AW: VC pots

[Don Tillman]

   Date: Wed, 25 Oct 1995 10:42:00 -0700 (PDT)
   From: Haible_Juergen

   JFETs:

   pro's: nonlinear (quadratic?) CV response is often desirable
	       (in compressors and in phasers)
	       small component, easy to layout the pcb
   con's: very small voltages needed to prevent distortion
		=> unually rather noisy circuits (at least together with
		cheap opamps as active elements)

The response isn't a quadratic curve, but I don't know that it's
anything especially useful.  The signal voltage doesn't have to be all
that small, 0.5 volt or so.  I think the noise level is really quite
low. 

   OTAs:

   pro's: cheap, easy to use, kind of "standard" in synths
   con's: SNR limited to about 65 ... 75 dB

Now *these* need a small signal voltage, .05 volt or so.  The big
feature is very accurate linear or exponential control over a very
wide range (several decades).  This is usually the big determining
issue.

Some OTAs can be very low noise.

   LDR + LED:

   pro's:  very good SNR
   con's: very slow

Not that slow, some LDRs can move pretty quickly.


   NTC + heater:

   (just for the record. Don't know much about that, only
   that this combination is quite common in ALC circuits
   of commercial telecomunication systems)


   motorized pots:

   pro's: very good SNR
   con's: very slow, very large, very expensive

Anything with motors is *cool*.

   special VCA IC's:

   pro's: good SNR
   con's: expensive and sometimes hard to get

I'm pretty sure all special VCA chips are in fact OTAs.

   DAC's, programmable pot ICs etc:

   pro's: good computer interface
   con's: staircase stepping

  -- Don