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Adapting Ripples VCA to standalone
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Adapting Ripples VCA to standalone
I was looking over some of the mutable schematics, and the vca on ripples has a really low part count - with an 074, 13700, 14 resistors and 4 caps one could build a dual vca out of it. Thought adapting this to standalone might be a good project to learn a bit more about otas.

From eyeing up the schem it seems like the output of the filter goes through an ac coupling cap and voltage divider to be a super low level signal going into the ota, and then gets reamplified and inverted at the output - what I'm trying to figure out is how the relationship is derived between the cv votage (which seems to get amplified and inverted) and the gain at the output of the vca.

So here's a few questions if anyone could help: would it be better to just massively attenuate any incoming signal to the vca and leave all the other values as they are? Or, leave the incoming signals as they are (say 5v p2p) and reduce the amplification of both the cv, and the output after the ota?

Here are screenshots of the schem, cv input:


The differential input of the LM13700 can't go about 20mV without getting pretty distorted; more like 10mV if you're not using the linearising diodes. So you do need to attenuate the incoming signal like that.

Note that the CV processing section is really a voltage to current converter, because the gain control input of the OTA is a current control. I wouldn't mess with that part smile

This is a pretty standard OTA-based VCA, by the way. This kind of design crops up all over the place, for example in Thomas Henry's VCA-1 which has a nice write-up:
It's covered in many Electronotes articles too.
I should add: Thomas Henry's book "Making music with the 3080 OTA" is well worth a read if you want to learn about these things.

And if you didn't know, the LM13700 is more or less a "dual 3080", with a couple of little differences which you can mainly ignore if you use circuits like the above.
Ah great, thanks! Will give that Thomas Henry VCO-1 page a read.
Should I worry about the impedance of the voltage divider getting the input down to 10mv?
I suppose if it's right at the input jack, and if the rest of your system expects to see 100k input impedances, you'll want a 100k input resistor. 100k/220R is what the electronotes circuits use for the input attenuator.
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