AMSynths Minisonik

Overview  Back in 1974 G.D.Shaw designed the Practical Electronics Minisonic 2 analog synthesizer. It was published as a magazine project to build, and 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 the keyboard and controls in a Roland SH3 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. 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 with hopeless keyboard tracking. An experience shared by many UK DIY synth enthusiasts!

Original Design The Minisonic 2 came with a 4 octave keyboard, two VCO’s, a diode filter, two basic AR envelope generators, two VCA’s with manual output panning. There was no LFO but VCO2 could be set as a drone, and there was a ring modulator. Listening to the music I wrote with the synth in 1979, you can hear the low drones and the VCS3 style filter.

AM8050 Prototype

Minisonik Replica I toyed with the idea of a replica for some years and built a copy of the diode filter as the AM8050. I was put off the idea of a full replica, as the VCA’s use rare 6-pin MFC6040 chips which are hard to find and expensive. There is no LFO, the two envelopes are AR and  only a sawtooth waveform from each VCO.

On the positive side there is lots of modulation routing via over 20x latching push button switches, the VCO’s can be set to drone and detached from the keyboard, there is bi-directional variable cross mod, variable oscillator sync, and VCO1 can FM modulate the filter as a LFO. There is also a ring modulator, are two outputs with level and panning.

Then in 2021 I found by chance, an 8-pin DIL version of the VCA chip; it was cheap and available as NOS. I knew I could sort out the tracking and stability of the VCO’s with an amended exponential converter design, so the project was on!

Minisonic 2 Routing

Features The Minisonik synth is based on the original design, with a some subtle changes and improvements:

  • New exponential converters in the VCO’s and VCF
  • Dual matched transistor pairs in the expo converters
  • Temperature compensation in the VCO’s.
  • Revised noise source using a transistor rather than a diode
  • Removed the +/-6V power supply
  • Retained the +/-9V power using LDO regulators
  • LED indicator for power ON/OFF
  • Tune control but no Span or Portamento
  • Eurorack Pitch and Gate inputs
  • External input to Ring Mod and added one to the VCF.
  • Color code push button caps
  • 60 HP black anodised panel

Minisonik Panel

Development The synth was laid out across two PCB’s, Panel and Main, in Autumn 2021. The front panel design was done first, so I could determine where the pots and switches would be. I have kept to the original left to right layout with the VCF, ES/VCA’s lined up vertically. The pan controls and envelope levels use 9mm trimmers, so I could fit the synth into 60 HP, with large knobs for the main controls.

The noise circuit is a traditional transistor design (ARP, Oberheim) using the 2SC828 which is particularly good at generating a wide noise spectrum (as used in the Roland TR808). The SG3402 based ring modulator circuit has been retained, as it has a slightly different internal schematic than the easier to find MC1496. The headphone feature has been omitted, which means we don’t need the rare MFC4000B chips. The Eurorack standard +5V gate signal is converter to -9V for the envelope shapers using a comparator chip and a transistor inverter.

Outcome  The two PCB’s were ordered in July 2022 and gradually populated over the next 3 months.

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