AM8071 Snowfall VCF

AM8071 Diode ladder low pass filter (Mark 1)

Summary
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. A fifth pole can be switched in to give a steeper 24dB response using the Slope switch. The control voltages can be smoothed out using the Lag 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 panel design is a mix of rotary and slide potentiometers. The filter was developed during the heavy snowfalls across the UK in February 2010, hence the name.

Detailed Description
The core of the filter is a diode ladder with unmatched diodes and high tolerance 100nF Ceramic 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. Two high quality analog switches on the main PCB are switched on and off from the front panel using momentary push buttons. These switch in a 5-pole response using an additional 100nF capacitor at the base of the ladder and CV lag by means of a 10uF capacitor provides a slewed filter frequency CV response which is essential for the bubbling sounds a diode filter is famous for.

The module has the following front panel controls:
Signal Input Level (x2)
Frequency
Resonance
Frequency CV Level (x2)
Resonance CV Level
SLOPE - Push Button: 18dB or 24dB
LAG - Push Button: CV lag on or off

https://amsynths.co.uk/home/products/filters/am8071-snowfall-vcf/