928 Sample & Hold

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.

The Moog circuit is quire different to the ARP FET design and it uses CMOS analog switches. It has two signal inputs for sampling, and two sampled outputs. There is the usual low frequency clock that uses an OTA and matched transistor pairs, and you can either use this as the sampling source or switch to the V-trigger and S-trigger inputs. The clock square and triangle outputs are also available on the front panel, but there is no clock rate LED.

AMSynths 928

AMSynths 928 I have replicated the circuit using THD components on two PCB’s, and in the same 8HP panel format that Behringer have used. The original uses early FET Op Amps (CA3130) which I have replaced with TL072.  The power rails have the usual Moog +12V, and -6V but also +9V, this is because the 3130 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 the rear PCB and I am going to have to use SMD components once I have the prototype running.

Copyright AMSynths 2019