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Old 17th January 2004, 06:41 PM   #1
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Default how to measure freq response

hi,

Is there a way i can measure the freq response of my amp if i dont have any oscilloscope?
If i need to build a sine-wave generator, is there any good design I can copy? One of those low distortion types....
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Old 18th January 2004, 09:06 AM   #2
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Do you have a good soundcard in your computer ?
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Old 18th January 2004, 05:25 PM   #3
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Default yes

My soundcard is built in-audio97 codec.
Do you mean to say that I can use an oscillator program?
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Old 18th January 2004, 05:45 PM   #4
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built-in ac97 probably won't do.

but to measure freq response, thd, etc. you can use the free http://audio.rightmark.org/

it's pretty simple but the quality of the results depend on your soundcard.

you would also need a shunt and/or selfmade preamp thing to measure high voltage of a power amp. soundcards usually only measure up to 2V.
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Old 18th January 2004, 05:49 PM   #5
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http://www.dazyweblabs.com/shannonsoft/page3.html
especially TMS-1.

Run a very short cable from output to input on your soundcard
to check first how good it is.
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Old 18th January 2004, 08:57 PM   #6
sam9 is offline sam9  United States
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To get a reasonable idea of freq resp using a soundcard, I would imagine the key thing is freq resp of the card itself. Unless the error is gross, freq accuracy wouldn't be that key and THD+N almost non-germaine. I would just take any free oscilator program and run it with a DMM reading AC volts at the soundcard output. Then two things: A set the level below clipping* on the amp and B: record the output at each frequency you are going to check. Next drive the amp with the same signal but attach the DMM to the amp outputs (I'm not sure if you need a nominal 8-ohm load on the output but don't use the speakers as test tones may harm them). If the the AC volt reading at the soundcard had dips or spikes, you can caculate correction factors for each freqwhich you multiply by the amplifier output. Plot these results against freqency. Results may not be as precise as using hundreds or thousands of dollars of test equipment, but it should pick up any serious anomomolies.


(* If you can determine this directly, assume your amp has a gain of 20, multiply the DMM AC volts reading by this, square it and divide by 8. If this is below the rated watts for your amp, you should be OK.)


If I've missed something, I trust another poster to add it or correct me.
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Old 19th January 2004, 04:31 PM   #7
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thanks, chris, ice, sam

sam, correct me if im wrong. your idea is that, first, get my souncard to generate signals so that I can measure my souncard's response itself.
then, i feed the tones from my souncard to my amp, and then I measure the amp output.
after that, I compare the amp output with my computer output, is it?
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Old 19th January 2004, 04:33 PM   #8
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I am downloading those programs now, gonna try them tomorrow.
one question i missed out: how do i feed the signals from my amp into my computer safely?
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Old 19th January 2004, 05:04 PM   #9
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Just out of interest, why do you need to measure the frequency response of your amp?
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Old 19th January 2004, 05:19 PM   #10
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1) to make sure there are no err...what u call it...... holes, and/ or spikes/resonance at certain frequencies. when i get a flat response, i hear music exactly as recorded in studio/ live
2) making sure i have my amp is working all the way to 20kHz, even though I may not be hearing them-)
3)checking for oscillations

correct me if im wrong
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