928 Sample & Hold

Moog 928 Clock

Introduction In August 2020 I bought a set of Behringer System 55 modules, as my first journey into Moog modular. My plan is to recreate the Klaus Schulze IIIP synthesizer and Dual Sequencer setup that Klaus bought on 22nd December 1975, and used successfully for many years.

Moog 928 The Moog Modular synthesizers were launched in the 1960’s without a sample and hold circuit, and only when ARP popularized this circuit in the 2500 and 2600 did Moog introduce one, around 1975. Klaus Schulze used his ARP Odyssey and 2600 for S&H sounds, which were evident in early albums before his use of sequencers.

Moog 928

The Moog circuit is different to the ARP design, as it uses CMOS analog switches rather that a JFET to sample the input signal. It has two signal inputs for sampling, with the X3 input having x3 gain for low level signals like Moog VCO’s, and two identical signal outputs.

There is a precision low frequency clock that uses an OTA and matched transistor pairs, and you can either use this clock as the sampling source or switch to the V-trigger and S-trigger inputs. The pulse (V-trigger compatible) and triangle outputs of the clock are available on the front panel, and there is a simple glide circuit after the S&H.

How does it work?  The 928 has two modes; Oscillator and Trigger. In Trigger mode the oscillator is stopped by putting +3.2V into the circuit, this is switched on or off by a 4016 analog switch. In Trigger mode the external input goes through a buffer and pulse shaper. Either the clock signal or the shaped trigger signal drives two monostable multi-vibrators (4001 chips) to create specific pulse durations to drive the 4016 analog switches in the sample and hold circuit.

Keith Emerson’s S&H Module

The Keith Emerson S&H The sample and hold module fitted to Keith’s Moog Modular is significantly different from a standard Moog 928, with integration to adjacent modules and to make it more useful in a live context. The module is twice as wide as the Moog 928 and has these features:

  • Clock rate and Glide potentiometers (like the 928).
  • Mixer potentiometer – mixes the clock waveform with noise.
  • S&H outputs; Control CV and S-trigger.
  • Switch to select sampled waveform (triangle or sawtooth).
  • Switch to select the S&H output to the internal bus (CV3).
  • Switch to select the S&H S-trigger out to the internal bus (ST3).
  • Glide ON/OFF switch.
  • Switches for sampling speed selection (Keyboard, Ribbon, S&H).
  • Switch to control whether sampling is ON or OFF.

The original module does not have an internal clock or noise source, just the basic S&H circuit on stripboard. It is internally patched to an adjacent 901A/B VCO and a 903 Noise Source. I may make a clone of this unique module, using the AMSynths 6-way bus to connect the S&H module to VCO’s and VCF’s on CV3/Trigger 3.

AM928 Module

AMSynths 928 I have replicated the original Moog circuit using THD components on two PCB’s, in a wider 12HP panel format that complements the Behringer System 55. The original Moog design uses early FET Op Amps (CA3130) which I have replaced with JFET Op Amps.  The power rails have the usual Moog +12V, and -6V but also +9V for the CMOS chips.

There is quite a lot of circuit to fit onto both the PCB’s and its a tight fit, but achievable in THD with the wider panel. There was space to add a LED indication of clock speed. I built a 8HP prototype in late November 2020, and developed the design during 2021.

Outcome & Availability A production run of modules in 2022, check webstore for availability.

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