VCA Circuit

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

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)

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.

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

This circuit seam to be among the best you can do with silicium. I like its simplicity. Thanks for sharing.

Have you ever considered replacing the op amp with discrete components here?

I'm using the LM324 and it's just current mirrors and bjts, you can certainly make something simpler that would work if you wanted to make a completely discrete analog theremin.

"Have you ever considered replacing the op amp with discrete components here?"  - innominata

I'm curious as to why you wouldn't want to just reach for the obvious, easy solution here?  I don't know about others, but every time I try to do anything like an opamp with discretes it doesn't end well.  Very often you need the enormous open-loop gain they can effortlessly provide in order to linearize feedback.

Whoa. I forgot about this topic. Meanwhile, I stumbled upon the info that the J11x FETs are more for switching than analog and have a really suboptimal curve for the latter. I realized that while building a "fetzer valve" based tube-to-JFET converted guitar amp "model", and I just couldn't get the bias adjusted properly with the trim pot using those finicky SOBs on a hair trigger.
I replaced with J201 after I actually found some online, and... it worked.
Now if I ever feel motivated, I should try the VCA with the J201 perhaps.

That's the circuit I was talking about:
http://www.runoffgroove.com/uno.html

"Have you ever considered replacing the op amp with discrete components here?"  - innominataI'm curious as to why you wouldn't want to just reach for the obvious, easy solution here?  I don't know about others, but every time I try to do anything like an opamp with discretes it doesn't end well.  Very often you need the enormous open-loop gain they can effortlessly provide in order to linearize feedback.

I'm happy with anything that works but I'm also curious whether a discrete VCA has to be prohibitively difficult.

I sometimes feel like op amps are cheating. It's just something I want to try and was curious if someone had already made headway into it.