AM8003A SH-3A Moog style Filter


AM8003 in 2009

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
  • ENV modulation
  • LFO modulation
  • x3 audio signal input levels

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.

Over the last 10 years 2SC1000GR transistors have become rare and expensive, who knows why! The module now uses low noise BC557B transistors which have the same gain and are low noise.

AM8003A Availability  The module was due to be released in 2018 but my schedule was too busy. In 2022 the module was updated with slide potentiometers, variable keyboard input and envelope signal inversion, and reduced to 10HP width. The module has the following front panel potentiometers:

  • Input Mix (rotary)
  • Resonance (rotary)
  • Frequency Cutoff (slider)
  • KYBD depth (slider)
  • ENV modulation (slider)
  • LFO modulation (slider)

Check the webstore for availability.

 

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