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13th August 2014, 06:27 PM  #1 
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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

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.
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13th August 2014, 08:34 PM  #3 
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"tugofwar" 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 
15th August 2014, 11:37 AM  #4  
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Quote:
Last edited by huseying; 15th August 2014 at 11:39 AM. 

15th August 2014, 11:40 AM  #5  
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Quote:
Just to split some hairs Jan
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15th August 2014, 01:07 PM  #6 
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Mea culpa The train of thought wandered while typing and wielding Windows on screen calculator.
Thanks
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15th August 2014, 11:55 PM  #7 
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That's how you do it in theory and practice:
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 
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Join Date: Aug 2011
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I want to test a new amp and an old one to see the differences in the 2040Hz 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 quasicomplimentary dinosaur? Would one test be safer than the other? 
16th August 2014, 06:58 PM  #9 
diyAudio Moderator

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.
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16th August 2014, 10:43 PM  #10 
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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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