AM8040 Module - E-mu Systems Low Pass Filter

Overview 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.

I 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. It's a classic slice of sonic engineering, and it is well worthwhile putting one of Dave's most famous designs into a module. I initially built this low pass filter into a 4U high E-mu Systems Modular style panel, this is the AM2100, see the picture on the right. I then built a couple more in FracRac format, as the AM8040.

AM8040 Module 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

The filter can be built without a 1K 3600ppm Tempco resistor, but the oscillation at high Q settings will not track accurately.

Front Panel I originally built this filter into a 4U E-mu Systems style panel for my Modular replica. I have also built the filter as a 3U 3" wide FacRac module, the AM8040.

 

E-mu Systems Audity

AM2100 Module

AM8040 PCB