Yet another variation of the simple reverse avalanche oscillator

I experimented with these about a year ago and recalled that the BC337 transistor worked well with a single +12V rail.
I tried this tonight:

I got a range of approx 44Hz to just under 400Hz a little over 3 octaves.

Output is quite low at around 1V pk-pk into Hi Z load ('scope and xtal earphone)

I intend to try other transistors to see which are able to oscillate with +12v supply

Incidentally I tried running it across +/- 12v (24v) and the output was slightly higher but the range was more limited. It also works of single + 9v supply but with even more limited frequency range.

Not sure how crucial this is but the capacitor was a film cap rather than an electrolytic type.

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Reading this has caused me to recall my reverse avalanche oscillator experiments with a few types of transistors, and I feel compelled to share an odd detail. :thinking: I specifically remember using bc548, and being a total beginner at the time lead to a few mistakes including getting the 548 pins connected backward. You wouldn’t know it with the 548 because the result is apparently the same. It oscillates connected either way according to my findings using LMNC super simple oscillator schematic (the only variation being the backwards transistor connection with the bc548)

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Ended up building 5 of these plus a simple circuit to control the overall pitch of all 5 oscillators.

Still experimenting with it but heres what Ive got so far:

Note that it is powered from + & - 12v supplies. Thr CV range is 0 to +12v

Opamp mixes signals from all 5 oscillators to a single output. (Note all are mixed equally)

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My thoughts on the choice of timing capacitors, ideally would be a polystyrene but I dont think they are available as high as 1uF+

With electrolytics, leakage can be a problem which tends to limit the maximum value of the corresponding timing resistances.

If you can get ceramics with low tempco that might be an option, my preference is film capacitors, which unfortunately start rising steeply in price beyond 1uF.

If you prefer to go for electrolytics probably need to opt for lower value timing resistors/pots, like in Sams circuit.

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Another variation using only 3 transistors:

The output is a sawtooth of around 10 V peak to peak. With a 4th transistor you could get a squarewave output too.

Input CV range is from around zero to +12v or more if available

The response is more or less linear (better at higher cv’s)

This one works well enough. I made a point to point circuit. It looked easy enough and I had all the supplies except for strip board. :100::robot:


I keep meaning to Breadboard this one to check it out.

Source for that, with more information:

(Spoiler: “So, you ask, how well does it track 1V/Oct? Well, for a synth VCO not very well at all, but for such a simple circuit I think it’s not bad.”)

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I like this circuit. I made something like this. It was supposed to be my own version of Sam’s drone oscillator of doom, even though I didn’t really use any of his circuit at all except for the transistors are the same (9018). It doesn’t have the overall pitch adjust but the rest of the circuit is very similar to yours. I had to figure out the capacitors from each oscillator into the mix (I had no idea what I was doing). I have mix pots for each input, also.