AM8071 Module - "Snowfall" Diode Low Pass Filter

    Play the Demo Sample

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 was 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 unmatched diodes and high tolerance 100nF Ceramic capacitors, just like the original. It will be interesting to build one using more contemporary 1% polyester or polystyrene capacitors. 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:

• Signal Input Level (x2)
• Frequency
• Resonance
• Frequency CV Level (x2)
• Resonance CV Level
• Push Button: 18dB or 24dB
• Push Button: CV lag on or off

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), KBD and MOD
• Resonance CV Input
• Signal Output

Outcomes The prototype filter went together very well (aside from some device errors in the CAD program - you can see some kludges on the prototype PCB) and after lowering the input gain on the Op Amp buffer to divide by 50, we have unity gain across the filter and signal levels working within the core ladder without waveform limiting. The FTRIM adjusts the frequency response of the filter, so that we have full cut off at low frequencies. The filter drives into self oscillation at higher RESONANCE settings, with that lovely EMS high frequency snow fall, especially when modulated with a VCO in lower range and with the LAG switched on. The SLOPE switch steepens the cut off curve and makes a pleasant difference. These additional features certainly make a difference and are well worth adding to the core design.

Overall the filter sounds extremely good and with excellent response characteristics, a lot better than any other diode filters I have built. The THAT matched transistors arrays certainly improve things, and the modern components make for a low noise and a stable design. The ceramic capacitors keep the sound close to the original, and there no need to match them or go for higher audio quality versions. One of the best filters I have ever made!

Front Panel The front panel is a 3" wide FracRac with Alpha rotary potentiometers, and push fit mixer style knobs.

EMS AKS Filter