Bob Cordell: Build A High Performance THD Analyzer

This is where we talk about testing, measuring and repairing things. Sometimes we have to repair the equipment we use to test, measure and repair other things. It's an endless cycle of fixing the broken things we need to fix other broken things.
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mediatechnology
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Bob Cordell: Build A High Performance THD Analyzer

Post by mediatechnology » Wed Jun 08, 2016 7:52 pm

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


Image
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
https://ka-electronics.com

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Re: Build A High Performance THD Analyzer: Bob Cordell

Post by Speedskater » Thu Jun 09, 2016 7:52 am

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.
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Re: Bob Cordell: Build A High Performance THD Analyzer

Post by lsburden » 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

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Re: Bob Cordell: Build A High Performance THD Analyzer

Post by JR. » Wed Jan 30, 2019 11:11 am

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

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Re: Bob Cordell: Build A High Performance THD Analyzer

Post by mediatechnology » Wed Jan 30, 2019 2:29 pm

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.
https://ka-electronics.com

SOCIAL MEDIA PLATFORMS should advance FREEDOM OF SPEECH. Yet too many Americans have seen their accounts suspended, banned, or fraudulently reported for unclear “violations” of user policies.
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Re: Bob Cordell: Build A High Performance THD Analyzer

Post by lsburden » Wed Jan 30, 2019 3:33 pm

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.

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