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, and two sampled outputs, both at different levels (x1 and X3).

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 square and triangle outputs of the clock are available on the front panel as 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.

AMSynths 928

AMSynths 928 I have replicated the original Moog circuit using THD components on two PCB’s, and in the same 8HP panel format that complements the Behringer System 55. The original Moog design uses early FET Op Amps (CA3130) which I have replaced with modern TL072’s.  The power rails have the usual Moog +12V, and -6V but also +9V for the 3130 and it can only cope with a maximum of 16V across the rails, with modern Op  Amps this is not a constraint.

There is quite a lot of circuit to fit onto both the PCB’s and its a tight but achievable in THD. There was no room for me to add a LED circuit but this may make it to production models. Prototype PCB’s were ordered in late October 2020 but caught in the Chinese holidays and didn’t arrive until November. The prototype has some errors and getting the S&H working has proved difficult, the schematics are partially hand drawn corrections.

Outcome & Availability Production modules due later in 2021.

Copyright AMSynths 2019