[sdiy] Dual Log/Linear Irwin-Gallo VCA

[Neil Johnson]

Hi David,

> 1.  The Irwin linearization trick is not "a lot of effort"; it's actually
> very easy, and it eliminates some of the not-so-nice features of the 2164
> when used as a gain block (the negative CV-to-gain relationship, and the 5k
> impedance on the CV input).

I guess its horses-for-courses.  I say 'a lot of effort' in the sense that you're using half of the SSM2164 just for linearising, plus an op-amp for each converter (granted, this may also function as the CV summer/scaler), plus a negative voltage reference.  Not a lot, I agree, and whether its 'a lot of effort' or not does depend on the individual.

> 2.  I can buy a THAT 2180 dual trimless VCA for $5.95, a THAT 2159 single
> deluxe VCA clone for $3.50, or an SSM2164 quad VCA clone for $3.50.  Hence,
> the 2164 is far more economical.  Plus, I'd still need to do the Irwin trick
> with the THAT VCAs if I wanted a linear response.

This is where the US/UK differences change the equations: SSM2164s are a lot more expensive here - I buy them in quantities from local distributors or from Digikey, and by the time you add in tax and postage the SSM2164 is about twice what you pay.  For high qulity audio VCAs THAT chips work out better value.

> 3.  Not all of us have the advantage of a fully developed modular with loads
> of VCAs already in it, and years of experience with electronics to see all
> the possibilities of a given chip straightaway.  I can attest, however, that
> the Irwin-Gallo VCA is a very nice and very elegant circuit, and an
> excellent way for us neophytes to get acquainted with the 2164.

I certainly don't have a fully developed modular!!  But I agree it is an interesting circuit to start playing around with the '2164.

Cheers,
Neil
--
http://www.njohnson.co.uk