AM8114 SH-101 Modulator

Description The AM8114 module is replica and expansion of the Low Frequency Oscillator from the legendary Roland SH-101 that was released in 1982 and proved very popular with over 10,000 sold. The AM8114 is a versatile LFO that helps create the unmistakable Roland monosynth sound, in a narrow skiff friendly Eurorack module.

The original SH-101 is a hybrid analog and digital design. It uses a dual Op Amp to create a Square and Triangle waveform from 0.1 to 30Hz. Both these waveforms are available in the original alongside a Random waveform digitally generated by the SH-101 microprocessor, and a noise waveform from the analog noise generator.

The noise signal for modulation goes through a fixed -3dB filter with a cut off frequency of 16 kHz, this removes some of the high frequencies from the signal which are not required for modulation. The random signal is generated by the microprocessor putting a random 6-bit number into the DAC every clock cycle and then amplifying this by x2, to achieve a 0 – +10V pseudo sample and hold signal.

The SH-101 bender PCB converts the triangle waveform to a sine waveform, which is used to separately modulate the VCO and VCF at the rate of the LFO. There is short delay feature on this waveform, which is fixed at 1.5 s.  The sine wave is active when the bender lever is pushed forward. The LFO square wave is used as the clock for both the arpeggiator and sequencer.

The SH-101 is designed to use battery power and therefore has a set of power rails all or above GND (GND, +5V, +14V). The circuits have to be adjusted to work with +/-12V Eurorack power.

Eurorack Module:    10HP wide
User Manual:             AM8220 User Manual

AMSynths Design The AM8220 module retains the Op Amp LFO core, with the addition of a 3-way Range slide switch (as in the Behringer MS-1). The three frequency ranges are;

  • LOW – 0.1 Hz to 30 Hz
  • MEDIUM – 0.2 Hz to 60 Hz
  • HIGH – 0.3 Hz to 120 Hz

There are slide potentiometers (with blue LED’s) for adjusting the LFO rate, sine wave level and noise modulation level. The sine waveform is activated by a front panel push button and/or a gate signal. A front panel slide switch determines whether the LFO is free running (INTERNAL) or is synchronised to an external clock (or gate) signal (EXTERNAL).

The noise generator is a traditional design with a transistor in avalanche mode, creating white noise. This is filtered (-3dB at 16kHz) as a modulation source, and is used to create a S&H waveform using the LFO square wave. This replaces the digitally generated Random waveform of the original.

Another option for the RANDOM waveform would be to use a digital LFO to generate the S&H waveform such as the Electric Druid STOMPLFO, synchronised to the LFO square wave. However the digital LFO does not go beyond 25Hz.

A 4-way rotary switch selects the output waveform; Square, Triangle, Random, Noise. Each waveform has been normalised to a consistent +/-5V.

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

  • Clock (LFO Sync)
  • Gate (Sine wave ON/OFF)
  • Sine Output
  • Noise Output
  • LFO Output

Outcome & Availability The AM8220 LFO module started development during late August 2022, with an initial prototype being tested in October. Production modules are expected to be available in November.

Copyright AMSynths 2022