[sdiy] Phasors FET vs OTA?

[Jay Schwichtenberg]

Howdy One and All,

I've been thinking lately about doing some work with phasors, at least 6
stages maybe more. In general I'm more into quiet vs freq tracking for this
type of device. After doing some research it seems to me there are about 4
different general architectures for phasor circuits. FET resistors, OTA all
pass, transistor ladders (Arp Omni) and LDRs. I'll ignore LDRs and
transistor ladders simply because of size (I'm into small and SMT) so that
leaves FETs and OTAs. In my mind (believe me it's a scary place) there are
issues with both. With a number of OTA stages I think there would be issues
of noise (OTAs aren't the quietest parts) and offset unless diode biasing
(LM13X00 & CA3280) and offset trim was used. Using the diodes wouldn't be a
problem but by the time you 6 or more trim pots on a board you've sucked up
a lot of space. Using a FET to ground at the op-amp input starts to seem
better noise wise. The major problem that I can think of with the FETs is
the control/freq range. A lot of FETs have a very small gate voltage for
full off/on operation some in the order of 0.1-0.2 volts. You can find other
FETs that are more in the volt range which might be better suited for this
type of circuit. Some other ideas for discussion might be AC coupling stages
and using OTAs as VC resistors like the FET but not directly in the audio
path.

I know that there are people out there that have some ideas and opinions.

Thanks for the discussion everyone.
Jay