AM8044 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! SSM2044 chips are easily found as many thousand were made for Korg but the large orders never materialized as Korg made their own chip the NJM2069 and then went digital with the M1. The NJM2069 is in the AM8069.

AM8044 Module The AM8044 module is based around the original SSM2044 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:

• Signal Input Level (x3)
• Frequency, coarse and fine
• Resonance
• CV Level (x2)

There are six 3.5 mm jack sockets mounted on the left hand side of the panel, these are:

• Signal Inputs (x3)
• CV Inputs (x2)
• Signal Output

Components The filter can be built with a 1K 3300ppm Tempco resistor, if you want the oscillation at high Q settings to track accurately. The filter uses 1% polystyrene capacitors, including 10nF for 3 of the poles.
The SSM2044 chips can be found on eBay and I have a large supply if you get stuck. The hard to find 5K reverse audio pot is available from Mouser. The spline shaft pot is 313-1550F-5K

Front Panel The front panel is a 16HP wide EuroRack professionally manufactured aluminum panel of 2mm thickness with Alpha potentiometers and the usual collection of plastic 15mm mixer style push on knobs.

Korg Poly 6

AM8044