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!
I posted in the other thread, but I've been hoping for exactly these mods. Can't wait to try them out, thanks!
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.. :)
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)
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.
Rather than hacking in extra resistors, a multi-turn trim pot might be an easier solution.
Quote from: Scruffie on March 20, 2016, 03:31:42 PM
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.