AM8071 Snowfall VCF

Overview  This module is a traditional diode ladder low pass filter, similar to that in the EMS VCS3, Roland SH-3, System 100 and Practical Electronics Minisonic 2. The filter has a 4-pole ladder with an 18dB like response, thanks to the unique characteristics of diodes. The Frequency control voltages can be smoothed out using the Lag switch, to give a more fluid modulation, and this is a very useful feature for creating “bubbling” resonance.

The filter has adjustable Cutoff Frequency and Resonance, along with a resonance CV input, 2x Signal inputs and 2x Frequency Cutoff CV inputs. The panel design uses all rotary potentiometers. The filter was developed during the heavy snowfalls across the UK in February 2010, hence the name! The filter was updated to a “skiff” friendly low depth module in 2014.

EuroRack Module:   14HP wide
User Manual:          Download PDF

AM8071 Rear PCB

Detailed Description  The core of the filter is a diode ladder with modern diode networks and 100nF polypropylene capacitors, a step up from the original ceramic capacitors from the 1970’s. The input and output signals are buffered using high quality Op Amps, with a THAT300 quad transistor array at the base of the ladder for signal inputs and CV control.

A THAT340 quad transistor array is used to amplify the signals at the top of the ladder, and a high quality Op Amp circuit is used to control the cutoff frequency of the filter. Resonance (RESPONSE) is provided by a FET acting as a variable resistor, controlled by a simple potentiometer and an external control voltage (CVQ).

A high quality analog switch (DG418) on the main PCB is switched on and off from the front panel using a momentary push button. This switches in a 10uF capacitor providing a slewed filter frequency CV response which is essential for the bubbling sounds a diode filter is famous for.

Module Availability  The original module from 2010 has gone through three revisions, with the Mark 3 being the most recently available and pictured above.

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