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AM8071 Module - EMS Diode Low Pass Filter
Overview Well I
had to replicate this filter at some point, and in 2010 I have designed an updated and enhanced version of this classic filter which was used in the EMS VCS3 and AKS synthesizers in the 1970's. This is a 4-pole diode ladder low pass
filter using all discrete and rather old components. The Roland SH-3 filter (AM8003) and PE Minisonic 2 (AM8050) filters are close copies of this legendary design.
EMS originally (1970) described the filter as Low Pass and with a cutoff rate 12dB for the first octave and then
18dB per octave thereafter, with band pass filter response at medium Resonance levels and a maximum Q of 20. From early 1974 EMS updated the filter with an extra capacitor at the base of the diode ladder to provide a steeper 24dB per
octave response, this appears to have changed the character of the filter. The filter frequency response is stated as 5Hz to 10kHz.
In terms of electronic design the standard filter is 4-pole but the characteristics of diode
ladders makes this sound more like 18dB, and the 24dB version is really 5-pole. No matter, the filter sounds fantastic...
AM8071 Module I have had the schematics for the EMS filter for a few years but only after
completing the AM8003 in early 2010 did I get round to putting them in Eagle CAD and revising the original EMS components into modern equivalents. The design fits onto a 100 x 100mm PCB.
The core of the filter is as a diode
ladder with matched diodes and 100nF Mylar capacitors. The original used large ceramic disk capacitors, so it will be interesting to build one using this less than pleasant sounding capacitor. The input and output signals are buffered
using standard Op Amps, with a THAT300 quad transistor array at the base of the ladder for signal input and CV control. A THAT340 quad transistor array is used to amplify the signals at the top of the ladder, and a standard Op Amp
circuit is used to control the cutoff frequency of the filter. Resonance is provided by a FET acting as a variable resistor, controlled by a simple potentiometer. A few additions have been made to the circuit:
Voltage controlled resonance using an Op Amp buffer. 5-pole response, using an additional 100nF capacitor at the base of the ladder controlled by a front panel switch CV lag by means of a 10uF capacitor provides the original slewed filter response, controlled by a front panel switch
The module has the following front panel controls:
There are six 3.5 mm jack sockets mounted on the left hand side of the panel, these are:
Signal Inputs (x2) Frequency CV Inputs (x2) Resonance CV Input Signal Output
Front Panel The front panel is a 3" wide FracRac with Alpha rotary potentiometers, and push fit mixer style knobs.
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