VCA Circuit

Posted: 3/16/2015 12:33:51 PM
carlmart

Joined: 3/16/2015

Has anyone designed or built an actual limiter using this VCA?

And tested, of course! To see how it sounds.

If possible compared to other limiter designs.

 

Carlos

Posted: 6/20/2015 3:01:12 PM
tinkeringdude

From: Germany

Joined: 8/30/2014

I built this VCA with parts I happened to have lying around, so I didn't have that fet, but rather two J113. And an NE5532P (at +/- 5V rails) as opamp, the second one in there used to invert a 0..+3V control voltage so I got the negative one for the gate.
I checked that indeed my input goes to 0..3V and the gate gets 0..-3V.

This circuit then only manages to attenuate the 1kHz @ 3V squarewave output of my scope to something like -30 dB, which is pretty poor I guess.

Now I'd like to remind you I'm a newb with only basic electronics knowledge and leraning something new here and there, but not the "sit down every day and work through a book XYZ chapter with excercises" way ;-)

What might be the reason for that? In spice it attenuated to I think -50dB which also doesn't seem great, although I don't know what the standards for simple analog VCAs are. But my real world assembly doesn't live up to that.
Is it simply differing characteristics of my J113's ?
What could I adapt to improve the performance?

(btw, I'm looking for a low parts count one for my synth stuff, not theremin. That's why I'm experimenting with this, it looks cheaper than OTA based ones I've seen, esp. when using some SMT JFETs I've seen for rather low price)

Posted: 6/21/2015 1:24:33 AM
dewster

From: Northern NJ, USA

Joined: 2/17/2012

tinkeringdude, did you try replacing the two input resistors with a 1k pot?  Matching the FETs for VT and IDSS helps, as does somewhat higher supply voltage.  Square wave as audio test signal in this scenario (measuring attenuation) isn't recommended due to the strong higher harmonics.

For real use I would crazy glue the FETs together to keep them at the same temperature.

Posted: 9/26/2016 4:01:03 AM
dony

Joined: 9/26/2016

Formal channels of distributors, more and better electronics components, http: //www.components2world.com

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