Mixtape Distortion, original design with a small problem...any ideas?

[diablochris6]

I designed a distortion on a whim to try and get the emphasis/de-emphasis sound of a tape player. I start with a boost circuit that can achieve op amp clipping if pushed hard enough which goes to a bass pre-emphasis gyrator centered around 100 Hz and a soft clipping gain section to add treble emphasis and sparkle around 3.7 kHz. The next section is the "tape saturation" section borrowed from the Sagan Delay that adds hard clipping when you play hard enough or have the preamp/pre-emphasis sections cranked. After that is the bass and treble de-emphasis section using simple filters. The final stage is a final tone control center (once again, pulled from the Sagan) and a master volume.

I originally forgot the 10k resistor between the junction of the bass EQ pot /C13 and pin 7 of IC3. Overall, it sounds fun. You could use it subtly, or create all sorts of woofy, splatty mess like a 14 year-old with their first TASCAM. The only problem I have with it is that the second half of the Bass EQ's pot turn, all the audio decreases, with a full CW rotation of the pot creating silence. I haven't quite figured it out yet. Any extra help on a possible remedy would be appreciated!

[anotherjim]

Bass pot should not kill all signal. I'd suspect a short to either VB or 0v at the end of the pot that makes silence.

[PRR]

DC voltages at IC3.

[diablochris6]

Bass pot should not kill all signal. I'd suspect a short to either VB or 0v at the end of the pot that makes silence.

I checked for continuity with ground and VB, and there was no connection.

DC voltages at IC3.

1: 7.85
2: 4.48
3: 4.41
4: 0.01
5: 4.45
6: 4.48
7: 2.28
8: 8.94

[reddesert]

I think the C10-R13 HPF after the clipping diodes needs to be DC-isolated from the next op-amp (R14 and IC3A) with a coupling cap. Right now, there is a DC path from the IC3A opamp output to ground through R15-R14-R13. That is forcing the DC level of the op-amp output high (pin 2 wants to be at Vbias and is roughly haifway between pin1 and ground), which is why pin 1 is at 7.8 volts.

It's not clear that this is the only cause of your problem, but it's clearly undesirable as that DC level is coupled all the way through the tonestack part of the circuit, so the whole IC3A/B section has basically no headroom.

[PRR]

Sorry, I was blind.

What Red said-- R13 is screwing-up the DC bias. It barely works flat-bass. When bass (and DC!) gain increases, IC3_B output goes low, slamming against zero V.

If you truly need the 141Hz low-cut there, snip R13, make R14 R15 2.5K. Or better: change C10 to 120nFd (makes the same low-cut against 10K R14).

[PRR]

R20 is not needed. R16 serves no point. R17 limits the bass boost (a Bax should be fed a very low Z). Make R17 short and move C11 4n7 over across R15 (make it now 3.5nFd). Same response.

[diablochris6]

Thanks for the suggestions, gents. The bass control works great now.