AM8003A SH-3A Moog style Filter

Overview  The SH-3 monophonic analog synthesizer dates from the very early days of Roland, it was launched in 1973 along with the preset SH-1000. It is a 4 octave keyboard synthesizer with a single VCO with octave dividing technology (like an organ), rather than a traditional VCO. The first version of the SH-3 has an “EMS” style diode ladder VCF, and the second version from October 1974 has a Moog style 24dB low pass filter. The design is very close to that used in the  Mini Moog and 904A.

AM8003A Module  I built this module in mid 2009 using the SH-3 Service Manual I managed to get from Roland UK. The original circuit design is followed but with some variations. The filter uses Mylar 68nF capacitors in the filter ladders, and I have matched these carefully within 1%. The ladder uses the original 2SC1000GR – although modern equivalents can be used. Dual matched transistor pairs are used at the base of the filter ladders. The module has the following front panel potentiometers: Frequency Cutoff Resonance 2 modulation CV levels 3 audio signal input levels AM8003A

Outcomes  I built the transistor ladder after completing the AM8003 VCF using almost the same PCB layout, but you’ll notice the larger 220 uF capacitor which sits at the base of the ladder and the use of 2SC1000 transistors rather than diodes. The build went well, after correcting a minor mistake on the path to the differential amplifier from the top of the ladder. You’ll notice some SIP sockets on the PCB as for this prototype I had to research what the right reduction in signal level should be before the ladder and the corresponding boost after the differential amplifier. The filter oscillates very nicely and I tried out both a log RESONANCE potentiometer as fitted to the SH-3A and a Reverse Audio pot. The Reverse Audio provided a far better control of resonance (as in the original Moog filter), and this is what has been fitted.

AM8003A Availability  This module and PCB kit will be available in 2018, this is a revised two pack PCB set.

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