During the exceptionally snowy weeks of early 2010 I built a redesigned version of this classic filter which was used in the EMS VCS3 and AKS synthesizers in the 1970’s. With snow falling and the wonderful bubbly sound of the filter, I named this module - Snow Fall. It 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 (in 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....
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The Snow Fall has a CV input for controlling Q as well as push button switches to provide Lag on the Frequency CV input and 4 or 5 poles. The prototype has been built as a FracRac module with a PCB front panel and SafMat lettering. Later this year I will have a 2 colour aluminium panel made in my workshop.
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