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.
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 three times the 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 (must be HEF4001 chips) to create specific pulse durations to drive the 4016 analog switches in the sample and hold circuit.
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.
AMSynths AM928 Module In 2021 I replicated the original Moog circuit using THD components on two PCB’s, in a 12HP panel format that complements the Behringer System 55. The original Moog design uses early FET Op Amps (CA3130) which I have replaced with modern JFET Op Amps. The power rails have the usual Moog +12V, and -6V but also +9V for the CMOS chips.
Outcome & Availability The prototype circuit worked (although it requires specific CMOS chips – HEF4001BE), and the module was in production during 2021/2022. An updated SMD component version of the module, was developed in the autumn of 2024. The updated module has a new etched aluminum panel with silver edges and lettering, just like the Moog original. Visit our webstore to buy one.
