Cordell SigGen vs the HP339 and SG505 - diyAudio
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Old 11th October 2007, 03:20 PM   #1
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Default Cordell SigGen vs the HP339 and SG505

By now everybody knows I love building oscillators (even amplifiers which oscillate.)

I was reviewing the Bob Cordell Audio Magazine THD Analyzer from Audio Magazine in July 1981 and comparing it to Cyril Bateman's sub-ppm oscillator in 2002 -- seems to me that Robert's article should be refreshed -- perhaps the AGC circuitry could be simplified with a voltage controlled amplifier like the SSM2018 -- perhaps the rectifier could be simplified with two rail-rail opamps. Run the final amplifier in Class A (isnt' that what they do in the SG505?) I wonder how many of the Cordell analyzers were built.

Hofer's paper "A comparison of Low Frequency RC Oscillator Topologies" is worth downloading from the AES website.
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Old 11th October 2007, 03:54 PM   #2
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Well, I built one, and it smokes any commercial unit except for Audio Precision. The floor of mine is 0.0002% midband, with very little rise over the audio range. The HPs won't touch it. IMO, the oscillator section is awfully good as is, and I don't know if an update would improve it much. I have considered trying the new National opamps instead of the 5534s, but like most of the things I consider, it'll never rise high enough on the priority list to get done.
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Old 11th October 2007, 04:05 PM   #3
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The paper is on Bob's website:

http://www.cordellaudio.com/papers/thd_analyzer.pdf
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Old 11th October 2007, 06:29 PM   #4
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This is the output section of the SG505. With the LME49710 you wouldn't need to do a balancing act.

Click the image to open in full size.
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