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.

Hi Everyone!
I’m pretty new to the forum and not an expert in electronics; I’m primarily self-taught. I have a question about the “super simple VCO - AVALANCHE” that is on the website and YouTube. I was looking at the basic drawings and trying to combine all the different versions.

My goal is to have a small PCB for each VCO afterward.

I chose the BC337 for this one because it’s what I have available. But I have many doubts about the CV IN with the Vactrol ( is this right in the schematics? Should the LED have a Resistor on CV?) How to add a CV Out, asBypass for other modules?

Also, does the LED1, which indicates the pulse, need a resistor?

I would appreciate it if you could have a look and make suggestions.

A lot of CV sources have a 1k resistor on the output, and that will serve as a limiting resistor for the vactrol LED. But if not, then yes, add a resistor. Whatever value gives a good CV response range and limits the current to the maximum the LED is rated for.

I’d also add a diode (1N4148) antiparallel to the LED to protect it against large reverse voltage (most LED datasheets specify a maximum of about 5 or 6 V reverse).

Not sure I understand your CV OUT question.

LED1 is OK without a resistor in this configuration. I think this is because it’s not connected to a voltage source, and the reversed transistor limits the current flow well enough. It also only lights up briefly and that helps.

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Thanks a lot @analogoutput for your comments! The CV OUT question is just something I would like to add as a direct bypass from CV IN, (just like MIDI IN and THRU) so any CV sources connected can be patched to other modules and replicated without being affected.


The problem with that is, you’d be trying to power multiple LEDs with a single source. If the LEDs are connected in series you won’t be able to connect very many before the summed forward voltage becomes too large for the CV to drive. If they’re in parallel then adding LEDs will add proportionally to the amount of current drawn, which isn’t sustainable. If the CV source goes to 10 V and has a 1k resistor, and the LEDs have a forward voltage of 2 V, then it can supply no more than 8 mA current, and that won’t drive very many LEDs. You could try it, just connect a jack in parallel with the input jack, but it may not perform as well as you’d like.

The usual way to send a CV to multiple modules is not to pass it through but to split it with an active or passive multiple module. A passive multiple wouldn’t work well here because it’d still amount to putting the LEDs in parallel and would still be drawing a lot of current from the CV source. An active multiple would work better.

This is a very simple circuit but that means it has greater limitations than more complex modules. Typically a synth module has high input resistance (~100k) which means it draws very little current from the CV source. In a less simplistic design the vactrol LED would not be connected directly to the CV but would be driven by an op amp that converts the CV to current without demanding that current from upstream. There also might be an op amp buffer on the output that would enable this module to drive a downstream module such as a filter or VCA in a way the present version would have trouble doing since its output resistance is high.

It’s a good educational circuit but not really suited as it stands to making a synth out of.

For a relatively simple way to drive multiple avalanche oscillators with a single CV see the 2000 Megadrone. It uses MOSFETs to control the oscillators.

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@analogoutput You’re awesome! Thanks for this fantastic explanation. I will have a look into the megadrone, maybe start simple with this one before moving forward. Great help, thanks!

There’s a big long topic here:

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@analogoutput thanks a lot! 2 things - stumbled upping your amazing channel on YT, great tutorials ! Following you now as user name HiLow.

  1. I,ve tried a protoboard with the circuit on the 2000 megadrone, worked very nice! Just waiting on the mosfets to drive the CV.

quick question - the readings show a Vmax of 328mV, so for 6 modules 1.968mV, meaning close to 2 amps to drive it? Is this correct? In case it is, it might be the leds? All I had were the super bright, blue. The circuit is with an s9018 and no mosfets.


I’m not following your calculation. In one oscillator:

All current flows through either R30 or R11. For R11 the maximum voltage at the top is 12 V and at the bottom — not sure, don’t know how to figure the MOSFET, let’s say 2 V for the LED forward voltage and maybe 2 V more, call it 4 V. Then the maximum current through R11 would be (12-4)/3.3k = 2.4 mA. Current through R30 would be (12-0)/(10k+47k) = 0.2 mA. So I estimate 2.6 mA per oscillator, or about 16 mA for 6 of them. I wouldn’t bet money on that being very accurate but correct order of magnitude I think. (Higher current flows through the LED when it lights up, coming from the capacitor, but that should be about the average.)


@analogoutput Thanks you do reply fast! I will measure again on the positions you mentioned, it might have been my mistake. The reading was taken between the 9018 before the led and ground. I will check also my equipment, as I mentioned I’m on a learning curve here,

Thanks again!

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Ok so now the first PCB for 5 oscillators!


Hello everyone!
Just keeping the spirit of “open knowledge,” I’m sharing here the simple PCB I’ve made for this VCO, of course giving all the credit to LMNC for the initial project. The idea is just to help facilitate for people who want to have the VCO in a simple etched DIY board or send them to places like JLCPCB and similar.

I’ve added the Gerber files (again, very simple) and a PDF (already mirror) with a BW version to etch on a single layer at home. PCB size is 45mmx34mm for each, as I have big hands for soldering. It’s not professional work, but it works after I modified a few problems from the initial board.

I’ve also added an extra 12V and GND on a few positions, so you can have a module just below or to the side of the next one.

In the end, I ended up using all BC337… for some reason, the 9018 did not work half of the time. Also, a “Dummies like me” placement in the PCB, just for reference.

Captura de tela 2024-02-19 195604

Mirrored PCB

Files: DIY_Synth - Google Drive

Now moving to the other parts for my first DIY VCO!