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 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 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 12MB PDF magazine article (here) and realised I could build a replica of the filter. (I have also replicated the Ring Modulator based on
the SG3402 chip).
Original Circuit
The original design is a traditional diode ladder filter, a 4-pole low pass similar to the EMS VCS3 and Roland SH-3 filters. The filter is the same as in the 1973 Minisonic, which was the initially keyboardless version of J.D.Shaws design, also published in Practical Electronics. 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 using a linear rather than reverse audio pot.
AM8050 Module The replica circuit is based on the original J.D.Shaw design, with the following changes and improvements:
Input and output Op Amp buffers to match 10V signal levels
New exponential CV converter that uses positive going CV's
Dual matched transistor pair at the base of the ladder
Optional 5th pole capacitor to steepen the response curve
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 module has the following front panel controls:
Signal Input Level pots (x3)
Frequency pot
Resonance pot
CV Level pots (x2)
There are six 3.5 mm jack sockets mounted on the left hand side of the panel, these are:
Signal Inputs (x3)
CV Inputs (x2)
Signal Output
Outcomes The build went together easily and worked first time. The initial prototype has been built with 4 poles and polyester "box"
capacitors, carefully matched to 1%. Diodes have not been matched. The new CV circuit works well and with some adjustment of resistor values has the correct Frequency range across the pot. 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. I have adjusted the input
and output buffer gains to ensure the oscillation volume sets well in the overall signal level, not too low or high. That about is, the BALANCE trimmer is set to get minimal offset on the signal output. A filter with lots of gentle high
end oscillation, different to both the EMS VCS3 and Roland diode ladders. The second prototype will have the 5th pole configured and I will use some Mylar capacitors.
Front Panel
The front panel is a 3" wide FracRac with Alpha potentiometers and push fit mixer style knobs.
