Voltage controlled APC--a question about the voltage inputs

Dr. K · Joined: Jan 15, 2020 Posts: 52 Location: wisconsin

I've got a couple APCs laying around, and I messed around a little bit with using the voltage control inputs on the 556 chip.

I was just thinking about something on the way home from the shop. Rather than run 2 separate CV inputs (as per the scheme I found--attached here), I should be able to supply 1 CV, but tie it to the wiper of a pot. Then, each leg of the pot to each of the CV inputs on the 556. (in the bottom, middle, the bold pot symbol I inserted in the original scheme)

That way, using the pot as a voltage divider, I should be able to dial in how much the the "base frequency" and how much the "divider" are influenced by the control voltage input.

That should work, shouldn't it? And since we are not really concerned with the current, the value of the pot (the bold black pot symbol I put on the scheme) shouldn't be too critical. I don't know take like a 10k or something?

Thanks for any thoughts on the matter. The APC if a cool and fun circuit, but operating by hand is a little, err, unruly! It would be fun to harness it in a more musical, sequencer controlled fashion.

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PHOBoS · Last edited by PHOBoS on Fri Feb 21, 2025 11:13 am; edited 1 time in total

PHOBoS · Posted: Fri Feb 21, 2025 11:09 am Post subject:

Tin Can Synth Jam APC demo, it's some kind of dual hybrid thingy
(link to the schematic in the video description)

Dr. K · Joined: Jan 15, 2020 Posts: 52 Location: wisconsin

Thanks for the info PhoBos. I'll think it all over and do some reading.

I am curious though, if a simple pot voltage divider setup didn't work, do you think the original circuit would work, with 2 CV inputs?

If so, can they only operate from 1 sequencer 1 at a time, or could they both be connected to the same sequencer at the same time?

PHOBoS · Posted: Tue Mar 11, 2025 11:36 am Post subject:

As long as you're connecting them to something that has a fairly low output impedance, which is usually the case with a sequencer, it should work.
Also, with this setup using a single pot between them as you've drawn will do something because the CV inputs don't have a very high impedance.
You just have to keep in mind that the pot value shouldn't be too high, 100K probably won't do a whole lot, 1K is likely too low or will at least effect
the range you can set manually.

Dr. K · Joined: Jan 15, 2020 Posts: 52 Location: wisconsin

I dug out ny old dub siren, and realized it has an op amp stage between the modulator (the first stage) and the cv input of the next stage. This weekend I want ro give that a try--run the sequencer output to separate pots, then through op amp (or maybe transistor?) Buffers, and then to the 555 cv inputs. That should provide the voltage, as well as a low impedance current source. It should also completely isolate the 2 cv inputs

These projects drive me nuts--I'm on the road working, and try to cram in my synth project on the rare occasions I'm home.

Anyway I'll report back whatever happens.