[sdiy] Discrete OTA

[David G Dixon]

> There are plenty of well (and not so well) documented ways to 
> use the 13700 without the limitations you mention. The 
> criticism is better aimed at the circuit design rather than 
> the 13700 itself, so it is more a personal preference rather 
> than technical merit as to which to use. After all, the 
> SSM2164 input circuit is still the same diff. pair/ current 
> mirror topology as the 13700, but has internal feedback 
> remarkably similar (in concept, not implementation) to the 
> Mike Sims linearisation technique for the 13700. 

Yeah, maybe.  However, another nice thing about the 2164 is that it is
exponential (nearly perfectly so), and therefore can replace not only the
OTAs but also the exponential converter in filter circuits.  In fact, it's
almost as if the 2164 was made for filter circuits.  All my filter designs
sounded better when I replaced 13700 with 2164, and they involved much fewer
parts.  I would never even consider using anything else for filters.  Having
four VCAs in a 16-pin package rather than two OTAs is also much more
convenient.

In fact, if it weren't for the 2164, I would never have gone so far with
synth design.  I owe that little chip a lot.  It really is the beating heart
of every module I've ever designed.