Simple Synth-Drum for Lunettas

synaesthesia

I am looking for a very simple synth-drum circuit for my next Lunetta project. Of course, there are plenty of circuits available, but my remaining board space is limited already and I am looking for a circuit with minimum parts. The circuit should run at 5V and the trigger should be a positive edge. And there should be no audible clicks on the negative edge. Here is what I am currently experimenting with.

R1 and C2 generate the envelope directly from the trigger signal. U1A makes sure that there is minimal load on the trigger signal. D1 is responsible for applying the envelope to the oscillator signal from U1B. D2 ensures that the negative edge from the trigger is suppressed. C2 can be 2.2uF or 4.7uF. The latter will generate a longer output signal. The pot can be replaced by a fixed resistor later. I also tried using a vactrol instead of R2 and triggered by U1A to have a decreasing pitch, but the result seems hardly worth the extra effort.

As-is this circuit makes three drums from one 40106 chip. I am sure that it can be improved. Any suggestions are welcome.

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piedwagtail · Posted: Sat Jan 31, 2015 11:29 am Post subject:

Some sort of Impact sidechain?
A filtered thump from the trigger.

There's an example in bottom left of

http://www.birthofasynth.com/Thomas_Henry/pdf/Bass_Plus/basspp_schematic.pdf

Robert

synaesthesia · Posted: Sun Feb 01, 2015 7:26 am Post subject:

Thanks for the inspiring suggestion, piedwagtail! I experimented a bit and here is a small but valuable change.

If you add a small capacitor between the oscillator cap and the envelope cap, you get a slightly decreasing oscillator frequency after the trigger. That gives a nicer impact characteristic. The effect is hardly visible on the scope picture, but definitely audible and worth the extra part. I have recorded the output of the updated schematic with fixed resistors of 100K, 220K and 47K (in that order) and an oscillator cap of 100nF.

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richardc64 · Posted: Sun Feb 01, 2015 10:35 am Post subject:

A definite improvement. See if a cap to Gnd at OUT can make the sound less square-ish.

synaesthesia · Posted: Mon Feb 02, 2015 2:31 pm Post subject:

Thanks richardc64, I tried that with several values. A cap to ground helps only a bit, but not enough to make it less square-ish. It also limits the amplitude significantly. I guess that is the price for the simplicity of the circuit.

piedwagtail · Posted: Mon Feb 02, 2015 10:23 pm Post subject:

You could run these drum oscillators in parallel as a sort of additive synth from a single trigger.
The main drawback seems to be your design lacks the simple velocity sensitivity that a twin-t has by its ringing nature.

The Synare was couple of parallel 4093 lunetta sections with an LFO switch option (and noise) mixing into a 24dB 3080 filter and a 3080 VCA.There were velocity sensitive envelopes for the filter and VCA. It's the complex version Smile

https://www.youtube.com/watch?v=sNzo2Jk77eU