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Old 17th March 2003, 09:05 PM   #1
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Default FFT Analysis

Hi,
It's my first time being able to measure the THD+N% of my amps using the FFT analysis. One question, when I measure the THD the harmonics jump up very quickly around 12VRMS (across 8ohms). At first I thought it was from clipping, but when I look on the scope I can't see any until around 20VRMS. Is this normal? If so, what is causing this sudden increase? Thanks
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Old 17th March 2003, 09:09 PM   #2
SY is offline SY  United States
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Can you give details of your test setup and the results? There are lots of possibilities, so let's start by eliminating a bunch of them.
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Old 17th March 2003, 09:25 PM   #3
tiroth is offline tiroth  United States
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First thing would be check if you are clipping the input amp of your analyzer...
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Old 17th March 2003, 09:27 PM   #4
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I am using a LynxOne soundcard and Spectra Plus software. I am sending a 1KHz tone from the soundcard to the amp and back into the card (full duplex). The amp I am testing is an integrated amp. The input is 1VRMS and I adjust the output with the volume control on the amp. I have set the reference voltage on the card using a multimeter (approx. 4V). I am not sure what other specs you are looking for. Do I need to adjust the input level on the card?
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Old 17th March 2003, 09:32 PM   #5
SY is offline SY  United States
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Check the rated overload voltage on that soundcard.
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Old 17th March 2003, 09:47 PM   #6
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It is not clipping at input and it is not overloading. Hmmm, thanks for the help though.
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Old 17th March 2003, 09:51 PM   #7
SY is offline SY  United States
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When you say that the harmonics jump up very quickly, how much do they jump? From more-or-less what to more-or-less what?
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Old 17th March 2003, 10:20 PM   #8
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from about .03% to around 20%
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Old 17th March 2003, 11:26 PM   #9
grataku is offline grataku  United States
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that's why I was advocating the use of 20k audio analyzer instead of $300 soundcards!

Robphill,
Can you put some attenuators in your return line and stay below the dreaded 12V? You care about the distortion figure not the value of the voltage, right?
Or maybe it just simply that your amp sucks. What is it anyways?
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Old 18th March 2003, 12:10 AM   #10
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The amp is a NAD C340 so it does not suck. I thought that distortion figures were taken just before clipping. I will see if I can attenuate the return voltage.
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