AM8009 SH05 LFO’s and S&H


SH-5 Modulators

Overview  This is replica of the two LFO’s, Noise Generator, and Sample & Hold circuits in the Roland SH-5 monophonic synthesizer introduced in 1976. They provide the versatile modulation capabilities that this synthesizer is famous for, and form the left hand section of the main panel. Whilst there are more sophisticated LFO’s in later Roland designs such as the Jupiter 8, it is nice to have transistor based designs with the unique waveforms and frequency ranges they have.

Euro Rack Module:   18HP wide
User Manual:               Download PDF

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Detailed Description  The AMSynths circuit is similar to the original and based around the same JFET’s and UJT transistors, with 7504, 741 and 1458 IC’s. LFO-1 is a basic affair with just a sawtooth and inverted sawtooth waveform, whilst LFO-2 has a wider range of waveforms; triangle, square and sine (which can be delayed).  There are red LED indicators for LFO-1 rate (its the slider LED) and LFO-2 rate. The LFO’s have a frequency range from 0.1 Hz to 20 Hz.

A small daughter PCB holds the noise source with a 2SC828R transistor in avalanche mode generating 10V white, pink and red (random) noise. The transistor is the same as used in the TR808, although only a R version is required in the SH-5 and not the specially selected R-NZ. There is an error in the Roland Service Manual with C12 stated as 10nF when it needs to be 1nF to give the correct brighter white noise spectrum.

The Sample and Hold circuit is driven by its own internal clock, with the rate set by the sample time pot. I have added a green LED and driver so that the clock rate can be seen, as in the Roland SH-7.  Three waveform sources can be sampled; either LFO-1 sawtooth, LFO-2 triangle or the random noise output. This approach enables the traditional random step waveforms as well as more predictable patterns. The S& H clock is faster and goes from 0.5 Hz to 75 Hz.

The panel design uses mainly rotary controls to obtain the resistor value matching, there is one slide potentiometer for LFO-1 Rate.

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

  • External Gate input
  • LFO-1 output (saw or inverted saw)
  • LFO-2 triangle
  • LFO-2 square
  • LFO-2 inverted square
  • LFO-2 sine with a delay of up to 4 seconds
  • White or pink noise
  • Trigger output (LFO-2 or S&H)
  • Sample and Hold output

Module Outcome & Availability The PCB’s  took a long time to design and lay out, and I eventually finished a prototype set in October 2018, with a full  test of all 3 PCB’s on 24 November. Overall the circuits worked well, albeit the usual 10V outputs were reduced down by 10-15% because the power rails are 12V rather than 14V.

Maximum output levels are:

  • LFO-1 Saw and inverted saw is 7.0 V
  • LFO-2 Triangle is 7.8 V
  • LFO-2 Sine is 8.8 V
  • LFO-2 Square is 8.0 V
  • LFO-2 Inverted square is 8.0 V
  • Trigger LFO-1 is 10.4 V
  • Trigger S&H (clock) is 10.0 V
  • S&H Output is 6.4 V

Production PCB’s have a small number of revisions:

  • Added a sample clock LED and driver
  • Adjusted the sine wave shaper to work with 12V rails
  • Added a gate input and booster for 5V Euro Rack gates

Module Availability The production module and PCB sets are due to be available in January 2019.

 

Copyright AMSynths 2017