via
this auction"EMS Dual Random Voltage Generator, perfect to add to your Synthi or VCS3. Also interfaces with other modular/cv synths. Fully working and in original wooden case. It comes with mini-Bulgin power lead and very rare Cinch-Jones to facilities cable for connecting with a Synthi/VCS3 down a single lead. The module is internally switchable between 110V and 220V.
from the original spec sheet -
TWO VOLTAGE RANDOM GENERATOR
EMS STUDIO MODULES
The EMS Range of Ancillary Studio Modules EMS Modules are designed not only for use with Synthi synthesisers but with any complex of voltage controlled equipment, of whatever manufacture. They are supplied in handsome afrormosia cases, but can be removed and mounted in standard 19" racks, occupying only 1.7" (44mm) of vertical space. They contain their own mains power unit with a very wide range of stabilisation to cope with poor mains line conditions, being operable at anything from +10% to more than—23% of nominal line voltage. Connections to and from the modules can be made either from the jack sockets on the front panel or via the multi-way socket at the back.
TWO VOLTAGE RANDOM GENERATOR
The EMS Random Voltage Generator produces a staircase type of output, the 'steps' of which occur randomly, but whose voltage range and time variance are controllable in the following ways:
Time: The internal clock which decides the mean rate of events can be (a) freerunning and subject only to manual control, (b) inhibited so that the next random voltage depends on a manual push button or an external pulse (equivalent to external clock control); (c) free-running but voltage controlled so that an external voltage influences the mean rate. In this state the device becomes a voltage/time converter.
If the time variance control is at zero, the mean rate is constant (though subject to change of steady rate if influenced by a control voltage). As the time variance control is tuned clockwise, the mean time random variance becomes greater, and when this control is at maximum the rate will randomly vary in a ratio of more than 100:1. The distribution of chances is rectangular, not Gaussian, so that no part of the variance range is particularly favoured.
Voltage: Two voltage outputs are available and separately adjustable, but both are subject to the same time control — i.e. the steps of the staircase occur at the same time from both outputs. At zero, there is no voltage variance at all, and as the setting is increased the variance about the mean voltage (set at the device being controlled) becomes greater, excursions being both positive and negative of the mean voltage. In addition, a trigger pulse is sent whenever a random selection is made, and a lamp on the panel glows at the same time.
The random voltage generator can be used for quasi-melodic purposes, for interesting timbre and other parametric changes, or at high speeds for randomly varying vibrato or tremolo (very near simulation of natural, performed vibrato), or for producing very dense textures of rapidly changing notes. A very versatile instrument, essential for any well-equipped electronic music studio.
SPECIFICATION
Power Supply:
240 or 115VAC 50/60Hz + 10% to —23%
Control Outputs:
+/- 2.5V max. (this excursion can be offset to accommodate non-EMS products).
Trigger Output:
+4V
Mean Time Range:
0.2 - 20 events per second.
Time Variance:
Max. approx. 100:1 (at mid-settings of mean rate — limited by possible clock range).
Select Input and Inhibit/Free Run Switch:
+ 5V at select input or manual push button operates and inhibits after one event if switch at INHIBIT. +5V will run clock at mean rate set while present at select input, if switch is to FREE RUN.
Voltage Controlled Mean Input:
0.5V/octave ( = doubling or halving rate) limited by possible clock range."
Doepfer has some nice images of their modules up. It's a bit hit and miss at the moment, but if you check their site you will find some in the individual module description pages along with their standard drawings.
Pictured: A-127 VC Triple Resonance Filter and A-128 Fixed Filter Bank 
