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
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
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
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?
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?
Soundcard FFT
Use an attenuator in at the input to the sound card. They tend to overload very easily I use a 10K pot and start out fully attenuated and increase the level until the distortion just starts to increase and then back the level down a bit. I can measure distortion products down to about 100 dB below the fundamental for my 1kHz lamp controlled wein bridge op oscillator. I can see differences in distortion for various opamps used for the oscillator using a lowly Soundblaster sound card. This really is a pretty good distortion analyser for frequencies below a few KHz. It has much better resolution than the typical used older HP distortion analysers used by many hobbyist and should be useful for many of the amp and preamp circuits posted here. They usually have distortion above this level for outputs of several watts for power amps or several volts for preamp and line level circuits.
Use an attenuator in at the input to the sound card. They tend to overload very easily I use a 10K pot and start out fully attenuated and increase the level until the distortion just starts to increase and then back the level down a bit. I can measure distortion products down to about 100 dB below the fundamental for my 1kHz lamp controlled wein bridge op oscillator. I can see differences in distortion for various opamps used for the oscillator using a lowly Soundblaster sound card. This really is a pretty good distortion analyser for frequencies below a few KHz. It has much better resolution than the typical used older HP distortion analysers used by many hobbyist and should be useful for many of the amp and preamp circuits posted here. They usually have distortion above this level for outputs of several watts for power amps or several volts for preamp and line level circuits.
Rob,
I didn't mean to make fun of your amp, I hope I didn't offend you,
and yes I meant to att. the input of your soundcard.
I used the plural because at work we have tons of minicircuits RF line attenuators laying around, usually the value you need disappears so when you are ready to do the experiment and we need to combine several in series to get the value we need.
Fred,
do you have a favorite soundcard based FFT analyzers?
I didn't mean to make fun of your amp, I hope I didn't offend you,
and yes I meant to att. the input of your soundcard.
I used the plural because at work we have tons of minicircuits RF line attenuators laying around, usually the value you need disappears so when you are ready to do the experiment and we need to combine several in series to get the value we need.
Fred,
do you have a favorite soundcard based FFT analyzers?
SY said:
Well maybe that holds for you.
With a soundcard + good software a lot can be done. I must admit that it is not comparable to an Audio Precision costing several tens of thousands. I already pointed in the "Math" topic to this thread Free oscilloscope,spectrum analyser.... were I showed what the performance can be of such a system of around 250 bucks (sound card + software).
For analysing amps and checking if you make progress, a soundcard-based system can be of great help even you can only investigate distortion accurately up to a few kHz. But you have to investigate the measurement system itself first to see if there is substantial distortion and what the causes are and how to fix it.
Cheers
The problem is not only bandwidth, but resolution. The cheap cards/software (and that's what I use) will do fine if you're measuring 1% distortion amps. But it's real easy to get below the threshold- I wouldn't trust my unit to tell me the difference between 0.1 and 0.01.
Hmmm, it might be interesting to build an outboard instrumentation amp as an accessory to cheap units that will extend their range downward. Might make a fun summer project for me.
Hmmm, it might be interesting to build an outboard instrumentation amp as an accessory to cheap units that will extend their range downward. Might make a fun summer project for me.
Yup.tiroth said:
Anyhow, soundcards can be useful in the right setup. Using outboard DAC and ADC I can get pretty decent measurements. Calculated residual THD+N is only 0.0028%, and you can infer more resolution than that.
An externally hosted image should be here but it was not working when we last tested it.
http://www.anidian.com/audio/misc/testbench.shtml
SY,
Maybe you are better off by buying a good soundcard. The layout of such a card counts also a lot regarding noise. I modded my M-Audiophile 2496 (http://www.m-audio.com/ ) a bit by putting some extra filtering in the PSU lines. That lowered the noise from the main board some dB. I also use an M-Audio USB DUO. That one has a neat instrumentation amp build-in (BurrBrown INA163) as a microphone amp. No, I am not pushing M-Audio but I am quite happy with it and it is good value for money.
TYROTH,
Do the measurement again at -10dB output. Great chance you get lower distortion figures. Also averaging helps to get noise levels lower.
Maybe you are better off by buying a good soundcard. The layout of such a card counts also a lot regarding noise. I modded my M-Audiophile 2496 (http://www.m-audio.com/ ) a bit by putting some extra filtering in the PSU lines. That lowered the noise from the main board some dB. I also use an M-Audio USB DUO. That one has a neat instrumentation amp build-in (BurrBrown INA163) as a microphone amp. No, I am not pushing M-Audio but I am quite happy with it and it is good value for money.
TYROTH,
Do the measurement again at -10dB output. Great chance you get lower distortion figures. Also averaging helps to get noise levels lower.
SY said:
A fast PC is unavoidable for doing real-time FFT’s. I am using a 1.8 GHz Atlon XP under Win-2K. Really good cards are the already mentioned Lynx cards (One or Two, http://www.lynxstudio.com). These are real studio cards, beware these cards are expensive. Another good card is the RME DIG196 series (http://www.rme-audio.com/english/). These are recently dropped in price.
Here are two links with a good overview of nearly all cards around:
http://home.earthlink.net/~rongonz/home_rec/soundcard.html
http://www.pcrecording.com/soundtable.htm
I have chosen M-Audio for its good performance and low price. Besides that it was one of the few with good drivers for Win-NT available some years ago.
You are right Sy, for measuring power amps you need a differential input (whether or not with a good instrumentation amp) to avoid ground loop problems. The minus terminal of the loudspeaker can have substantial signal with regard to the low-level signal ground. This should not be overlooked. Good wide band INA’s are available from BurrBrown at reasonable cost.
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