AM8071 Snowfall MK2 VCF

AM8071 Snowfall Diode Filer (Mark 2)

This module is a traditional diode ladder low pass filter similar to that in the EMS VCS3, Roland SH-3, System 100 and the PE Minisonic 2. The filter has a 4-pole ladder with an 18dB like response, thanks to the characteristics of diodes. The Frequency control voltages can be smoothed out using the LAG momentary push button switch, to give a more fluid modulation.

The filter has adjustable Cutoff Frequency and Resonance, along with a resonance CV input, 2x Signal inputs and 2x Frequency Cutoff CV inputs. The Mark II panel design is all rotary potentiometers, which reduces the price. The filter was developed during the heavy snowfalls across the UK in February 2010, hence the name.

The core of the filter is a diode ladder with unmatched diodes and 100nF capacitors, just like the originals from the 1970's. 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.

Voltage controlled resonance is available with an external CV input (CVQ). A 10uF capacitor can be switched into the Frequency CV circuit using the Blue push button switch on the front panel. This provides a slewed filter frequency CV response which is essential for the bubbling sounds a diode filter is famous for. A blue front panel LED (in a lens) is illuminated to show when this feature is activated.

The module has the following front panel controls:
Signal Input Level (x2)
Frequency
Resonance
Frequency CV Level (x2)
Resonance CV Level
LAG - Push Button: CV lag on or off

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