Looking for vca recommendations

[Cfish]

Hi all. I am hoping you all may be able to shed a little light on the VCA subject for me.
I have scratch built a few different vca circuits from different schematics. I have ben looking for something to use in my second mono synth project.

Circuits I have built so far were all based around the action of a single transistor or single fet .

When I look at more advanced circuits I think I may be missing something in the (cosmic uses and purposes of a vca) in the synth world.

I use my VCAs for the usual gate the audio path with either the gate signal or an envelope generator.

A lot of the schematics I am seeing seem to have additional purpose in there grand scheme. (AC and DC SETUPS) (odd control inputs)

I would appreciate any guidance on this topic.

[synaesthesia]

I am currently working on a small Lunetta for a workshop and tried many VCAs as well. There are of course very good circuits available, but I was looking for a minimalistic design that still has acceptable characteristics. This is what I finally settled on. It is similar to the single transistor VCA. In this case I used a FET, as it showed slightly better performance than a bipolar transistor and also has a very high input impedance. I use the XOR because I had one gate left and because it can provide positive and negative trigger inputs. You can use a single inverter as well for a positive trigger input. The 100K pot gives you a range from a very short pluck up to about 2 seconds release time.

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[Cfish]

Thanks Synaesthesia. I'm digging for an or gate now to give this a run on the breadboard. Only enhansment FET I have on hand is 2n7000. Data sheet looked close enough to keep the magic smoke inside where it belongs.

Thanks again. I'm always looking for new circuits to try out

[synaesthesia]

When you use the FET, you do not need to buffer the input. However, the signal becomes more or less a square wave at the output.
You can try the following alternative with a bipolar transistor. It is much nicer to the signal and preserves the waveform, but you might need to buffer the input because of the load it presents.
I am using both circuits only with the envelope generated by the capacitor. Haven't tested them with a control voltage.
Please mind that both are intended for use in a Lunetta when your requirements are pretty low.

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[Cfish]

i like that concept really well. I used something similar in a past project, using LDRs to ground and used LEDs to trigger. A lot of signal still made it through. Made a nice tremolo though. Will give that one a shot on the breadboard. The first was rough on a sign wave.

[Cfish]

No hex inverter on hand so I will try inverting with an opamp. I really like the simple sample and hold capicitor layout for the decay.

[synaesthesia]

You don't have to use an inverter, you can use any gate. All you need is a brief low pulse at the 1K resistor.

[elmegil]

I like Thomas Henry's VCA from _Analog Synth for the 21st Century_. Uses two stages of a V2164/SSM2164, so one chip (and the other various op amps etc) can provide a dual VCA.

[Cfish]

Thanks Elmegil, looks like I need to order the book.

[elmegil]

[AlanP]

Where is the paper copy?

I've heard somewhere it's only on magic smoke, which I don't entirely trust to not be a money pit.

[Cfish]

I had read the stories about (magic smoke). Found several second hand copies available on line.

[elmegil]

Lulu.com is not affiliated with Magic Smoke directly, and they sell both the PDF and the paper copies. You don't need to worry about their fulfillment process, Tim's not in charge of it