Taming the Rustbucket

[lars]

I don't know if anybody else noticed there seems to be a lot of background noise/distortion in the Rustbucket. I did some hunting around with an audio probe and pinpointed the source right at pin7 of IC1, which in conjunction with IC2 fellow madbeaner Scruffie identified as a compressor circuit.
I found some mods that definitely quieted down my build, and also seemed to improve tracking and decay response.

Here are the mods:
1) Change R6 and R7 to 47k (stock is 470k - too high of gain)
2) Change C3 to 150n (this will help clean up bass notes and provide a more abrupt decay cutoff, although it will thin out the sound a bit and cause bass notes to cut out more quickly than mid/high notes. This is the least important of these mods to try)
3) Add a 100 - 120pf cap across pins 6 & 7 of IC1 (this will roll off a lot of the high frequency hiss and noise that gets amplified through the rest of the circuit)
4) This is the "must do" mod:  the bias trimmer for the BBD has an extremely narrow sweep for settings that sound good. I've found that by making a "custom" trimmer, it helps greatly in getting the bias just right.
Take a 20K trimmer and solder 39k resistors to legs 1 and 3 respectively. This gives you a trimmer that the circuit will "see" as 100k, but gives you much greater control over the sweet spot. I've been able to eliminate a lot of distortion in my circuit with just this mod.

After doing these mods, I noticed a huge improvement in S/N ratio and playability of the circuit. You do have to adjust the sensitivity knob higher than maybe what you were used to before, but it has that extra range for a reason!

[lincolnic]

I posted in the other thread, but I've been hoping for exactly these mods. Can't wait to try them out, thanks!

[Jebus]

Do you consider these mods something that should be definitely implemented on the circuit? I'm still building mine and I'm just thinking if I should go with stock and add these later or should I go with the mods immediately..

[lars]

Definitely run some jumpers from the R7 and R6 positions to a breadboard, and try the mods out that way. You can then easily compare the sounds between stock values and see what you like better. I've added a 4th mod to the list that I've tried, and I consider that one to be a "must do". (Explained above)

[BrianS]

Would you mind posting a picture of how you have the trimmer hooked up and where you put the cap on IC1? I would really appreciate it.

[Scruffie]

Rather than hacking in extra resistors, a multi-turn trim pot might be an easier solution.

[lincolnic]

Rather than hacking in extra resistors, a multi-turn trim pot might be an easier solution.

Might be a big pain to actually get it to the right position in the first place, though.