Simple Random Voltage Generator / Sample & Hold

Here's finally a nice and neat mod that brings a Simple Random Voltage Generator or a Sample & Hold to the Synthi A and VCS3.
As numerous players, I find that such a useful feature is missing on these wonderful instruments... A pity.
So I had to imagine a good solution to implement it.
I think I succeeded with this new upgrade.

It uses 2 push-pulls on both noise pots.
No extra holes needed and 100% reversible, as usual.

As long as the mod is not enabled by pulling the push-pull pot, the Synthi works exactly as unmodified.

2 wiring options are possible.

1. The "RVG" hard wiring option

Colour knob selects the trigger : osc 3 or trapezoid.
Level knob selects the output to matrix : noise or random voltage.
The pots are still active and make possible to adjust the output random voltage (more low or high CV and intervals).

With the "RVG" wiring, the noise is routed to the S&H input when the mod is on and thus not available anymore because it's disconnected from the matrix and replaced by the random voltage. The S&H is used as a Random Voltage Generator only.

To keep the noise available with the mod on and sample other sources than the noise, the "S&H" wiring option will be prefered.

Here are some audio demos (I recommend to download the ogg files) of the prototype with elaborated patches.
What you hear is only Synthi A playing live, no external device, no KS, no sequencer.
Panned left - right because recorded at the phones out of the Synthi A.

And a basic video with a basic patch, sorry for the quality (when I patch and film together, both results are so poor), but it shows how it works...
In the first part, patch is osc1 sine > output ch and RVG > osc 1 frequency.
In the second part, osc1 > filter > envelope > output channel and RVG > osc 1 frequency, filter frequency (with inverter pin), decay and osc 3 frequency.
The LEDs won't be on the offered PCB.

Wiggler dan_p uploaded this cool video of his Synthi with the RVG, thank you Dan !

2. The " S&H" patchable wiring option

The circuit's input is connected to the "meter" column and output to the "input ch. 2" row in place of the hard wiring.

Colour knob selects the trigger : osc 3 or trapezoid.
Level knob connects the S&H output to matrix row 9 ("input ch. 2") when pulled.

With this wiring the noise is still available when the mod is on and any signal can be sent into the S&H sample input via the "meter" column.
When the mod is off the S&H output is physically disconnected from the "input ch. 2" row to prevent any interference if this row is used in its former way.

If another mod already uses the "meter" column and/or if user wants to keep both "input ch." lines always available the "RVG" wiring option will be prefered.

The "S&H" wiring, used together with a KS.
As usual, the video is poor and you almost don't see the KS on the right.

0:15 S&H with noise controls osc1 frequency
0:25 S&H with osc3 triangle controls osc1 frequency
1:10 KS sequence controls osc1 frequency
2:00 S&H controls filter frequency
2:10 S&H with noise patched into the ringmod with osc1 and osc2 plays percussive sounds


User will simply have to replace the 2 original pots with this kit and run a few wires to the trigger sources and power rails.
Not exactly plug and play but it should be quite easy, even without a big experience in soldering.

The PCB offers choice between both "RVG" and "S&H" wirings and is available for both Synthi A and VCS3 pots spacing.

The upgrade kit costs 140 and includes
- populated and tested ready-to-install PCB mounted to the 2 push-pull pots
- clear and detailed installation instructions for both possible RVG and S&H wiring
- cables and cable ties for the connections
- worldwide registered shipping


No hole
Push-pull pots
Matrix MKI/MKII layout
Voltage controlled shape and response
Hi/lo range switchable oscillators

Attack and decay time extension

Documents, manuals, dopesheets, links

Filter 18/24 db
Filter slew/unslew
Oscillator synchronization
"Synthi 2" interface