I made a Barton Analog Drum in Eurorack a few years ago, and I currently have one on breadboard. I’ve found a few things I want to change.
I noticed in my Eurorack module if I tried triggering with a gate it sometimes would trigger on both the rising and falling edges. I never chased down the reason but I suspect it was that there was some ringing on the falling edge, taking it up above the comparator threshold again. That threshold is pretty low, about 120 mV from a 100k/1k voltage divider on +12 V. Changing that to a less extreme ratio might help, or better yet changing it to a pot for an adjustable threshold.
The 220Ω fixed resistors in the envelope generators are too small. The op amp can’t supply enough current when the decay times are turned down. Changing them to 4.7k, and the pots to 1M and the caps to 470 nF, still gives you a 200:1 decay time range but makes a big enough load for the op amp to handle.
I looked at the VCO output on my scope (yellow trace):
VCO (yellow) and VCA (blue) outputs
It’s presumably intended to be a triangle wave but at the top it goes to +10 V and at the bottom it clips at -7 V. The amplitude’s too large (and, see below, doesn’t need to be this large). Replacing the 470Ω from LM13700 pin 13 to ground with 1k gives a good ±5 V triangle wave. But it also roughly doubles the VCO frequency, so the VCO capacitor needs to go from 22 nF to 47 nF.
The output of the VCA (blue trace above) saturates at ~ ±10 V until the envelope comes down far enough. I don’t know, perhaps that’s deliberate, to get a louder sound, but I don’t like it. If needed I’d rather amplify into saturation externally.
Per Electric Druid’s discussion of LM13700 VCAs the LM13700 input should probably be no larger than about 50 or 60 mVpp to minimize distortion. With the VCO output at 17 Vpp the 22k/220Ω voltage divider gives about 170 mVpp! Reducing the VCO to 10 Vpp helps, but you need another factor of about 2. Changing the 22k input resistor to 39k gets the input down to around 56 mVpp.
Also per the Electric Druid article, it’s best to keep the control current below about 500 µA. Here there’s a maximum 10 V control voltage so about a 9.3 V voltage drop across a 10k resistor connected to a PNP transistor, giving 930 µV. Changing that to a 20k resistor brings it down to 465 µV. Those two VCA changes bring the amplitude down to where it no longer saturates.
Modified VCA (blue) output
How does this affect the sound? Hard to say. I haven’t really been able to make an A/B comparison. I think the modified version sounds good though. And looks nicer on the scope.
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