AM8050 Minisonic Diode Filter

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.

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 hopeless keyboard tracking. 35 years later and I tracked down the original magazine article and realised I could build a replica of the filter.

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.

AM8050 Availability  This module and PCB kit will be available in 2018, this is a revised two pack PCB set.

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