Why does my circuit suck?

[jcpst]

Let me preface with I don’t really know what I’m doing.

I put 3 little circuit blocks together. The preamp from the Chelsea Muff, the HPF from the Karma Suture, and the Harmonic Jerkulator.

When I probe from the jack in to the preamp out, it’s fine. When I probe from the Jerkulator in to the jack out, it’s fine. The problem seems to be the HPF. There’s oscillation/feedback at lower input levels. As you increase the input, the signal takes over, but the oscillation comes back anytime the signal is low enough.

I really don’t know, but if I was forced to guess, I’d say one of these:
- just too much gain
- something is missing, like a capacitor or resistor to ground

Here’s the circuit blocks.
Preamp (muffin crumb)

HPF



Harmonic Jerkulator (input / harmonics pot omitted)

 

[Aleph Null]

The Percolator is particular about what it sees at the input. I had a similar issue when prototyping the Moka Pot Fuzz.

You could...

1. Set a minimum resistance to ground that's high enough to prevent the oscillation. This limits the range of your boost.
2. Lower the value of the input cap feeding into the Percolator (probably the larger of the two caps). This will limit bass response.
3. Play with the value of the 1M resistor across the 2N3906. I can't remember if it needs to go up or down, but changing that value will effect the bias and the motorboating. I've seen values as high as 2.2M and as low as 150k in that position. I'd probably start with a 470k and keep going down until the noise went away, then play with the extreme of the input and HPF to make sure it behaves.

 

[jcpst]

Thanks @Aleph Null ! This gives me plenty to try. I’m curious to see how idea 3 goes, I’m currently getting a little bit of that sub-octave that I’ve read about before.