Audio Precision Sys 1 Problem - Low Frequency Disortion - diyAudio
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Old 25th January 2015, 05:23 PM   #1
nickm1 is offline nickm1  United States
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Default Audio Precision Sys 1 Problem - Low Frequency Disortion

I've got an AP Sys 1 and the loopback distortion gets very high at low frequency. At 20Hz it is 0.22% and it decreases at about 20dB/decade so by 2kHz it is normal around 0.002%. I tested the generator with another analyzer and the generator is fine at about 0.001% at 20Hz. So I'm pretty sure the analyzer is have trouble. The other clue is both the A and B analyzer channels have the same distortion problem.

Any ideas whats going on?
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Old 28th January 2015, 08:20 PM   #2
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Check the "Reading" Monitor Output. This may give you a clue as to what is causing the abnormal readings. Mine picks up noise if I set a monitor on top of the unit. Also could be the fundamental is not fully nulled out.
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Old 28th January 2015, 08:36 PM   #3
kevinkr is offline kevinkr  United States
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Sure you are not picking up some 60Hz?
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Old 29th January 2015, 04:46 AM   #4
nickm1 is offline nickm1  United States
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I found that if I left it sitting for long enough it would slowly settle down. So after about 2 hours it finally got down to about 0.002%. The waveforms I've attached were taken just after I turned it on. Top trace is Input A Monitor, and the bottom trace is the Reading Monitor. The one with the tall spikes is 1Vrms 20Hz and it read a distortion of about 1%. The second was at 10Vrms and the distortion was 0.05%. These are all done reading the GenMon. Thoughts? Any help is appreciated.
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File Type: jpg IMG_20150128_202644480 - Copy.jpg (228.4 KB, 38 views)
File Type: jpg IMG_20150128_202806812 - Copy.jpg (222.7 KB, 37 views)
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Old 29th January 2015, 11:21 PM   #5
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My first thought would be to look at biasing of the output buffer, but crossover distortion should affect more than just low frequencies. You might try looking upstream towards the oscillator to see if distortion occurs there.

Edit: there is an adjustment R4601 "20Hz THD".

Last edited by Loudthud; 29th January 2015 at 11:24 PM.
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Old Yesterday, 09:50 PM   #6
nickm1 is offline nickm1  United States
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I went through to find where the distortion is introduced into the system and it isn't all the way until the bandpass filter section of the distortion board. I'm not sure how to go about diagnosing that section, I think I'll get a can of compressed air and see if any of the components are temperature sensitive.

Where is the 20Hz THD pot at? That sounds like it could be it.
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Old Today, 12:17 AM   #7
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Sorry, that adjustment is on the generator side. Re-reading your original post it would seem that the problem is on the analyzer side.
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Old Today, 09:54 PM   #8
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I fired up my Sys 1 last night and I would like to know exactly the settings you used for the scope shots above. I just set the Analog Analyzer input to "GenMon" and looked at the scope outputs. At 20Hz the Reading output was all noise. Generator amplitude had no influence but THD+N rose at low amplitudes. No 2nd harmonic or crossover notch. Filters were set to <10Hz, >500KHz.
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