AM 8015 Module - Utility Module
ARP 2600 Sample & Hold, VCC, Electronic Switch & Lag
Overview This is a replica of the ARP 2600 Sample & Hold(S&H), Voltage Controlled Clock(VCC), Lag and Electronic Switch (ES). The original circuits
appeared in the ARP 2600 in 1971.
The AM8015 Module For my FracRac ARP 2600 synthesizer I wanted to recreate as many of the special ARP modules as possible, as well as the standard VCO, VCF, ADSR, VCA. For these
"utility" circuits I designed two PCB's;
Voltage Controlled Clock This is a simplified version of the the VCC in the ARP 2500, which uses a Unijunction transistor and 2.2uF capacitor
to form a relaxation oscillator. I have retained this older design as it has voltage control of frequency - which the ARP 2600 version does not have. The ARP 2600 version was also simplified in later versions of the 2600, probably
to save cost.
I have also added a simple Op Amp LED driver , so you can see the clock frequency. Other than these two modifications it is the same design, with both a square wave output, and a pulse output for triggering the
Sample & Hold and the Electronic Switch.
There are two front panel potentiometers; Frequency and FM. The clock will oscillate from one period every 15 seconds to 60 Hz. There is a trimmer to adjust the frequency as at higher
values it will stall.
Sample & Hold
This S&H module provides a stepped voltage by sampling and storing the instantaneous voltage level of the signal input each time it receives a trigger pulse. The stored voltage is held until the next trigger is applied. The S&H trigger is hard wired to the VCC but the S&H can be triggered by any sharply rising waveform applied to the S&H Sample jack socket. The S&H signal input is prewired to the AM8015 Noise Source signal output. The S&H potentiometer attenuates the input signal before it is sampled. The circuit uses a modern FET Op Amp, so the dual FET transistor can be dispensed with.
Lag Processor The Lag Processor smoothes out sharp rising and falling voltages. This delay can be adjusted from 50us to 500ms by the Lag
potentiometer. The Lag Processor is used to filter or round off a low frequency signal. The input and output are available as front panel jacks.
Electronic Switch The Electronic Switch is driven by the Voltage Controlled Clock, and it takes a common signal (C) and alternatively switches it
between the A and B jack sockets. The circuit is a simple monostable which switches state on the pulse from the VCC. The original uses simple FET's to switch the audio signals.
Front Panel The front panel is a 3" wide FracRac with Alpha potentiometers and push fit 15mm mixer style knobs. The module has the following front panel controls:
S&H: Sample Level for the signal that is being sampled
VCC: Frequency and FM, and a LED to display clock rate
LAG: Amount of Lag
There are eleven 3.5 mm jack sockets mounted on the left and right hand side of the panel, these are:
VCC: FM input, Clock Output Square Wave and Pulse
S&H: Signal Input, Sample & Hold Input, Signal Output
LAG: Lag Input, Lag Output
ES : Signal In (C), Signal Out (A), Signal Out (B)