FORMANT 24dB VCF

[Haible_Juergen#Tel2743]

Hi!

I took a brief glance at this one this morning.
Each stage has 2 Opamps and one (selected) 3080.
One Opamp serves as buffer (unity gain), the other one
as input adder for the OTA. The idea is that you don't use
the OTA input as a summing node for input and feedback, but
use an inverting Opamp adder instead. This way you can
use the adder output for HP response.
Resistor divider is 39k/100, which they consider fine for
linear operation (but then again, the FORMANT's standard
signal amplitude is lower than usual, if memory serves.)
The feedback is from stage 4 back to stage 1, and there
is a RC-lowpass in the feedback path with a cutoff of 19kHz.
This feedback loop is used for both, LP and HP mode of
the filter.
To change between LP and HP, the output of each stage
is switched between adder output and buffer output with
a 4p2t switch.

JH.