FFT Analysis

RobPhill33 · 17th March 2003, 08:05 PM

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

SY · 17th March 2003, 08:09 PM

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.

tiroth · 17th March 2003, 08:25 PM

First thing would be check if you are clipping the input amp of your analyzer...

RobPhill33 · 17th March 2003, 08:27 PM

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?

SY · 17th March 2003, 08:32 PM

Check the rated overload voltage on that soundcard.

RobPhill33 · 17th March 2003, 08:47 PM

It is not clipping at input and it is not overloading. Hmmm, thanks for the help though.

SY · 17th March 2003, 08:51 PM

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?

RobPhill33 · 17th March 2003, 09:20 PM

from about .03% to around 20%

grataku · 17th March 2003, 10:26 PM

SY
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?

RobPhill33 · 17th March 2003, 11:10 PM

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.

17th March 2003, 08:05 PM	#1
RobPhill33 diyAudio Member Join Date: Sep 2002 Location: Milton Ontario	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

17th March 2003, 08:09 PM	#2
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	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. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

17th March 2003, 08:32 PM	#5
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	Check the rated overload voltage on that soundcard. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

17th March 2003, 08:51 PM	#7
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	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? __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

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17th March 2003, 08:25 PM	#3
tiroth diyAudio Member Join Date: Dec 2001 Location: Pittsburgh, PA, USA	First thing would be check if you are clipping the input amp of your analyzer...

17th March 2003, 08:27 PM	#4
RobPhill33 diyAudio Member Join Date: Sep 2002 Location: Milton Ontario	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?

17th March 2003, 08:47 PM	#6
RobPhill33 diyAudio Member Join Date: Sep 2002 Location: Milton Ontario	It is not clipping at input and it is not overloading. Hmmm, thanks for the help though.

17th March 2003, 09:20 PM	#8
RobPhill33 diyAudio Member Join Date: Sep 2002 Location: Milton Ontario	from about .03% to around 20%

17th March 2003, 10:26 PM	#9
grataku diyAudio Member Join Date: Dec 2000 Location: -	SY 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?

17th March 2003, 11:10 PM	#10
RobPhill33 diyAudio Member Join Date: Sep 2002 Location: Milton Ontario	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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