[sdiy] SSM VCA ICs, was Re: BA 6110 & LM13600/13700

[Neil Johnson]

Tom,

On 29 Nov 2007, at 19:39, Tom Wiltshire wrote:
>> No, they are not pin-compatible.  And they have different control  
>> transfer functions - the SSM2024 is linear, while the SSM2164 is  
>> exponential.
>
> Can you tell me any more about that, Neil? The SSM2164 datasheet  
> describes the SSM2024 control response as "log/log" whilst the  
> SSM2164 is described as "linear/log". That confused me somewhat, so  
> if you understand it better, I'd like to know.

The SSM2164 control response is -33mV/dB, so a linear change in  
control voltage produces a log (or expo, depending on your point of  
view) change in the gain of a gain cell.  This is eminently suitable  
for audio VCA usage so that you get the required log response from a  
linear CV.  Ideal for automated mixing desks, for example.

The SSM2024 had very similar behaviour to the usual OTA-style devices  
(CA3080, LM13600, etc).  To use those for audio VCA use you either  
need a log-scale CV or a converter.

The control transfer function of the '2164 is actually quite useful,  
as it becomes very easy to make VCFs and VCOs with proper response  
curves, without the need for an expo converter.  The only downside is  
that there is no built-in temperature compensation, so you need some  
way to compensate for the -3300ppm/C temp.coeff of the control  
input.  There are ways round this, from using temp.co's to using one  
of the four gain-cells in a compensation loop.  In many applications  
this is not actually that necessary (e.g. VCF or VCA).

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