[sdiy] 1496 VCA?

[Dave Manley]

John Mahoney wrote:
> Ian Fritz wrote:
>> The 1495/6 approach was almost instantly dropped when OTAs became 
>> available.  They require quite a few external parts, they have to be 
>> carefully trimmed, and unless you hand pick selected units they typically 
>> don't null out very well.
> 
> To "not null out" means to allow CV feedthrough, is that right? 

I think here, "null out" means to set 0 gain for 0 CV.

Also there is the issue that a balanced modulator is a 4-quadrant 
multiplier, while a VCA is a 2-quadrant multiplier.  This means if the 
CV is negative, the 4-quadrant multiplier inverts the input signal, and 
so requires an additional circuit to prevent output when the CV is 
negative (IC1+diode form a rectifier clamping negative CV to 0).  VCAs 
don't have this problem.

-Dave