My implementation of the Cordell Distortion Analyser - Page 2 - diyAudio
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Old 1st November 2009, 08:32 AM   #11
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Great job, Patrick!
Thanks for the info on the case,

jd
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Old 1st November 2009, 12:46 PM   #12
mlloyd1 is online now mlloyd1  United States
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kudos, patrick
great job!

other than balanced, did you make other enhancements?

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Old 1st November 2009, 01:42 PM   #13
EUVL is offline EUVL  Europe
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Only changed one or two opamps to LM4562, etc.

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Old 1st November 2009, 08:55 PM   #14
Dave is offline Dave  New Zealand
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This is probably a silly question but where can I find the schematics?
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Old 1st November 2009, 10:35 PM   #15
jcx is online now jcx  United States
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I wouldn't bother today - add a few filters to supress fundamental and use a sound card - indirect IMD measurement resolution can reach way below human hearing noise floor with averaging

the ESI Juli@ is ~US$120 and has great resolution and low distortion by itself
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Old 1st November 2009, 11:44 PM   #16
Bonsai is offline Bonsai  Taiwan
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JCX, is the problem with a sound card not the fact that the bandwidth is limited to c. 22kHz? How do you do a wide bandwidth measurement (e.g. 80Khz like the AP)? Another thing is frequency response (although not covered by Bob's instrument) - sound cards do a great job of measuring 5Hz to 22KHz - but after that what do you do?
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Old 2nd November 2009, 12:25 AM   #17
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Great job- mine isn't as pretty!

I built my Cordell analyzer over a period of a couple years, starting right after the Audio magazine article came out. Etched my own boards from the published patterns. It's a super instrument and I wouldn't trade it for the world. Sound cards are great as far as they go, but bandwidth, calibration and certainty of the result are all issues. You can get wildly different results depending on sample rates and bit depths. The only way I'm sure of what my sound card is doing is to compare it with a conventional THD analyzer. Also, at least with my Emu, the ADC quality is great, but the DAC snr is unimpressive. One would never mistake it for a high quality analog signal generator. The noise isn't obvious in a spectral plot, but sticks out like a sore thumb on a scope.

The only downside to building a Cordell these days is the expense and difficulty of building up the switches. Sometime I'll post a photo of mine. It's been trouble free and the floor is about 0.0002% midband.

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Old 2nd November 2009, 01:02 AM   #18
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Bravo, Patrick
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Old 2nd November 2009, 03:00 AM   #19
jcx is online now jcx  United States
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I appreciate fine analog circuitry and good dedicated function test equipment as much as anyone - but the lazy cheapskate in me can't ignore the possibilities of today's good&cheap digital hardware

the ESI Juli@ has up to 192K sample rate so it can make measurements comparable to the classic 80 KHz BW audio distortion analyzers

with a few active filter circuits to cut the fundamentals of the test signals and boost the error signal you can resolve with very good accuracy with better soundcards - spurious free dynamic range before the added filters can be ~100 dB, indirect measurement tricks with two channels generating IMD test tones and added gain/filtering can allow resolution of nonlinear test circuit behavior to exceed 160 dB

Free SciLab or other processing software can extract anything you can write the equations for from the .wav files
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Old 2nd November 2009, 03:55 AM   #20
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The link to Bob's article :

http://www.cordellaudio.com/papers/thd_analyzer.pdf


Patrick
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