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AM3044 Module - 24dB Low Pass Filter
Overview This
module is designed around the infamous SSM2044 VCF chip that appeared in a variety of analog synths (and drum machines) in the early 1980's, including the Korg PolySix, PPG Wave 2.2 and the E-mu SP1200.
A direct descendent of the
legendary SSM2040 chip, and designed by Dave Rossum and Ron Dow in 1980, the design was patented in the USA (4,404,529) and described as a low pass filter circuit, employing feedback current mirrors as dynamic resistive elements,
and characterized by high accuracy, low noise, and low distortion. Whatever the specification, its a very nice and fat - low pass filter!
AM8044 Module The AM8044 module is based around the original datasheet and an
article that appeared in Polyphony Magazine in 1982. The chip delivers a 4-pole voltage controlled low pass filter, with the added feature of the resonance being voltage controlled, but this is a reverse log response. Designed to be
used with micro processors this is not a problem in poly synths such as the Korg Poly Six. However when used in a analog module a micro processor is not really cost effective, although I did consider this option initially.
I
then tried out the Polyphony design (using an OTA), which was rather useless. The Q was voltage controllable but the response curve was dreadful. Other designs on the Internet attempt to replicate the reverse log control with Op
Amps...but I have found that the original datasheet design of a reverse 5K log pot and some resistors to define upper and lower voltage ranges works extremely well - and its cheap! So that's what I use...
The module has the following front panel controls:
There are six 3.5mm jack sockets mounted on the left hand side of the panel, these are:
Signal Inputs (x3) CV Inputs (x2) Signal Output
The filter can be built without a 1K 3600ppm Tempco resistor, but the oscillation at high Q settings will not track accurately.
Front Panel
The front panel is a 3" wide FracRac with Alpha potentiometers and 15mm mixer style push on knobs.
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