This 1U rack unit is rather more interesting than a traditional noise source or even a “sample & hold” circuit. The schematics are available for the EMS Synthi 100, and it is possible to work out what the circuit does. An interior screen shot of the PCB is available (see below) which is a VCS3 style “card” PCB – as used in the Synthi 100. It has an edge connector on one end and a FACILTIES patch connector on the rear to enable connection to the VCS3/AKS.
How it works At the core of the module are three sample & hold circuits (called ramp and hold) that use an internal clock and noise source to create two sets of random voltage levels. The twist is that the third S&H adjusts the frequency of the clock, which means both the height and length of the first S&H are randomly varied.
The clock rate is adjusted by a front panel potentiometer (MEAN RATE) from 0.2Hz to 20Hz, and the step length randomness varied by the VARIANCE potentiometer – this controls the amount of control the second S&H has on the clock rate. There are two signal outputs (V1 and V2) each which its own LEVEL control.
A trigger output is driven from the signal output, rising to +4V when a step starts and lighting a front panel lamp (NEW SELECTION).
The module has two modes selected by a toggle switch:
- INHIBIT – disconnects the internal clock, with manual stepping from the RESELECT push button or +5V on the SELECT input.
- FREE RUN – connects the internal clock to a voltage plugged into the V.CONT MEAN input (0.5V/octave).
Connections & Controls There are four pots on the front panel, two have blue caps (MEAN RATE and VARIANCE) and two have silver caps (for output voltage adjustment). There is a toggle switch to engage the internal LFO (INHIBIT/FREE RUN), and a push button selector for manual selection (RE-SELECT). A red LED indicates when a new random voltage is selected (NEW SELECTION).
There are five 6.35mm jack sockets for connecting the module; three are outputs (V1, V2, TRIG) and two are inputs, V.CONT MEAN which controls the mean rate of the internal LFO and SELECT which disengages the internal LFO and applies the SELECT signal.
Recreating the RVG AJH Synths have recreated the random noise generator in Eurorack as the RVG, but re-engineered the sample and holds with JFET op amps to reduce voltage droop. They have also added V1 and V2 voltage offsets and mapped the design to all SMD parts, except for the UJT.
I have decided to recreate the Random Noise Generator as per the original 1U wooded rack, using THD components through out. A Meanwell power supply supplies the +/-12 V power and a FACILITIES socket has been added to the rear of the unit.
