6581 Filter overview
Quote from an interview with Bob Yannes found in the internet: "The Filter was a classic multi-mode (state variable) VCF design. There was no way to create a variable transconductance amplifier in our NMOS process, so I simply used FETs as voltage-controlled resistors to control the cutoff frequency."
Two wikipedia links which might be interesting:
- https://en.wikipedia.org/wiki/Electronic_filter_topology#Biquad_filter
- https://en.wikipedia.org/wiki/State_variable_filter
Now to toss in two very nice pictures from Tommi Lempinen's site:
The amplifiers are not real OpAmps, of course, but just inverters, and we have +12V instead of +9V.
Some part of the filter, including one of the FETs which work as a voltage controlled resistor:
Schematic:
Putting a voltage divider at the gate of the voltage controlled resistance FETs is a trick to reduce distortion.
Looks like the filter curve will go off with temperature. Unfortunately, there was no space on the silicon to implement something like a temperature sensor circuitry which modifies the voltage in the frequency control DAC to compensate for this, but we are getting there later in the 8580.