Riffing on the Harmonic Percolator topology  [documentation]

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Post by lykwydchykyn »

Many kilobytes have been spilled attempting to recreate the legendary Harmonic Percolator, and there are some that enjoy going to great lengths to find just the right transistors to recapture its classic sound. I'm more the type that wants to just shove any old BJTs in there and then jank with the rest of the circuit until it goes BRRRRRR...

I started with the Escobedo Harmonic Jerkulator:

Harmonic_Jerkulator.gif (6.23 KiB) Viewed 912 times
Tim already did the work of finding sane values for a 2n390x transistor pair, so this was a good starting point to riff on the circuit.

Next, I started making changes:

- First to go is that rather unfortunate "harmonics control". A voltage divider at the input? Come on interfax. After some experimentation, it seems the gain of the circuit is dependent on both feedback resistors; a dual-gang pot might be interesting here, but I found that by dropping the PNP feedback to ~100k and putting a pot in series with a 10k on the NPN I could get a decent gain control. Most of the interest action happens between 0 and 250k, but if you go up to a 1M you get a little extra sizzle from it. Maybe a C1M would be good here, to emphasize the lower range.

- This circuit is abysmally noisy, though. I found I could tame the noise considerably by adding reverse-parallel diodes in series in the feedback loop (the cap that goes from the PNP collector to the NPN base). I put them on the PNP side of the cap and the noise is practically gone. I've used this trick in the signal path before and it usually screws up low-gain sounds because of the crossover distortion, but in the feedback loop it seemed to not do this. I can still roll back the volume and get a decent light drive sound.

- I've become convinced that there is no dirt circuit that can't be improved if you somehow shove a bazz fuss into it, so just for giggles I tried replacing the PNP feedback resistor with an LED. Yowzers, it kicks it into the stratosphere! Super buzzy, saxy, synthy tones. Decided to keep this, but put the resistor on a switch so I could toggle between low and high gain modes.

- At this point, though, I'd invented a fart pedal. At highest gains, there would be this weird, low "yuurrrrrp" sound after I played and silenced a heavy chord. The higher the gain, the longer it lasted. It was like, motorboating, but rising in pitch and then cutting off. I found I could eliminate this by dropping the 22µF cap to 100µF at the expense of a bit of low-end grunt. Putting a 10k in series at the input helped a bit too, though I think it just lowered the gain really.

The final schematic looks like this:
This is my vero layout, which I've verified:
No sound demo just yet, I still need to box it up. Any thoughts on improvements are welcome.

EDIT: I boxed it up in a puzzle tin and made a demo:

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