AM8200 – SH02 VCO


Description The AM8200 module is faithful replica of the voltage controlled oscillator from the legendary Roland SH-2 that was released in 1979 and proved very popular, with over 7,300 sold. The AM8200 is a fantastic sounding traditional analog VCO with that unmistakable Roland sound, in a narrow skiff friendly Eurorack module.

The original SH-2 uses a UA726 chip as a heated exponential generator. JFET Op Amps, which were finally available in the late 1970’s made the VCO design a lot easier. The JFET VCO design was used in a variety of Roland synths around 1980; including the SH-2, SH-09, Jupiter 8 and 100M. It was replaced by the CEM3340 in the Jupiter 6 and SH-101.

The SH-2 design is a slimmed down version to keep costs at a minimum, although the sine wave converter is considerably more complex and elegant. The two VCO’s have slightly different setups;

  • VCO1 – sawtooth, square, pulse noise
  • VCO2 – sawtooth, square, pulse, sine
  • VCO2 also has a sub oscillator

Eurorack Module:    10HP wide
User Manual:             AM8200 User Manual

AMSynths Design The AM8200 module follows the original design closely except for the expo generator, where a modern SSM2212 matched transistor pair is used, along with a Tempco resistor. There are a set of precision 10V power rails to improve pitch accuracy and stability.

The sawtooth, pulse and sine waveforms have been retained along with the sub oscillator, which uses an AMSynths clone (a TC4013 in SMD) of the obsolete DN819 frequency divider chip. The noise generator has been omitted, as it appears in our SH02 LFO module. An additional square waveform is available making a total of 5 waveform outputs. This provides the option to mix (and sequence) the waveforms using the AM8102 Advanced Mixer module.

The module has both coarse (PITCH) and fine (TUNE) frequency controls. The rotary PITCH control spans +/- 12  semitones, whilst the Tune slide potentiometer enables  micro tuning of +/- 100 cents. This enables multiple VCO’s to be set at thirds, fifths, sevenths and more complex chords whilst still retaining the ability to beat the notes for that analog sound.

The AM8200 module has these unique features;

  • Precision voltage rails for frequency stability
  • Five separate waveforms
  • Square in addition to Pulse
  • Trimmers to accurately set 50% square waves
  • A really nice smooth Sine waveform
  • Pitch and Tune controls.

Outcomes & Availability A prototype was tested in September 2022 and the square and pulse wave converters adjusted for 12V operation. The pulse setup proved tricky to get right (50% to 5%), but we got there in the end, by adding an additional 2M2 resistor to GND. The sine wave converter uses both the sawtooth and square waveforms as inputs, an exact 50% square wave is critical to getting a pure sine waveform. To ensure exact 50% duty cycle on both the square and pulse waveforms I have added trimmers to the design.

The waveform outputs have been set at:

  • Sawtooth = 0 to +10V
  • Square = 0 to +10V
  • Pulse = 0 to +10V
  • Sine = 5V to +5V
  • Sub = 0 to +10V

Production modules will be ready in late 2022, check the webstore for availability.

 

Copyright AMSynths 2022