Big Muff Pi squeals

[Beta2.0]

Hey all,

I have this....http://gaussmarkov.net/layouts/opbmp/opbmp-schem.png                                                                                                                                                                                                                                 on my bread board. It sounds good, but the sustain pot has no adjustment. It has to be cranked all the way, else it squeals.

Any thoughts?

Brad

[Cliff Schecht]

A gain of 330, when combined with filters on the input and output, can make a helluva oscillator. It could be a number of things however, including microphonic pickups, a microphonic breadboard (do you hear audible thumps in the output when you tap the breadboard), a high gain mixed with bad breadboarding technique, or a flurry of other problems. Try changing that 8.2k resistor (R9) to a higher value, say 47k. This will cut down the gain of the output stage significantly (gain of 10 instead of 54) but will increase the op amps input resistance decently. 8.2 k is a pretty small input impedance, even when being driven by another op amp.

[Beta2.0]

Thanks Cliff,

I will test the things you mention tonite. One thing I did not mention, and coincidently is directly connected to R9, I used 4 1N914's rather than 6. My local supplier only had 4. I did not think it would make that much difference, but maybe it does?? The diodes are in the feedback loop, and R9 supplies the signal from IC #1 to IC #2. So, even though they are directly connected, R9 has little affect on quantity of diodes. Am I correct?? I could be wrong
I just trying to make it sound better and learn more about the circuit.

Thanks for you help.
Brad

[Beta2.0]

Okay...I think I have it the way I like it. Switched out R9 for a 47K, got about a eigth of a turn on the sustain pot...hmmmm....stuck in a 62K got more sustain and the tone was better, but still a littel harsh. Just for kicks I stuck in a 92K and wow, what a difference. The tone is very warm, and I get about 75 to 80 percent of a turn on the sus pot. At about 50% the tone starts to deteriorate, but I can live with that cause by then it's pretty lowd.
Just wondering, with that high of an impedence on the input of IC 2 is it at unity gain?

[Cliff Schecht]

It's at unity gain when R10 is equal to R9. I'm glad changing R9 helped, that was my first suspect.

The three diodes in your feedback path determine at what voltage you start clipping (before the op amp saturates, that is), which in your case is 1.4 V instead of the 2.1 V with two diodes instead of three. It's a good thing that you reduced the gain now since you were probably saturating the crap out of those diodes. I suspect this is what caused you to have such a squealer!