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AM8050 Module - Minisonic 2 Diode LPF
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 Phonosincs. I was lucky enough to get this kit at Xmas 1974 as a present and I gradually built the whole syntheszier 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. 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 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 VCO's and keyboard
tracking. 35 years later and I tracked down the orginal 12MB PDF magazine article (here) and realised I could build a replica of the filter.
Original Circuit
The original design is a traditional diode ladder filter, 4-pole low pass similar to the EMS VCS3 and Roland SH-3 filters. The diodes are not matched in the original design and a 741 Op Amp is used as the ladder buffer. There is a trimmer to balance the signals from the top of the ladder into the Op Amp, and there is an exponential converter based on a BC184 and 741 to enable frequency control. Resonance is provided by the usual feedback loop.
AM8050 Module The replica circuit is exactly the same as the original, although I have added an input and output Op Amp buffer to match the signal levels to a modern module design. The ladder is powered from +9V and the
exponential generator has been modified a little to use positive going control signals rather than the negative ones used in the Minisonic. There is no temperature compensation and the filter can be built with the original 741's and
BC184 transistors, or upgraded to modern components.The module has the following front panel controls:
There are six 3.5mm jack sockets mounted on the left hand side of the panel, these are:
Signal Inputs (x3) CV Inputs (x2) Signal Output
Front Panel
The front panel is a 3" wide FracRac with Alpha potentiometers and push fit mixer style knobs.
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