AM8050 Minisonic Diode Filter

Minisonic VCF

Overview This is a 4-pole diode ladder filter from the Practical Electronics Minisonic 2 analog synthesizer. It was designed way back in 1974 and published as  a magazine project to build an analog monosynth – the Minisonic 2. Complete kits were available from a company called Phonosonics.

I was lucky enough to get this kit as a Xmas present in 1974 and I gradually built the whole synthesizer with a 4 octave keyboard and controls in a Roland SH1000 style casing. My first synth! The Minisonic 2 synthesizer proved disappointing, as the VCO’s drifted and I could not get close to the keyboard playing octaves accurately.

VCF Schematic

Some History The filter was not at all what I was expecting, as I was aiming for a Minimoog Wakeman sound rather than what turned out to be a squelchy sound like a VCS3. The envelopes were tricky and I ended up using the Minisonic as a sound effects synth (as in Silver Machine), which it did very well. Little did I know that the VCF was a 4-pole diode ladder filter very similar in design to the EMS VCS3 and AKS filter. No wonder it sounded the same as these filters, and nothing like a Moog!

I recorded a few songs with this synth but sold it in 1979 to help pay for a visit to the USA as I never managed to sort out the two VCO’s which drifted terribly and had hopeless keyboard tracking. 35 years later and I tracked down the original magazine article and realised I could build a replica of the filter.

AM8050 Prototype

Description  My replica circuit (October 2009) is based on the original J.D.Shaw design, with a series of changes and improvements, including input and output buffers, a new exponential CV converter that uses positive going CV’s, dual matched transistor pair at the base of the ladder and temperature compensation The diode ladder is still powered from +9V and the filter can be built with the original 741 Op Amp differential buffer.

The original Parts List suggested Polyester capacitors for the ladder, which I have matched to within 1%. The filter does resonate and go into self oscillation, the oscillation frequency is higher than transistor ladders, with oscillation from 2 kHz up to beyond an inaudible 15 kHz (at least for my older ears!). This is right on the original specification. A filter with lots of gentle high end oscillation, different to both the EMS VCS3 and Roland diode ladders.

Outcome & Availability  This module works really well and reminds me of my old Minisonic 2. I sold a couple of modules/PCB’s back in  2009.

2024 Update The filter has been re-engineered and released as a new module, the AM1007, to complement the existing filters in the ARP 2500. The original Shaw design has been faithfully reproduced after listening to my recordings of the Minisonic 2 in 1979.



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