Tech Square

Posted by rick_m (A) on November 21, 2007 at 13:58:23

In Reply to: RE: Distortion analyzer recommendations posted by shermanr@prw.net on November 02, 2007 at 15:28:46

Since you likely have a computer, just use it's sound card and a free FFT program such as wavetools or Visual Analyser. Far more information, no cost.

If you've already acquired a distortion analyzer you can still use it in conjunction with your computer to enhance the dynamic range of the residues.

Sorry to be so late, I don't normally look at this forum.

Rick

• RE: Save your money (if it's not too late). - shermanr@prw.net 11:03:55 02/14/08 (0)
  In Reply to: RE: Save your money (if it's not too late). posted by rick_m on November 21, 2007 at 13:58:23

  OK. I haven't spent a dime yet... the PC aproach sounds awesome. I have an old desktop PC that uses Windows 95 and has a Pentium 150. Maybe my buddy can scrounge an old sound card ans I can leave this permanently on my bench. Will look around... great suggestions. This is out of the box thinking completely!
  
  

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• RE: Save your money (if it's not too late). - beermanpete@socal.rr.com 19:45:36 11/21/07 (2)
  In Reply to: RE: Save your money (if it's not too late). posted by rick_m on November 21, 2007 at 13:58:23

  Using a computer sound card and FFT software is not a great way to go for measuring audio equipment. The bandwidth is limited to 20kHz, including harmonics. This means that harmonic distortion measurements of higher frequencies will be limited and perhaps misleading. Also, the input voltage capability is limited to only a few volts and is not tolerant to overloads. To use it with a power amp will require building some type of input attenuator.
  
  

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  • RE: Save your money (if it's not too late). - rick_m 08:33:06 11/22/07 (1)
    In Reply to: RE: Save your money (if it's not too late). posted by beermanpete@socal.rr.com on November 21, 2007 at 19:45:36

    Well, your point about the power amp is a good one, but it's just takes a pot, nothing fancy.
    
    The upper end isn't much of an issue, most sound cards will sample to at least 96KHz so you can see out to ~40KHz which is well beyond what your ears will respond to on a continuous basis.
    
    As far as misleading, nothing on the face of the earth is more misleading than a distortion analyzer. The rub is that it gives you a pat answer to two decimal places which on it's own hook means absolutely nothing if you are doing design or repair. It is of some value for production. You end up always having to have a scope on the output so you can get a clue if it's measuring hum, residual noise, 2nd, 3rd, what?
    
    The sound card/FFT scheme is outstanding by comparison, one glance and you see what's going on. And the cost/benefit is beyond belief. About 20 years ago I was doing a project where I needed to see the signal more clearly than I could with a distortion analyzer. So I bought an HP dynamic signal analyzer. It was a 50KHz, ~14 bit FFT machine. $12,000 used. The price of a good new car. Now you get better performance for essentially nothing. Some things in life really are better than the "good old days".
    
    Rick
    
    

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    • RE: Save your money (if it's not too late). - rdf 16:38:58 11/30/07 (0)
      In Reply to: RE: Save your money (if it's not too late). posted by rick_m on November 22, 2007 at 08:33:06

      100% agreement. With free software like sinegen coupled with WaveSpectra and a pair of ten turn pots fine harmonic analysis down to <-120 dB is a pip with little more than a $150 M-Audio Audiophile Firewire.
      
      

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