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Old 9th October 2021, 11:43 PM   #11
Mark Tillotson is offline Mark Tillotson
diyAudio Member
Join Date: May 2018
Location: Cambridge UK
I was tinkering with my distortion-measuring Python script and thought to do a THD test on my modular preamp via an inverse RIAA network through a 3rd-gen Scarlett Solo, getting a quite pleasing 0.00068% figure (including rumble filter, so 5 opamps in the signal path)
Click the image to open in full size.
The software takes in a .wav file and iterates to a best-fit model sinusoid to match the data (in amplitude, frequency and phase), then subtracts it off to get the residual, and measures the correlation of the residual against harmonics of the model to get the red peaks and thus can measure THD and SNR separately.

I pre-processed with a 200Hz high-pass filter to knock off most of the mains hum components (wasn't measuring in a screened box, just on desktop) - otherwise this dominates the SNR (but doesn't affect the THD measurement).

[ The SNR ratio is fairly meaningless as I failed to record the actual level ]

I also measured the current consumption at 36mA per rail, which is as expected with 5 opamp stages per channel. Next step is boxing it up.
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Old 10th October 2021, 07:45 AM   #12
DRONE7 is offline DRONE7  New Zealand
diyAudio Member
Join Date: Aug 2008
my distortion-measuring Python script
Did I miss that somewhere !!

Would be keen to try it...link ?
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Old 10th October 2021, 10:09 AM   #13
Mark Tillotson is offline Mark Tillotson
diyAudio Member
Join Date: May 2018
Location: Cambridge UK
Yup, here's the current version, Python 3

You need numpy/scipy/matplotlib/soundfile modules, and I forgot a few files, hopefully I've got all of them in the zip now.
Attached Files
File Type: zip measure_distortion.zip (6.9 KB, 3 views)

Last edited by Mark Tillotson; 10th October 2021 at 10:12 AM.
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