AM8040 Low Pass Filter

E-mu Systems Low Pass Filter

This module is based on a Dave Rossum filter design that never made it into the E-mu Systems Analog Modular – a SSM2040 Low Pass VCF. The stock Modular filter was a very clean transistor ladder dating from 1972, but whilst the HPF was updated to use the SSM2040, the original low pass VCF module escaped this upgrade. AMSynths have filled in some history, by taking Dave’s design from the SSM Voice Card and Eµ Systems Audity and produced a low pass 24dB VCF based on the SSM2040. The SSM2040 is famous for being at the heart of the SCI Prophet Rev01 and 02, providing the best filter ever – for some people!

The filter design is taken straight out of the SSM application notes, and is as used in the SSM Voice Card and the awesome Audity. The SSM2040 is joined by a OP177 Op Amp on CV summing duties, and a couple of OPA2134 Op Amps to bring the signal down and then back up to normal modular signal path voltages. The circuit is temperature compensated with a 3600ppm 1K PTC resistor, as the chip will oscillate quite nicely at high Q settings. The module has the following front panel controls: Signal Input Level (x3) Frequency, coarse and fine Resonance CV Level (x2) 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.

Due to the scarcity of the SSM2040 chip this module was only released as a PCB and panel for SDIY. The module was discontinued in 2016.

http://www.amsynths.co.uk/mysynths/filter-bank/am8040-e-mu-systems-low-pass-filter/