AM8110 – SN-101 VCO


Overview  This module is replica of the VCO 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. The re-issue of the CEM3340 in THD from both CEM and Alfa Rpar enabled me to start production of the module, alongside the VCF and VCA.

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

SH-101 VCO PCB

Detailed Description This module is an accurate reproduction of the SH-101 voltage controlled oscillator, using a CEM3340 (or AS3340) chip in the 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 in the SH-101.

The change to 12V power rails is simple enough, but I wanted to make this VCO accurate and easy to setup. The AS3340 Application Notes describe a 4-trimmer setup which is far easier to use than the traditional 2-trimmers, with the constant adjustments between them. This approach has been implemented using 20 turn trimmers, and the VCO can be quickly aligned at the factory, or when being built as a SDIY kit. Some minor changes were made to the resistor values in the application notes to ensure that the 32′ range is set at 32.3Hz for 0V keyboard input.

Precision +/-5V voltage regulators have been used, so that the 3340 chip is fed a stable -5V power, and the Range switch uses +5V with 0.1% precision resistors, so there is no need for trimmers. The precision -5V rail also stops the interaction of PW modulation and VCO frequency.

The AM8110 module stays true to the original, with both Saw and Pulse waveforms produced, which can be mixed with three types of sub oscillator waveform. The output signal level has been set at 0 to +10V, with the sub oscillator below this level (as in the SH-101) so that it does not dominate the sound. The VCO pitch range is selected using a slide switch across three octaves; 8′, 16′, and 32′. The original has an octave up/down transpose switch and 4 octaves of range switching. The module includes the first three octaves on the Range switch and then relies on the attached keyboard to transpose up or down further octaves.

The sub oscillators are based on the 4013 dual flip flop chip and are unusually generated from the sawtooth. The waveform is buffered and voltage adjusted to ensure the 4013 triggers at the correct voltage. With a 12V power supply the sawtooth is only 8V and needs a bit of amplification to get over the 8.4V high level input threshold of the 4013. This means changing the value of the resistors at R183 to 56K. The circuit will only work with Toshiba TC4013 chips.

There are two rotary potentiometers for frequency modulation control and macro Pitch, plus a small front panel trimmer for micro tuning of the VCO. There are four slide potentiometers for Pulse Width and the three waveform levels into the final audio output.

The SH-101 Pulse Width slider goes from 50% down to “minimum” (stated as 0% in the service manual, but 5% in practice). This is lower than the Behringer MS-1 which only goes down to 10%, possibly due to how the V3340 chip is engineered.  The AM8110 goes down to the correct 5% pulse width (and a bit further). A 2-way slide switch is used for pulse width source selection, either internal or external control.

Potentiometers:

  • Frequency Modulation
  • Coarse Pitch
  • Tuning
  • Pulse Width
  • Sawtooth Level
  • Pulse Level
  • Sub Level

Connections [3.5mm jack]:

  • CV inputs – KYBD, MOD, PWM
  • Audio Output – OUTPUT

Outcomes & Availability The AM8110 was designed in July 2022 with an initial prototype tested in August. The production version was successfully tested in October and released with a red aluminum panel. 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 own the trade mark SH-101 and therefore it can not be used.

 

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