[sdiy] Linearized 2164 VCA Warning

[Neil Johnson]

Hi David,

> *** THIS IS A MESSAGE FROM THE EMERGENCY BROADCAST SYSTEM ***
>
> If you are linearizing the SSM2164 (or CoolAudio V2164), be aware  
> that the
> input voltage offset of the linearizing opamp will influence  
> whether you
> will be able to completely shut off the VCA or not.

[snipped]

> *** WE NOW RETURN YOU TO OUR REGULARLY SCHEDULED PROGRAMMING ***

Or simply... don't use a cheap-ass op-amp like the TL074 for a  
critical DC application.  Its "good enough" for non-critical use  
(typical audio path) where DC accuracy is not paramount, but for  
important CV circuits you really need to choose components that  
favour DC operation (low offsets, low drift, etc).

Try an LT1012 instead (max 25uV offset).  Or if you don't want to  
pony up the $$$ for the op-amp then you need to provide some manual  
means of trimming out the offset, for example as you suggest with a  
large resistor to a stable bias voltage (suitably decoupled else any  
noise will amplitude-modulate the audio).

Did you also include the series resistor that George Hearn wrote  
about from his 2164 experiments?

http://search.retrosynth.com/synth-diy/search/lookit.cgi?-v0906.246

Otherwise you'll be cooking those 2164s nicely (perhaps not a bad  
thing at the moment tho' ;).

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