VCA based Guitar Volume Controller

[marman]

Hello Everybody,

Due to the circumstances i have more time on my hands then usual and i came up with the idea of building a VCA based Volume Controller to control the guitar volume with an expression pedal.
The whole idea is based around a THAT Corp 2180 VCA. 12VDC Supply Voltage from a Truetone PS and +/-5VDC for the VCA and the OPAmps are gerated by Recom RS3-1205D DC/DC Converter.
I Came up with the following:



basically the 2180 is controlled by the voltage generated via the voltage deviders R9/R10 and RP1/Exp.Ped Resistance (0V=unity gain, 0,55=-90db attenuation). RP1 is an adjustable Pot to match different Exp. Pedals.
Since im quite new to the guitar pedal world (not electrical engineering) i wanted to ask for feedback from more expierienced DIYers.

Anybody seeing any fundamental flaws or issues with the design or things that could/need to be improved.

Thanks in Advance

[Jarno]

As someone who switched to building a modular synth from stompboxes (and back again), steer away of the 2180, they are expensive and really not that easy to use.
Another IC that you might find is the 2164 (quad vca) and also that one is finicky to use.

I'd simply use an LM13700 based VCA like this one:

http://musicfromouterspace.com/analogsynth_new/DUALVCA/DLLVCA001.html

Leave everything off you do not need (CV inputs, circuitry for linear operation).
An alternative might be this one, even without the OTA:

https://yusynth.net/Modular/index_en.html

[marman]

Thanks a lot for the Info - will have a look into it!

One reason i went for the 2180 is that i already have one - what issues could i run into with it?

[Mark Hammer]

There's always this circuit:  http://www.tonepad.com/getFile.asp?id=2

[Jarno]

Didn't know that one, but if you are going to build that, double it and use a 13700 OTA, 3080's are expensive and often faked (there are a few new replacement parts being manufactured by alfapar and coolaudio, I think? Not sure how easy it is to get those).

Regarding issues with the 2180, I haven't seen a good working circuit for a VCA that uses it, there are several to be found:
http://www.acxsynth.com/vca/vcafr.html  (this one does not work)

But also this one:
https://manhattananalog.com/products/vca?variant=33847987846

Is not completely right (although mine works)

[marman]

There's always this circuit:  http://www.tonepad.com/getFile.asp?id=2

Thanks a lot! That seems like a good and easy solution especially since it just requires 9V - haven´t had the time to really look into it yet, but any reason why that wouldn´t work with a LM13700 instead of CA3080 (since that is long gone)?

Didn't know that one, but if you are going to build that, double it and use a 13700 OTA, 3080's are expensive and often faked (there are a few new replacement parts being manufactured by alfapar and coolaudio, I think? Not sure how easy it is to get those).

Regarding issues with the 2180, I haven't seen a good working circuit for a VCA that uses it, there are several to be found:
http://www.acxsynth.com/vca/vcafr.html  (this one does not work)

But also this one:
https://manhattananalog.com/products/vca?variant=33847987846

Is not completely right (although mine works)

Thanks for the links as well - looks like you´re right and the 2180 isn´t gonna make it!

[StephenGiles]

There is info about the THAT 2180 here http://www.thatcorp.com/datashts/dn137.pdf, which appears to be a drop in replacement for the THAT 2150, for which there are many circuits floating about.

[Jarno]

I couldn't find an easy and confirmed working circuit with 2150, the one that marc bareille did might work.

Just came across an article on the actual nuts and bolts of a design with LM13700:
https://electricdruid.net/design-a-eurorack-vintage-vca-with-the-lm13700/

[Jarno]

Not much happening here, but Rod Elliott recently did an article about a simple discrete vca:
https://sound-au.com/project213.htm

Close enough for rock n roll.