Hi folks, I've been working on 4 channels of mic preamps using John Hardy's Jensen 990's with parts I had lying around, and am stumped with a problem. It's based on the simple circuit as published in Hardy's 990 datasheet:
The issue is that when adjusting gain with signal present, there's noise that sounds like there is DC across the pot (which are new dual deck clarostat's from Hardy like the M1 uses--one 500 ohm deck for high gain and 10k for low). However, there shouldn't be a path for the bias voltage to ground because of C3 (and I measure no DC across the pot, down to 100uv resolution). So far, I've tried without success: switching caps for C3, changing pots (new blue Alps), removing the bias compensation circuit (which works well when wired in). I must be overlooking something basic?
I am using Jensen JE-110's on 2 channels with Jensen outputs and Triad HS-66's with xfmr-less outputs on the others, so the bias current on the non-inverting input is slightly different than the schematic shown, but that shouldn't matter so much.
Thanks in advance for any ideas.
James
The issue is that when adjusting gain with signal present, there's noise that sounds like there is DC across the pot (which are new dual deck clarostat's from Hardy like the M1 uses--one 500 ohm deck for high gain and 10k for low). However, there shouldn't be a path for the bias voltage to ground because of C3 (and I measure no DC across the pot, down to 100uv resolution). So far, I've tried without success: switching caps for C3, changing pots (new blue Alps), removing the bias compensation circuit (which works well when wired in). I must be overlooking something basic?
I am using Jensen JE-110's on 2 channels with Jensen outputs and Triad HS-66's with xfmr-less outputs on the others, so the bias current on the non-inverting input is slightly different than the schematic shown, but that shouldn't matter so much.
Thanks in advance for any ideas.
James
