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AM 4036 Module - ARP 2500 Sample & Hold
Overview This
is the Sample & Hold and Random Voltage module from the ARP 2500 Modular Synthesizer of 1970. It is the first circuit of this type that ARP every produced, and the ideas were later incorporated into the ARP 2600 and Odyssey
synthesizers.
Original Circuit
The ARP 1036 module is unusual in having three circuit blocks, all duplicated as a "dual" module. First there is a sample and hold circuit based on FET's, LM301 Op Amps and a 15nF Mylar capacitor (to store the voltage). Then there is a voltage controlled pulse generator for internal sampling. This is a transistor based relaxation oscillator, with a frequency of 0.1 to 1kHz. Being voltage controlled, the sampling rate can be externally varied.
Finally there is a white noise generator that can be feed into the sample and hold circuit as an audio signal. This creates random stepped volatges. The noise is generated from the usual analog design of a "transistor in
avalanche mode".
The sample & hold has two modes - TRIGGER and GATE. In TRIGGER mode the signal level is sampled and held at the same level until the next trigger pulse. In GATE mode the output tracks the signal input as
long as the sample pulse is positive. Once the pulse is at zero, the signal level is held until the next positive sample pulse.
The circuit has a number of nice features:
- There is a manual SAMPLE button
- The sampling pulse is available as an output (CLOCK OUT)
- The CLOCK OUT sums all sample inputs
- Sampling is a sum of the EXTERNAL INPUT, INTERNAL CLOCK and MANUAL SAMPLE
- The second S&H unit can be clocked by the first
- The RANDOM signal can be mixed with an EXTERNAL signal
AM Circuit The AM module closely follows the original, except being a single rather than dual circuit.
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