AM8003 SH03 Diode Low Pass Filter

Roland SH3

Overview  This is replica of the diode filter in the Roland SH-3 mono synth. The original  filter was quite a simple and basic design copied from the EMS VCS3. Roland used a variety of early Op Amps, including the low quality 741 and 1458, and a very old 14-pin dual Op Amp – the NEC UPC33C complete with heat sink. This chip was usually found in early Japanese stereo amplifiers such as the 1974 Trio KT9007.

The SH-3 diode ladder is essential a copy of the EMS diode ladder right down to the FET controlled resonance circuit. Whilst the ladder buffer and CV control circuits are slightly different, the only other addition is a 5th capacitor below the first pair of diodes which introduces another pole to the filter. and a steeper cutoff response

EuroRack Module:   14 HP wide
User Manual:           
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Project Notes:           
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Panel Dimensions:   
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AM8003 Module

Detailed Description  I first built a prototype in 2010 using a standard Op Amp to replace the UPC33C control voltage circuit. The filter did not initially self oscillate but after some modification it was oscillating in a very smooth and subtle way with a 10K linear RESONANCE pot providing just the right feel.

The production version was developed in the winter of 2017 and uses a dual PCB skiff friendly design with the filter on the rear PCB and the pots and jacks on the front PCB. The circuit uses the original 1S2473 diodes, Mylar ladder capacitors, all carefully matched. Matched transistor pairs are used at the base of the ladder with the AS394 replacing the ITS1276. We may offer a “vintage” version with the NEC UPC33C…

There are five 3.5 mm jack sockets on the base of the front panel:

  • 2x Audio inputs
  • 2x Frequency control voltages
  • Audio output

Module Availability  The AM8003 Euro Rack module was launched in August 2018, along with PCB’s and a Hard To Find component kit for SDIY. The module is in production, check the webstore for availability.

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