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Bob Cordell: Build A High Performance THD Analyzer

Posted: Wed Jun 08, 2016 7:52 pm
by mediatechnology
Build A High Performance THD Analyzer Bob Cordell, Audio, July, August, September 1981.

Build A High Performance THD Analyzer Bob Cordell, Audio, July 1981, Part 1: https://www.proaudiodesignforum.com/ima ... y_1981.pdf
Build A High Performance THD Analyzer Bob Cordell, Audio, August 1981, Part 2: https://www.proaudiodesignforum.com/ima ... t_1981.pdf
Build A High Performance THD Analyzer Bob Cordell, Audio, September 1981, Part 3: https://www.proaudiodesignforum.com/ima ... r_1981.pdf


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Build A High Performance THD Analyzer Bob Cordell, Audio, July, August, September 1981.

PC Board Layouts at Precise 300 DPI

Cordell THD Analyzer CP1 PC Board Layout https://www.proaudiodesignforum.com/ima ... 00_DPI.jpg
Cordell THD Analyzer CP2 PC Board Layout https://www.proaudiodesignforum.com/ima ... 00_DPI.jpg
Cordell THD Analyzer CP3 PC Board Layout https://www.proaudiodesignforum.com/ima ... 00_DPI.jpg

Re: Build A High Performance THD Analyzer: Bob Cordell

Posted: Thu Jun 09, 2016 7:52 am
by Speedskater
I think that I might still have a set of the original pc boards up in the attic. Getting the selectors switches brought the project to a halt.

Re: Bob Cordell: Build A High Performance THD Analyzer

Posted: Wed Jan 30, 2019 10:38 am
by lsburden
Sorry to resurrect an old thread...
This has interested me for years. There is quite a thread at diyaudio.
Has anybody built this?
The number of multi-deck rotary switches is a bit of a deterrent but using OTS rotaries and relays or even solid-state switches in some locations may be a good alternative. In the end it probably isn't cheaper but could simplify the build and reduce wiring.
I have experimented with replacing the FET Amplitude Control with a THAT Corp VCA. I was using a 4301 but since the EOL announcement I would ultimately go with a 4305. Anyways, where it was left I was having trouble maintaining a constant level at different frequencies.

Lucas

Re: Bob Cordell: Build A High Performance THD Analyzer

Posted: Wed Jan 30, 2019 11:11 am
by JR.
Cordell's JFET multiplier used for the gain control loop in his sine wave oscillator is quite good. I am not sure you will easily improve upon that. There are several very low THD designs floating around the WWW.

The rectifier part of the AGC may not be trivial especially at HF.

JR

Re: Bob Cordell: Build A High Performance THD Analyzer

Posted: Wed Jan 30, 2019 2:29 pm
by mediatechnology
lsburden wrote:
Wed Jan 30, 2019 10:38 am
Sorry to resurrect an old thread...
This has interested me for years. There is quite a thread at diyaudio.
Has anybody built this?
The number of multi-deck rotary switches is a bit of a deterrent but using OTS rotaries and relays or even solid-state switches in some locations may be a good alternative. In the end it probably isn't cheaper but could simplify the build and reduce wiring.
I have experimented with replacing the FET Amplitude Control with a THAT Corp VCA. I was using a 4301 but since the EOL announcement I would ultimately go with a 4305. Anyways, where it was left I was having trouble maintaining a constant level at different frequencies.

Lucas
Lucas - Glad you did resurrect this.

I have used the 2180 VCA in Vicktor's oscillator and did some proof-of-concept here: viewtopic.php?f=6&t=887

I'd love to see a modernization of this design that eliminated the multi-pole multi-deck rotaries.

Re: Bob Cordell: Build A High Performance THD Analyzer

Posted: Wed Jan 30, 2019 3:33 pm
by lsburden
Wow. You have given me quite a bit of reading to do and a collection of circuits to review that I have not seen before.
At one point I was really looking at an oscillator on a Linear Technology App Note, AN43, written by John Williams.