, another amp on the other end driven with your test signal such as a frequency sweep measure the V a your amp's output a soundcard is good down to its crosstalk limit for such a test
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Quote:
 Originally Posted by Mooly Well if you look it up you'll see damping factor is the ratio of the load impedance to the amplifiers output impedance. So a rough and ready way to estimate... if you set the amp to give (say) 10 volts rms output (with no speakers attached) as measured on a DVM (use something like 400Hz which any DVM should cope with), and then load the amp with a known resistance as a load (say an 8 ohm or 10 ohm resistor of suitable rating) you will see the 10 volts drop a little as measured at the speaker terminals. If we assume the amp internally is "perfect" as a voltage source then you can work out the damping factor. For example if you measured 10 volts and connecting a 10 ohm gave 9.8 volts then the internal resistance within the amp is dropping 0.2 volts. The perfect amp is delivering current I of 10/10 (I=V/R) which is 1 amp. The internal resistance is R=V/I which is 0.2/1 = 0.2 ohm. So the ratio 0.2 to 10 is 50 which would be the damping factor. That is a bit simplistic and there are other ways to measure damping factor more accurately and over a wider frequency range but if you only have a DVM it will get you in the right ballpark Try the same test at the end of the speaker leads too such that they are in circuit as well and their resistance is included. Now that amplifier with a claimed 500 damping factor isn't so much different to one of only 30 or 40.
Thats a good way of testing amps' damping, i usually use this method but shredhead please keep in mind that the longer the cables wich you use to connect resistor to amp, the more wrong damping value you get , and also to decrease the cable's resistance use tick cable as posible as you can.
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Last edited by huseying; 15th August 2014 at 11:39 AM.

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Quote:
 Originally Posted by Mooly For example if you measured 10 volts and connecting a 10 ohm gave 9.8 volts then the internal resistance within the amp is dropping 0.2 volts. The perfect amp is delivering current I of 10/10 (I=V/R) which is 1 amp. The internal resistance is R=V/I which is 0.2/1 = 0.2 ohm. So the ratio 0.2 to 10 is 50 which would be the damping factor.
Mooly, agree with most of your post but not on this. If the voltage drops to 9.8V with a 10 ohms load the Iout is not 1A but 0.98A...
Just to split some hairs

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 15th August 2014, 01:07 PM #6 diyAudio Moderator     Join Date: Sep 2007 Mea culpa The train of thought wandered while typing and wielding Windows on screen calculator. Thanks
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That's how you do it in theory and practice:
Quote:
 Originally Posted by jcx "tug-of-war" with a resistor load, another amp on the other end driven with your test signal such as a frequency sweep measure the V a your amp's output
Note that you can basically do this with two channels of one stereo amplifier as well, i.e. the resistor goes between R+ and L+, with playback on L and measurement on R. The resistor should be >10x the expected output impedance (current source approximation), but that shouldn't be an issue since you need something in the usual loudspeaker impedance range anyway. Sufficient load handling advised, shouldn't need to be anything too extreme though.

 16th August 2014, 05:38 PM #8 diyAudio Member     Join Date: Aug 2011 Location: NC I want to test a new amp and an old one to see the differences in the 20-40Hz area but for the old one I'm scared that because it doesn't really have much in the area of protection circuits, it might release the magic smoke. Does anyone here have any experience in doing these 2 kinds of tests to an old quasi-complimentary dinosaur? Would one test be safer than the other?
 16th August 2014, 06:58 PM #9 diyAudio Moderator     Join Date: Sep 2007 Just do the test at a couple of volts and then there is no danger of anything overheating. If you add a low value resistor (say 0.1 to 0.47) in series with the speakers then you automatically decrease damping factor down to at least 80 with a 0.1 ohm, and to just 17 with a 0.47 ohm. That assumes 8 ohm speakers and a perfect amplifier with a damping factor of zillions. Try it.
 16th August 2014, 10:43 PM #10 Banned   Join Date: Jul 2014 Location: In from of my workbench There is no chance getting a good measurement with an multimeter in the range of 400Hz and up to 20k or 40kHz. This is an area only for modern Oscilloscopes

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