Welcome to the AMSynths Blog

I post articles with text, photos and videos about the amazing AMSynths analog synthesizer modules.
You can buy them at www.amsynths.co.uk

Saturday, 18 September 2010

Korg Poly 6 VCF Replica

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 in 1984. But these large orders never materialized as Korg made their own OTA 4-pole filter chip the NJM2069 and then went digital with the M1.

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...

AMSynths Panel
I built this module around 2006 using a hand made PCB panel sprayed with blue car aryclic paint and using SafMat for lettering. However in September 2010 this module has been fitted to the new AMSynths professionally manufactured alumnium panel, which you can see in the photograph. The panel is EuroRack sized and fully compatible with many other EuroRack modular synthesizer systems such as Doepfer.

More Information
Visit the AM8044 web page on my web site for more information.
A video will be uploaded soon. I plan to manufacture a short run of 10 of these modules in 2011 as the AMSynths AM8044, so please drop me an email if you would like one reserved.