AM8101 – SN-101 VCF


AM8101 Module

Overview  This module is replica of the low pass VCF from the legendary Roland SH-101 which was launched in November 1982 and became one of Roland’s most popular analog synths with 8,000 sold over 5 years. In 2022 Alfa Rpar released the AS3109 in SMD, a clone of the IR3109 filter chip which is at the heart of the SH-101 filter. This new chip enabled me to restart production of the Jupiter 6 and 8 filters but also add a new module – the AM8101.

This is a warm sounding filter with smooth frequency control and a resonance that gradually increases up to full self oscillation, a sound very reminiscent of the early 1980′s. The filter is a simplified version of the Jupiter 8 implementation, with a single mode (24dB), Q compensation but no High Pass Filter.

Euro Rack Module:   10HP wide
User Manual:            Download PDF
Project Notes:           Download PDF

SH-101 VCF PCB

Detailed Description This module is an accurate reproduction of the SH-101 low pass voltage controlled filter, using an AS3109 OTA chip in the filter core. The design has been updated to use +12V and -12V power rails, as the original is designed to operate from +9V battery power. This means the audio signals are positioned as symmetrical around +4.5V and the IR3109 is connected to +9V via the positive supply pin, +4.5V via the GND pin and GND via the negative supply pin.

The change to 12V power rails is simple enough, except in the resonance feedback circuit, which now needs to pass a negative going waveform. The circuit takes the resonance feedback signal from the RESONANCE potentiometer  and inverts it using TR26 and feeds this signal into the final Op Amp to boost the overall audio level as resonance is increased (Q compensation). TR27 inverts the signal again and drives the the diode clipping circuit to stop excessive levels at self oscillation.

R131 needs to be connected to -12V rather than GND and the mystery resistor on the emitter of TR27 is R137 with a value of 68k, which also needs to go to -12V. R113 and R136 are reduced to 10k and R135 reduced to 10R. R135 and C54 are providing a conditioned positive supply to the transistors. These changes enable resonance to work correctly and it is smooth and goes into controllable self-oscillation.

The sound of the filter is shaped by the use of ceramic disk capacitors and the filter will self oscillate down below 20Hz and well beyond 20kHz. The AM8101 has a range of 10Hz to 20kHz and watch out the sine wave is 20v peak to peak. When playing the self oscillation it is wise to turn the level of the VCA that the VCF is plugged into.

There are two front panel slider potentiometers for adjusting the Cutoff Frequency and Resonance, with red LED illumination. There are rotary potentiometers for 2x control voltages (MOD and ENV) and 2x audio signal inputs (INA, INB), they have Roland TR-808 style knobs. The ENV control voltage can be inverted with a slide switch, and there is a 1V/octave keyboard input (KYBD), which is directly connected to the filter.

Potentiometers:

  • Cutoff Frequency
  • Resonance
  • Frequency Modulation (MOD, ENV)
  • Audio Inputs (INA, INB)

Connections [3.5mm jack]:

  • Audio Inputs – INA, INB
  • CV inputs – KYBD, MOD, ENV
  • Audio Output – OUTPUT

Outcomes & Availability The AM8101 was designed in early March 2022 using the new AS3109 chip and developed into a production version in April. It was released onto the market with a red aluminum panel in May 2022. Check the webstore for the module, panels, PCB’s and Hard to Find Kit. The SN-101 lettering on the panel is not a typo, Roland have trade marked SH-101 and therefore it can not be used.

 

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