[sdiy] Dual Log/Linear Irwin-Gallo VCA

[Neil Johnson]

Hi,

Andre Majorel wrote: 
> Interesting. I wonder how stable it is WRT temperature. The 2164
> was designed to pass audio, not to drive VCOs.

The spec on the temperature dependence on the control input is -3300ppm/C, about the same as for the usual expo converter, and can be compensated for either with a temp.co resistor somewhere in the control path, or you can use one of the other gain cells as a compensator.

As for driving VCOs, it is perfectly suitable for that: my 18dB/oct filter oscillates quite nicely over the audio range, and I have a design on the bench for a multi-phase VCLFO which with some capacitor swaps it will easily go way up into the audio range.  As it is now it goes down to around 30 minutes.

Personally, I think its a bit odd to build a plain VCA out of an SSM2164.  A THAT corp dedicated audio VCA would give better audio performance, and you'd get better separation between channels than you would with the SSM2164 (same package, shared supply rails, cross-coupling between cells on the silicon, etc).

Sure, Irwin spotted up a neat trick, by turning a '2164 into an LM13700 through a lot of effort.

No, for me the real magic of the '2164 is having four easy-to-use gain cells in one package.  That opens up a lot of interesting possibilities...

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