Sunday, 10 April 2016

VXP1 Voyager Output Expander for Eurorack

(BLOG UPDATED 21st July 2016)
Two years ago I designed and built a Eurorack interface for my Moog Voyager. I built it in a week to take to the Brighton Modular meet to see if there was any interest in such a module. I have a VX-351 which is perfectly good but I wanted something that had 3.5mm minijacks to make it easier to use with my Eurorack and Roland 100m modulars. It was a simple breakout panel that had one jack per signal available on the Voyager's Accessory Port (a 25 pin D connector). Most signals were wired straight through but one or two included active electronics to provide scaling to give a more useful 5V range, in particular Keyboard Pitch had a scaling amplifier and a trimmer to set it exactly to 1V/octave. LEDs with driver circuits were also added to the LFO and the two gate signals.

I called the prototype CVGT2, which in hindsight was a mistake as I use cvgt or CVGT in my Eurorack to Buchla interfacing products; e.g. CVGT1, cvgtMM, cvgtElements. Anyway the production version has been renamed to VXP1.

Most modular users don't have a Voyager (and sadly it's now out of production) so I didn't get too much interest at the Brighton meet but I did get some interest on Twitter and Muffwiggler and in particular from one wiggler whose frequent enquiries about its development has spurred me on to develop it. It has been very slow going due a number of reasons (not all connected to the module itself).

Output Adaptor Retrofit

The first reason for procrastination was due to an error in the design of the Voyager's Accessory Port which was not suitably buffered; which means that when a cable is connected to the port some of the signals become unstable and oscillate. Moog's remedy for this was to use a small adaptor board that has to be retrofitted inside the Voyager between the main circuit board and the Accessory Port's ribbon cable (my understanding is this just affects backlit types, not the Old School version).

The Output Adaptor is a small circuit board that adds a 330 Ohm resistor in series with many of the signals; here is a link to an installation guide. The 330 Ohm resistance is enough to isolate the capacitance of the extra cable length which maintains amplifier stability (ED Note: If you want to understand about amplifier stability then this series of 5 short videos by Analogue Devices is the perfect intro, in particular video 4 which describes this problem nicely. Ideal for anyone designing with op-amps).

As you can see from the installation guide that fitting the adaptor is a bit much for some people to take on and I didn't want to encourage inexperienced Voyager owners to open up their expensive synth and potentially damage it or harm themselves (there is an AC mains hazard on the left if you don't disconnect from the mains). Unfortunately there is no other solution than to use an Output Adaptor (like that supplied with the VX-351) and fit it if you are confident or get a music shop tech to fit it for you. You might already have one fitted if your Voyager has been used with the Moog VX-351. I tried to find a source of Moog Output Adaptors but alas no joy so I designed one from scratch; problem solved! Here it is:-

Smaller, Better

The second reason for delay is I increased the scope of what the VXP1 can do; I didn't want this to be a Eurorack rip-off of the VX-351. Rather than just a simple breakout panel it now handles each signal with active electronics as follows:-
  1. All signals are filtered, buffered and have the necessary gain and drive to give more Eurorack-friendly output levels.
  2. The smooth and step Sample and Hold outputs are replaced by just one output and a variable control to give an adjustable slew to Sample and Hold.
  3. There are more LFO waveforms: ramp, saw and 5V clock.
  4. There are more noise options: white, red and random pulses.
  5. The panel is narrower than the first prototype: 16HP down to 14HP.
Here is the second prototype panel I have used during my development (a simple but mechanically accurate panel with engraved text made by Schaeffer):-

Here is the panel design now in production:-



Current Status

March 15th 2016
All the circuit design, PCB layout and mechanical drawings are complete. I will be checking, double checking for the next week before I commit to manufacture.

March 22nd 2016
Checking is complete and I found a few errors; well worth switching your mind off and coming back to it a few days later! The PCBs and the panel are now ready to get quotes and place orders.

April 9th 2016
The PCBs are now on order; expected delivery 22nd April.

May 26th 2016
The first set of PCBs have been built and tested and, in the main, work fine. I need to tailor the slew pot to optimise the adjustment range to give better control around the more pleasing portamento effect but that is minor. There are two problems: The first is I cannot get a quote out of my panel supplier to make front panels - they seem mega busy with far bigger orders - I'm trying elsewhere reluctantly. The second is the need for an output adaptor board fitted inside the Voyager (as the VX-351 has for backlit Voyagers). My alternative approach was not successful. I'm now designing my own output adaptor board.

July 21st 2016
Nearly in production. This will be just a small quantity initially, 25-30, to see how it is received. I now have panels, PCBs and components in stock, just waiting on the assembly company to return from summer shutdown around 8th August. The Output Adaptor is designed and I have PCBs and components ready to build. Six foot long 25-way D cables are in stock. I have also devised a modification for Old School Voyagers to make use of the redundant TOUCH sockets - they become a 3-input gate combiner. These VXP1-OS Old School front panels are now in stock; busy times!

VXP1 Module, Cable and Adaptor     |     VXP1-OS Front Panel


ATTENTION: The first and most important thing that I need to get across here is that the VXP1 has been designed for the keyboard versions of the Moog Voyager. The Old School version has the same Accessory Port but does not have a touchpad so therefore does not support the Touch X, Y, A and Gate outputs. The RME (Rack Mount Edition) has an Output Accessory Port and does support the VX-351 and is probably OK BUT I have not been able to verify the extent to which it is compatible as there is virtually no information about the RME Output Accesory Port's signal set. Logic suggests it is very similar but I cannot yet guarantee how well the VXP1 will work with the RME. I will try and find out (maybe borrow an RME for a weekend - any offers?).

TOUCH Outputs (X, Y, AREA, GATE)

This function produces TOUCH X, Y, AREA CV and GATE outputs in response to touching the Voyager's touchpad. X and Y vary according to where the pad is touched, AREA varies according to how much area on the pad is being pressed. The GATE turns on when the pad is touched.

X, Y and AREA are CV outputs that have an effective range of ±5V.

GATE is a 0V off and 10V on signal. The GATE LED illuminates when the pad is touched.

NOTE: The Touch outputs are not supported by the Old School Voyager; they will remain at zero volts. Consequently for the Old School the TOUCH X, Y and AREA outputs are turned into diode connected inputs and the TOUCH GATE output is the combiner output. This acts as a logic OR gate. This would be ordered as a VXP1-OS (this comes with a cable but no Output Adaptor as it is not needed).


This function produces KEYBOARD PITCH CV, VEL CV (velocity), PRESS CV (pressure) and GATE outputs in response to playing the keyboard. PITCH CV is determined by which key is pressed, VEL by how fast a key is pressed and PRESS by how hard a key is pressed. The GATE turns on when a key is pressed.

PITCH is a CV output that is trimmable to 1V/octave (using the screwdriver adjustment on the front panel) and has a nominal range of -0.916V (low F) to +2.667V (high C).

VEL and PRESS are CV outputs that have an effective range of ±5V.

GATE is a 0V off and 10V on signal. The GATE LED illuminates when a key is pressed.


This function produces WHEEL PITCH and MOD CV outputs in response to operating the Pitch and Mod wheels. The Pitch wheel is centre sprung to give 0V when not operated. The Mod wheel is not sprung and can be left in any position. Normally when the Mod wheel is fully rotated towards the player no modulation occurs (in the Voyager). However this position equates to -5V output. A jumper can be set on the VXP1 circuit board to provide either the standard ±5V range or a 0V-10V range. A 0V-10V range will allow it to operate a modular VCA to control modulation depth for example.

PITCH and MOD are CV outputs that have an effective range of ±5V but MOD is jumper selectable to have an effective range of 0V to +10V.


Saturday, 13 June 2015

New Euro to Buchla CV Translator - CV1.2

The CV1.2, one of the cvgtElements range of DIY circuits, is available and its data pack is ready to view/download on the support page. CV1.2 is ready built and tested, you have to DIY them into your own modules/panels.

CV1.2 takes a Euro 1V/octave CV and translates it into a Buchla 1.2V/octave CV - ideal if you want to take a Euro pitch CV into the Music Easel's pitch KEYBOARD CONNECTION

The GTPulse datapack is also available to view/download on the support page. All modules are actually available - it's just the support info that's lagging behind but should be done very soon.

Pricing for the cvgtElements range is as follows:-

The pulsers (GTPulse, PulseGT) are 21.60GBP each, the scalers (CV1.2, CV.83) are 24.00GBP each and the chicBus is 9.60GBP. Prices include VAT but if you are outside the UK or EU then the VAT free prices are 18GBP, 20GBP and 8GBP respectively. Shipping is extra. 

Please enquire at

Friday, 22 May 2015

Synovatron Documents Downloads

EDIT 31May 2015:

For those who have tried to download from links printed on kits, fliers and data sheets etc without success: I apologise but the original file hosting site has shut down without warning in recent weeks. All Synovatron docs have now been rehosted onto Google Docs where they can be viewed or downloaded. The new links are shown on the Support tab above where all downloadable documents will be maintained from now on - if you experience any problems with file downloads please email me at Keep an eye out for new and updated documents. Any additions or significant updates will be announced on the blog as they occur.

The documents added to Google Docs are:-

CV Tools User Manual 
CVGT1 User Manual 
cvgtMM User Manual 
GTPulse Data Pack 
ASM2DB Adaptor Fitting Instructions & uZeus Tech Note 
DM2ASB Adaptor Fitting Instructions 
5ASM2DB Adaptor/Expander Bare Board Data Sheet and Fitting Instructions

Saturday, 17 January 2015

ASM2DB Tech Note for use with uZeus ribbon cable bus

Here is a guide if you are using the uZeus ribbon cable backplane.

(apologies I didn't have a real uZeus backplane to hand but hopefully you get the idea)

Saturday, 8 November 2014

Custom Half-width Buchla Modules

I was asked to design and build a pair of custom half-width Buchla modules for some clients. They each had two half-width slots spare in their custom built cases and wanted to fill the spaces with Euro to Buchla CV and gate translator modules.

The brief was simple (and the same for both clients): 5 channels of 1V/oct to 1.2V/oct CV translation and 5 channels of gate boosting to Buchla levels; the Buchla stepped pulse was not required, just big pulses (10V or more).

This was a great opportunity to use the Synovatron Proto range of prototyping boards which accomodate 5 channels and offer stackability if more circuitry is required than will fit on one board; see below.

The Proto range of boards
As the Proto boards are designed primarily for Euro there was space on the 4U Buchla front panels to add some extras in the form of a passive attenuator. Here are the front panel designs that we agreed on:-

CV and Gate modules
To keep costs down it was decided not to have panel marking as the functions are obvious.

The CV module design used a straightforward precision amplifier with gain of 1.2000 (yes that accurate), 1M Ohm input and 0 Ohm output impedances. The gate module used comparators and output and LED drivers. The outputs each have diodes in series allowing them to be wire-ORed by just connecting outputs together (common to Buchla, wish it was used on Euro).

The CV module electronics fitted nicely on a ProtoJax board with plenty of space as you can see:-

For the gate module I decided to put the input comparators and sockets on the ProtoJax board and output buffers and LEDs on the ProtoXp board and stack them together (it  probably could all have been put on one board but it was simpler to make use of the space). The stacked ProtoJax and ProtoXp boards can be seen here:-

The gate board was powered from an MTA connector and the CV board took its power from the gate module (hence the 10-pin ribbon headers for a linking ribbon cable); this was at the client's request as they only had one spare connector (these are very low current modules and daisy chaining them didn't cause any crosstalk issues). 

The finished panels looked pretty good; they were made by Schaeffer using Front Panel Designer

If you are interested in the Proto range of PCBs or in having a custom module built then please contact me at

Saturday, 27 September 2014

Synth Repairs

Analogue Systems RS-40 Noise, Sample and Hold, Osc

This came in as 'defective'. After a while it became obvious the noise output was not working. This was traced to an open-circuit LEVEL contol pot (10k lin) which was replaced. Still it didn't work as a sample and hold - i.e. I was expecting to see a stepped output from S\H OUT. Not having used the RS-40 before it took me a while to realise that the noise and clock signals are not normalled to the Sample and Hold circuit like the front panel markings suggest; the lines linking NOISE OUT to EXT SRC IN and INT CK OUT to EXT CK IN suggest to me these are linked in some way but no. 
After looking at the jacks it was clear that these are not normalled (I'm not sure if this is the case with all RS-40s but this one looked clean and unmodified so I'm assuming it was factory standard). I used external patch leads and to link the noise and clock outs and ins and hey presto it worked. The first thing that I saw on the scope was at each sample point the sampled signal is present for what I guess is the duration of the sample pulse (about 0.2ms). When using the RS-40 to modulate a VCO I could not tell that the noise spikes were a problem, in fact it is probably above hearing range (at least mine). 
Noise spikes

However I didn't much like it and I didn't much like not having the noise and clock not normalled to the S/H circuit. I spoke to the owner and he agreed to have a couple of simple mods applied.

The noise and clock signals are present on the jack board close to where they would have to go for normalled connections. The switched contacts were connected to ground on both EXT SRC IN and EXT CK IN jacks by solder bridges. I removed the solder bridges and fitted links as can be seen here. This worked a treat allowing it to work without patching but still allowing external signals to be patched in if need be.

Solder removed from red arrows and links applied at yellow arrows
The noise spikes 'problem' although not audible was fixed by placing a 10nF cap across the S/H output amplifier's feedback resistor (27k) making it a low-pass filter with a cutoff of 590Hz. This got rid of the spikes without adding noticeable slew.

10nF capacitor

Spike free steps

Saturday, 13 September 2014

5 into 1 Analogue Systems to Doepfer Bus Adaptor/Expander

The new 5ASM2DB Analogue Systems Module to Doepfer A-100 Bus Adaptor/Expander is now in stock and available to order. 

SchneidersLaden have stock of 5ASM2DB which can be ordered from their website at

For DIY enthusiasts it is also available as a kit or as a bare PCB direct from Synovatron (please email 

The 5ASM2DB offers five Analogue Systems sockets on one small circuit board with low-current LED indicators to show that ±12V and +5V power is available. It comes with a ribbon cable to connect to the A-100 bus and four self-adhesive nylon PCB mounting pillars (you can of course use screwed in pillars so a drilling template will be provided).

If you have several Analogue Systems modules in a Euro rack then the 5ASM2DB could be a better alternative to using several ASM2DB Adaptors because it expands the bus and frees up A-100 Bus connectors for Euro modules; it costs roughly the same as three ASM2DB Adaptors. ASM2DB and DM2ASB Adaptors are still available.

5ASM2DB Adaptor/Expander
What you get with the 5ASM2DB Adaptor/Expander
What you get if you order the 5ASM2DB DIY Kit

Price info and ordering (2018 prices)

For sales, support and combined shipping enquiries please contact 

The base price is without delivery and VAT.
The UK and EU prices include delivery and VAT @ 20%
The ROW price only includes delivery; VAT is not applicable. 
Model                                        Base            UK              EU                ROW
5ASM2DB Adaptor/Expander    25.00 GBP   33.00 GBP   34.80 GBP   29.50 GBP
5ASM2DB DIY Kit                      16.50 GBP   22.80 GBP   24.60 GBP   21.00 GBP
5ASM2DB Bare PCB                     6.50 GBP   10.80 GBP   12.60 GBP   11.00 GBP
UK delivery by Royal Mail First Class; elsewhere by Royal Mail International Standard.