AM 8003A Module - Roland SH-3A 24dB LPF

Project Status: Potential EuroRack and FracRac module - email if interested.

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. I may replicate the VCO later on, as it is quite unusual - providing a form of additive synthesis. The first version of the SH-3 has an "EMS" style diode ladder VCF, and the second version (contrary to popular myth) has a Moog style 24dB low pass filter. Okay, so on to the reasons to build one:

• The SH-3 VCF is a 4-pole 24dB diode transistor ladder, similar to the EMS VCS3. It also appeared in the SH-1000, and the filters in the SH-5 and System 100 are similar - but they use transistors as diodes in the ladder.
   
• The SH-3A VCF that replaced the diode version is a traditional Moog transistor ladder filter with a 24dB response. The design is very close to that used in the Minimoog and 904A. Once again this is a unique Roland design, only seen again in the Roland SH-2000.

AM8003A Module The original filters are quite simple and basic designs. Roland used a variety of early Op Amps, including the low quality 741 and 1458. The VCF in the SH3A is a direct copy of the Moog low pass ladder filter, so the AM8003A is very similar.

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 signla 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 orioginal Moog filter), and this is what has been fitted. You can see in the photo that I have yet to fit this pot onto the PCB.

Front Panel The front panel is 16HP wide and the standard 128.5 mm EuroRack height. It is machined out of 2mm thick aluminium (same as Doepfer modules) and it has the standard EuroRack mouting holes. The panel has been manufactured in Germany and has Black lettering.