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.