[sdiy] OTA VCA

cheater00 cheater00 cheater00 at gmail.com
Tue Mar 21 01:32:06 CET 2017

If multiple (read: more than 4) caps draw enough current during attack
to sag the 5V rail, I'd say that's a feature rather than a bug.

On Mon, Mar 20, 2017 at 11:10 PM, Tom Wiltshire <tom at electricdruid.net> wrote:
> 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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