Turning the Arturia MiniBrute into a MegaBrute! BETA VERSION

Turning the Arturia MiniBrute into a MegaBrute! BETA VERSION TAKE CARE
A guide to hacking the MiniBrute. Leafcutter John January 2014
email leafcutterjohn@gmail.com with corrections and additions.
The MiniBrute (MB) is an affordable analogue mono-synth which offers several Control
Voltages (CVs) for use with modular synth systems.
As standard you get:
1. Pitch out
2. Gate out
3. Gate in
4. Amp in
5. Filter in
6. Pitch in
7. Main audio out
8. Headphone out
9. Audio in
10. Midi in
11. Midi out
This is amazing for a synth of this price but inside the MB lurk many more useful CVs!
Opening up the synth you will see there are many test points, these are all numbered and
will make the task of modifying your MB a piece of cake.
Here you can see the 2 test
points which I've soldered
thin stranded wire onto.
These points carry the +12v
& -12v power for the synth.
They are quite small but
they are very easy to solder
to and as long as you keep
the amount of bare wire at
the end of your cables down
to a minimum and you are
not an absolute soldering beginner you should have no trouble making the modifications.
USE SRANDED WIRE, solid core will not be flexible enough and you'll come unstuck!
I would recommend a decent temperature controlled iron with a fine tip for this work. Be
VERY CAREFUL not to overheat the circuit board and don't splatter solder over the board.
The surface-mount components are tiny and you don't want to cause difficult to spot
solder bridges. If you tin the test point and the stranded wire you should be able to make
all connections in 1-2 seconds (I set my iron to 350 Deg C and used leaded solder)
Working on the MegaBrute
I unscrewed the bottom plate which houses the keys. Disconnected the two connections
between keys and the smaller circuit board and put the keys/backing plate to one side.
Remove the thin metal section under the front of the keys – it's not attached by screws,
and if you try use it a carrying handle your synth will hit the deck like mine did!
Next I removed the three black screws from the back panel of the synth which frees the
smaller circuit board. The board can be lifted up which will give you more room to work on
the main board. If you intend to power up the synth like this make sure you don't short the
little circuit board to the case (it is GROUND). I put some thick tape over the back panel so
that the little circuit board could hang out and be safe (see below).
Ok on to the fun bit the patch points!
I found the following points using an oscilloscope, there will be more.....
PT numbers are marked on the MB PCB's
Power (small board middle, ground main board lower left)
PT742 +12v
PT743 -12v
PT720 / PT 605 3.3v
PT175 Ground – there are loads of ground points to choose from, try and stick with one
though.
Envelopes (large board upper left)
PT105 Filter Envelope
PT112 Volume Envelope
LFOs (large board lower middle - these all respond to tempo and are MIDI beatclockable!)
PT59 Random Steps
PT58 Smooth Steps
PT35 Sine
PT32 Triangle
PT34 Saw
PT33 Square
Oscillators (large board mid right - respond to pitch)
PT13 Square
PT158 Sub (square or sine as selected on MB panel)
PT22 Triangle (+metaliser if activated)
PT19 Triangle (clean)
PT20 Saw (the modulating wave for this is PT65)
PT57 Noise
PT151 some kind of folded triangle ( |\/|/\| )
Clocks (large board bottom left)
PT136 Clock out
PT145 Clock in (see special tap emulation circuit)
Modulation (small board bottom right)
PT656 After Touch
PT655 Mod Wheel
PT654 Pitch Bend
These are 0-3.3v and need a little care to use as outputs because they MAY connect
directly to the main CPU, you don't want to kill that now do you? Read on for a solution.
Making Connections
For all output CV's apart from the Modulation CV's and the Clock in I buffered the signals
from the MB using non-inverting buffers (TL074 - you get 4 op-amps per chip) the circuit is
very easy and looks like this. You need one buffer for each signal you want to send out. It's
possible to not use the buffers but there is a chance that it'll affect the operation of the
MB depending on what you plug into. I would not try and use the envelope outputs
without buffers, it'll make your MB quieter and we don't want that! Don't forget to power
the TL074's from the MB +12v & -12v patch points.
For the Clock in connection you are simulating pressing the tap-tempo button which
shorts the switch. You can do it using a square wave from another modular synth like this.
I've not fully tested the modulation outputs yet but I'd be inclined to be very cautious with
them. There is a chance that if you accidentally get voltage into these test points you
could fry your main processor. Using a diode clamp will probably make it safer. I'll update
this soon. Here is an image of a diode clamp I pulled of the internet – I'll replace it in the
next update of this document. Green square is the modulation patch point on the MB.
VCC should be the 3.3v connection on the MB and Ground is ground on the MB. Instead
of a push button send the output to a non-inverting buffer then out to modular land!
Here is what the insides of my MB looked like after my mods.
The wiring is not super tidy but the soldering is good and that counts! The beige protoboard in the top left holds 4 x TL07's for buffering outputs and the transitor circuit for the
clock control mod. It's bolted to the MB's smaller board using a short standoff.
I cut a hole in the side cheek of the MB to
allow me to connect a break-out box
I ran all the CV's and the MB's ground
connection down a ribbon cable that
connects to a break-out box made of
aluminium
Hope you enjoy your MEGABRUTE!