[sdiy] usage of 3080 in micromoog

[Benjamin Riggs]

The output of the ladder needs a balanced hiZ input with a gain ~ 100-1000.
to do that with an opamp, you need to load the top ladder rung with the
balanced resistor network (of 4 resistors) with an opamp to set gain (5
components), or without loading the ladder use a more complicated
instrumentation configuration (3 op-amps, 7 resistors), or a discrete
circuit using a bunch of transistors and resistors) or use a OTA with one
resistor setting gain and 1 resistor doing the I-V conversion at the output
(1 OTA and 2 resistors). There are probably more ways of doing it, but with
so few parts?

Moog liked elegance, the best/simplist solution with the least number of
parts. That after-all was the motivation of the ladder filter in the
firstplace.

-----Original Message-----
From: owner-synth-diy at dropmix.xs4all.nl
[mailto:owner-synth-diy at dropmix.xs4all.nl] On Behalf Of Dave Manley
Sent: Friday, 6 October 2006 8:52 AM
To: synth-diy at dropmix.xs4all.nl
Subject: [sdiy] usage of 3080 in micromoog

There are a few places in the micromoog where a 3080 is used with the 
bias current fixed.  For example look at the output of the ladder 
filter.  Why was a 3080 used here instead of an opamp?

-Dave