[sdiy] OTA VCA

Tom Wiltshire tom at electricdruid.net
Mon Mar 20 23:10:30 CET 2017


On 20 Mar 2017, at 20:13, Elain Klopke <functionofform at gmail.com> wrote:

> Tom: I had initially trimmed it down to the schmidt trigger, diodes, pots, capacitor, and the buffering op-amp. I'm guessing that the buffer isn't really needed since one AR generator is going to one VCA, but shouldn't I keep the schmidt trigger there to give the envelope a nice 0-5 volt gate pulse when the key is pressed? Or is just a five volt rail getting connected through the key to the diodes enough?

I'd hope that just the 5V rail getting connected through the key would be enough. After all, the caps aren't going be drawing a huge current, especially if you've got a preset attack resistor in there.
Similarly for the output - the buffer is required if you don't know what going to be coming next, but if you're hard-wiring the output to a known VCA, it ought to be possible to get it to work without. Guidelines for the 13700 are a control current of not more than a mA or two max for audio. If you went for the simplest-possible voltage-to-current convertor (AKA "a resistor") between the envelope and the VCA control input, that'd imply a 5K value, which is probably low enough to affect the release. So try it first. You could also give 10K a whirl and add some gain later if the OTAs aren't loud enough. If you *can* get away with it, great. If not, you'll have to add the buffers.

Tom






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