Kassutronic avalanche vco

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Hello smart people,

I’m currently building the Avalanche VCO from Kassutronic:

I’ve built three on my breadboard, and they all work really well. Since I’m only using a 9V battery and don’t need 1V/oct tracking, I’ve only implemented the expo converter.

My problem now is that I want to mix these signals together. However, passive mixing isn’t working. When I put a 100k Ohm resistor on the output, I lose the signal completely, but with 1k Ohm, it works. Still, passive mixing doesn’t work properly.

So, I thought I would buffer the signals, as I have a few LM358 and LM324 lying around. I built three buffers and connected them, but I get no signal at the output. The inputs aren’t affected, as I can still output sound from them, but there’s no signal at either the negative terminal or the output of the LM324 or LM358.

Up until now, I’ve always managed to solve my problems while tinkering, mostly thanks to this forum, but now I’m at a loss.
I know that an op-amp doesn’t work optimally with a single supply, but I haven’t had any problems building buffers with the LM358 or 324 in smaller circuits before. Where is my mistake in thinking?
I hope you can help me.
Thanks in advance this community is awsome!

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That’s odd, a 100kΩ resistor is a smaller load than a 1k; I’d expect it to work better.

A mixer based on an LM324 or LM358 should work well, provided the input signal has a positive DC offset — which the Kassutronics Avalanche VCO does not, because it has an AC coupling capacitor at the output. That means the output goes positive and negative, and an op amp with a single (positive) supply can’t handle the negative inputs. In fact the datasheet says the op amp inputs are supposed to be above a minimum of -0.3 V. What happens if you go below that isn’t defined. That includes the possibility of damage, though that would surprise me in this case.

Still — half the signal is positive going, and the op amp mixer should be able to handle that, so I’d expect some (clipped, distorted) signal at the output.

If you’re using an inverting amplifier configuration to sum the signals, as is usual, you should not expect to see a signal at the inverting input of the op amp. The op amp feedback loop keeps that point at zero voltage. But you should have a signal on the output. Post the circuit design you used.

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Thanks for your reply.

I didn’t build an active mixer; I just wanted to buffer the outputs, and then i wanted to mix the signals. Let me show you the schematic I used:

AvalanceVCO1

This is the circuit I built three times on a breadboard. I’m using a Bluetooth speaker with an AUX input to listen to the VCOs.

Each oscillator works fine on its own, but passive mixing doesn’t seem to work. I suspect it’s because the oscillators are affecting each other, or current is flowing where it shouldn’t. While experimenting, I connected a 100k resistor to the output of one oscillator on the breadboard and noticed that I got no sound at the end of the resistor. With a 1k resistor, I did get sound, which as you mentiond i didnt understand because of the load. So, I thought it would be a good idea to buffer the outputs, and that’s where the trouble started.

Here are two pictures illustrating the issue:
AvalanceVCO2

AvalanceVCO3
AvalanceVCO3737×431 24.6 KB

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Try using a 1uf bipolar cap on the output of each avalanche into each mix channel. I’ve built several attempts at mixing multiple reverse avalanche signals on breadboard and some on strip board. I’ve done single and double supply, passive mix and with opamp. I’m a total noob and through trial and error I made a single supply lm358 opamp work with 3 reverse avalanche oscillators and attenuators for each input. Had to have the bipolar caps to make it work. I used the mix part of the original devil’s triangle circuit by Paul in the lab.

DTD schematic~2
DTD schematic~21600×1465 458 KB

The more channels I added (I tried up to six) the more I didn’t like the mix results. All six channels and oscillators worked. The pitch was affected somehow as well as much bleed through to other channels. Three channels is the most I could get a decent result with a single supply opamp. Couldn’t say why. Still a noob. :100::robot:

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The Kassutronics schematic already shows a cap in series on the output, though it’s not in the diagram shown above. But there should not be such a cap going into a voltage follower of the kind shown, because that puts a negative voltage on the non inverting op amp input, which it can’t handle due to its single supply. If it goes into an inverting configuration, through a resistor to the inverting input which is a virtual ground, then there is no negative voltage on the input, but that still can’t work because the op amp output can’t go negative. In short, no, you can’t use a single supply op amp with a signal centered on zero volts — unless you’re doing something else to bias the op amp output positive.

Without an AC coupling cap the oscillator core output has a positive DC component, and that can go into a voltage follower with a single supply and give a good output. But not into an inverting configuration, because, again, that would try to make a negative output, which it can’t do.

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I said previously I needed the bipolar caps for the three oscillator mix. After looking at the actual working circuit I have I can tell you I did NOT need the bipolar caps for the three channel mix. I only used the attenuators and 100k resistors as shown in the schematic I posted. However it was necessary to have the 1 uf bipolar caps in order to get more than three channels working. I did get up to six working with the 1uf bipolar caps. Once again I couldn’t tell you how. Also I was just using sam’s simple reverse avalanche schematic.

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Thanks again for your help.

Try using a 1uf bipolar cap on the output of each avalanche into each mix channel.

I tested this with a single VCO, using both a 1uF and a 10uF capacitor (I only have ceramic capacitors available). However, the result was the same—I couldn’t get any sound with my audio probe. I’m not even attempting to mix the signals yet.

Without an AC coupling cap the oscillator core output has a positive DC component, and that can go into a voltage follower with a single supply and give a good output. But not into an inverting configuration, because, again, that would try to make a negative output, which it can’t do.

Just to clarify: even if I’m only using a 9V battery, can the capacitor output a negative voltage?

Yesterday, I was testing with my oscilloscope, and I could only see the signal when my audio probe was connected to the output. As soon as I disconnected the audio probe, the waveform disappeared. I’m struggling to understand why this happens.

I can send a picture of the breadboard, but I don’t think it would add much since the diagrams I posted reflect what I’ve built.

IMG_20240906_075907772
IMG_20240906_075907772740×555 230 KB

I initially built it this way to simplify the CV input since I was too lazy to make three vactrols, but with the amount of time I’ve spent on it now, it feels like a really stupid decision! ;D

But now that I’ve reached this point, I really want to understand what the problem is.

I built the Devil’s Triangle a few months ago without any issues. In that case, I also buffered the outputs with a voltage follower and mixed them passively—not for any particular reason, just to experiment.

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So, guys, it seems I’m really an idiot. I connected the expo converter wrong—specifically, I connected the emitter of the 2N3904 to +9V instead of ground. That explains why I only get a signal when I connect the speaker, as that’s when I finally have a connection to ground… unbelievable.

But the best part is, when I connect the transistor to ground, my frequency is much higher than when it’s connected to +9V. Also, the bandwidth is slightly higher with +9V.

I’m using 1uF capacitors on the SSO because with the 10nF ones from the original schematic, I only get high frequencies, probably because I’m missing the negative component from the rest of the circuit that Kasstronic built.

Now I actually prefer the sound of my incorrectly wired circuit. So, with my basic knowledge, what did I do? I built a voltage divider with two 10k resistors at the VCO output, and guess what—now my buffer works too!

Thanks again for all your effort! If anyone has an explanation for this, I’d love to hear it just to understand what’s happening. Otherwise, I’m just happy it works =D. And once again, thank you so much—this really is the best community for this kind of stuff. For all the Germans here… never look for answers in German electronics forums, haha xD.

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Yes. The capacitor’s an uncharged device; it doesn’t know if you’re using a battery or a wall wart or a nuclear power plant. It just knows there’s a varying voltage on the input side so it charges up and discharges via a resistor to ground, and the voltage drop across that resistor is positive when it’s discharging and negative when it’s charging.

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Alright, thank you very much, that absolutely makes sense since it’s essentially a “change of direction.” Thanks for clarifying!