There are quite a number of modifications for the Minibrute (leafcutter john) and the Microbrute (by Mr. Usson himself and others), so I first felt there’s not anything exciting for me to add to the Arturia mono synth modding cosmos. Well, blame an offer on local ads that I could not refuse… 🙂
Schematics: http://hackabrute.yusynth.net/index_en.php
Modwiggler thread: https://modwiggler.com/forum/viewtopic.php?t=152071
Modifications
Rather than following the temptation to modularize everything inside the Micro, I focus on some select changes I find musically sensible. One line of changes is aimed at making the sound engine yet more flexible and versatile (although there are already many good things under the hood!). A second line of changes is aimed at increasing the Microbrute’s power as a CV converter/utility synth. Further down the line, there might be some experiments with the oscillator circuits, so drop by regularly as this page is bound to grow.
Let’s kick off with some waveshaper fonk:
Placing Controls
The Microbrute sporting an high function-to-space ratio, placing extra controls is actually not as simple as it may seem at first glance. The extra CV sockets on the picture at the top of this page, for instance, only fit with some additional internal modifications (the sockets need to be quite shallow unless you want to chip off a bit of the front PCB as I ended up doing). The sides and rear a are a tad easier for placement, yet you want your extra bits accessible. As I don’t really need audio in on mine, I decided to remove the corresponding socket and knob in order to have space for other controls.
Some of you might have come across pictures of hacksawed Minibrutes, where the keyboard was taken off. That’s also possible for the Microbrute, and while you’re at it, should you go down this dark path, also (don’t!) think about removing the now useless octave buttons and LEDs.
Variable ’master’ volume for VCO
I really like how the individual waveforms on the Microbrute add up, but thought it might be nice to be able to bump up or reduce the overall volume of the waveform mix while keeping the individual level ratios of sub, square, saw and tri as they are. Firstly, this is useful as the waveform mix can mush quite a bit if you turn every waveform fully up, and secondly, when you actually want to overload the filter input a wee bit, the changes this introduces to resonance behaviour are not half bad either.
The modification is easy: simply replace R76 (it’s near UB6 – check if you got the right PCB side here, see pic of UB6 just below this section!) ) with a B100K potentiometer. This way you can dial the waveform mix level from zero to double its original maximum level. Of course you need to place that potentiometer somewhere. NB ch
Buffered VCO mix output
One opamp of the waveform mixer circuit is unused, so I repurposed that as a buffer, since it was set up as a voltage repeater anyways, albeit with the positive input tied to ground. So, what you need to do is to remove the ground connection from pin5 of UB6 first. Remove UB6 and scratch the copper off of the PCB terminal for pin 5 (positive input, pulled to ground). Put UB6 back on the PCB and connect pin5 to TP30 and pin 7 to your new VCO mix output socket via a 1K resistor. If that’s too fiddly, just use an additional opamp.
Here’s an example of inserting Erica Synths’ Black Polivoks VCF (modded a bit too, of course…) between the Microbrute’s VCO and VCA. As the Erica is quite sensitive to input volume, you can hear very well how boosting the waveforms results in overdrive – delay fx applied in the end.
VCO hard sync input
This is already documented, so no repetition here, just some audio. Easy and efficient modification, this! (I’ll probably write this ten times, but how sweet is it that Arturia published the schematics – way to go!).
Square wave fix
On the Minibrute, there were some phase isssues with the square wave, resulting in phase cancellations when several waveforms are mixed together. The Microbrute has the same setup, so the same things happen here – if that’s a problem or not, as lame as this sounds, depends on what you like best. Sometimes we can just embrace happy little accidents (Bob Ross much?).
Here’s a demo of the ex factory setup – you can hear cancellations as soon as you dial in the saw in addition to the pulse wave.
This is the result of the corrected circuit:
If you want to change his behaviour on your Microbrute, you ‘simply’ need to swap two inputs of an opamp. As described in the post linked above (see the little schems at the bottom of that post), you can do this by rewiring four resistors. I tried a different take that allows you to keep all resistors in place but requires you to take out the opamp and put it back in. So, if you are confident in meddling with SMD ICs, try that one.
First you need to remove UB15 on the rear board (just above the two blue trimmers). Then you need to cut traces between UB15, PIN6 and R293-R295 (the connection between the two resistors remains intact, only the trace to UB15 is severed!); also cut trace between UB15, PIN5 and R298-R300 (same thing here – only trace to IC is cut).
Then you put the opamp back in place and solder the node R293-R295 to UB15, PIN5. And you solder the node of R298-R300 to UB15, PIN5. Swapsies!
Harness the metalizer
This wavefolder we have here (metalizer, they named it) is a beauty to my ears, and since I don’t have any extra folder/multiplier modules, I wanted this modularized, i.e. with dedicated I/O. Since the original octave buttons and LEDs on my unit could go, I used that section for an input insert, input boost switch, output socket, and timbre switch.
Input boost switch: wire a 100k resistor to an ON/ON switch and that switch across R216 (or its replacement, if you do the input insert), so that you can switch the 100k in parallel. I went for a different setup (switch between 120k and 75k attenuation) but am still experimenting a little. A potentiometer could also be an option.
Timbre switch: the folding stages in the metalizer are fourfold. Switching a resistor across the capacitors at play here changes the folding timbre. I switch a 30k resistor across C111 by means of an ON/ON switch.
Now, this is only one of many timbral possibilities here. Theoretically, you could use several switches across each wavefolder stage (C107, C106, C111, and C110 -although I find switching C110 not so super interesting). An alternative to a switch could be a 20k resistor + B100k potentiometer in series replacing the respective 120k resistors in each folder (that’s R203, R201, R225, R224) – though I didn’t test that. For starters I’m quite happy with input boost and a switch across C111.
Input insert: as per schematic, R216 (120k) is where the triangle wave enters its wave folding journey. Remove that resistor and wire the left hand terminal to the NC lug of a switched socket, and wire the tip lug of that socket via a 120k resistor to the right hand terminal where R216 used to be.
Output socket: wire TP109 via a 1k resistor to an output socket. That socket might come in handy if you want to “borrow” the metalizer for external synths. Here’s an example of MI Ripples serving as the VCF-VCA back end of a Microbrute voice (VCO output to metalizer input, then playing different waveform combos):
Increase range of tuning pot on the back
Sure, the tuning potentiometer on the back is meant for fine adjustments, yet I like a wider range, especially when operating with external CV converters. Having a wider sweep range is as easy as replacing R309 (1M, located near those blue trimmers) with a 100k resistor.
Reduce portamento time
I personally find the portamento time too long overall and like some more control in the shorter regions, so I replaced C38 (4,7uf, see pic below) with a 470nf film box capacitor.
Portamento that also works on external pitch CV
One annoying design choice in the Microbrute is that portamento only works on internal cv, which means using an external cv/gate converter requires you to use an additional glide module… I wonder who started this, but this is something I find super annoying with the mono synths of another company (whose case designs and use of ziff connectors are also not unlike Arturia‘s choices). Anyhow, as per pictures below, you can reroute the internal cv through the “pitch cv“ socket. Patching in external pitch cv, then, has two effects: first, this breaks the internal connection (i.e. only your external cv and the LFO can modulate pitch, and secondly, any cv you send in passes the portamento circuit.
NB: the way this is setup, the LFO is hardwired to pitch. If you rather want a setting where LFO-to-pitch is disconnected when plugging a jack into the LFO output socket (as is ex factory), then no worries – simply wire as per last picture above but connect the LFO wire to the NC lug (just above the one I used in the pic above) instead of the tip lug of the LFO socket.
There is a super sweet side-effect to this modification: plugging external CV bypasses the notes from the internal sequencer and keyboard while the gates from the sequencer keep running. Good for ”gated” sounds a la di-du-dunn, di-du-dunn … An expanded version of this is described in the VCA section below.
VCA signal input insert
While the internal filter of the Brute is very much to my liking, I’m a big fan of variety, so I wanted an insert at my VCA input that, together with the VCO output, can serve for using a different VCF altogether. The mod is super easy: remove R23 (100k) and wire the PCB terminals for R23 as follows: filter out (the one facing away from U7) to the NC lug of a switched socket, and the tip lug of that socket via a 100k resistor to the other terminal.
More gate/env impact on VCA
I wanted the VCA to have a bit more heft but did not like so much how things sounded when driving the audio input harder (you do that by reducing R23 from 100k to something lower). Instead, I bumped up the VCA control signal by removing R28 (100k) and putting in its place a 50k resistor and 50k pot that are wired in series. (NB on the picture above you see R28 still in place, which is not correct for the final version of this mod.)
VCA CV input socket!
Just do this super easy and helpful mod already! Its as easy as putting in a jack socket since everything else inside is already there (see hackabrute site and pic above)! Merci, Yves!
Decouple internal gate when plugging external gate
Aleyas on Modwiggler wondered if the internal sequencer could be separated from the sound engine entirely, so you can sequence another synth while still playing the Microbrute itself. An interesting idea that turned out more challenging than assumed for the gate part, but I found a way. Now, in this setup the sequencer *and keyboard* send gate and pitch to the respective outputs *only*, while the synth engine of the microbrute is driven independently per external CV/Gate setup (read sequencer *and* keyboard are decoupled).
As I did not want a bazillion extra switches for this modification, I rearranged everything so the existing CV and Gate Input sockets do the switching when you plug in a cable. The pitch CV aspect of this is easy – just do the portamento mod detailed above. When you plug external CV, the keyboard and sequencer still spit out the ‘internally‘ generated pitch to the CV output, while the synth engine pitch is played by the pitch CV input only.
Although we can do the same for the gate, the wiring is more complicated on account of a slightly more complex setup. Background: internal gate line is post CPU (of course, otherwise sequencer and midi interface would not work), but external gate input socket is pre CPU. So, in order to have this setup where we can disconnect internal gate when plugging a cable into external gate input, we need to: rewire and slightly redesign the external gate input circuit, partly disconnect and rewire the internal gate bus. Off we go!
First we need to turn the gate input socket into a switching socket. The existing socket is a switching type but its NC (“normal”) lug is tied to ground, so desolder the socket, scrape off the PCB connection to ground for that lug, and solder the socket back in.
Now, let’s rewire the internal gate bus first. Cut the trace between TP82 and the ziff (leading to front board TP56), then wire TP82 to the freshly “ungrounded” NC lug of your external gate in socket.
Now we need to remodel the external gate input circuit a bit. “As is” the circuit fires a 3,3V gate to the CPU when external gate is present, and this connection needs to go first (by removing R155, R156 and lifting the collector of Q7 from 3,3V supply). Moreover, the setup we have post CPU is sensitive to different gate levels, so we need to make sure, the resulting gate fires as hefty with an external gate of +5V as it does with the internal gate of ca. 10V. For this, we take some inspiration form the ole Roland modular envelopes and build a little NPN-PNP transistor switch. So, …
5 (without picture): Place a new PNP transistor nearby. The pnp collector is connected to TP56 of the front board (see pic below). Its base is connected via a 22K resistor to the collector of Q7. Finally, the emitter of your pnp is connected via a 10k resistor to +12V. Voila, done! Any gate now makes your Micro go boom.
Main Output Protection
In this thread here, some users reported the main outputs of their Microbrutes going zoinc: https://modwiggler.com/forum/viewtopic.php?t=145556. It appears that in the original design there is no protection between the main output socket and an opamp input, so when there’s some load on the output, this can zap your output circuit. Tony Allgood, ace as ever, suggested a couple of solutions in this thread, the easiest of which I did on my Microbrute (also because it reminds me of the ARP designs).
Simply cut the trace between UB5, PIN5 and the output socket on the rear board as shown below. Then wire the tip lug of your output socket via a 10k resistor back to UB5, PIN5 (best use some wire and go the ‘long way’ around to the other side of the PCB as using the shorter route might result in some squeezing and bulging.
Last but not least thanks to greendino for proofreading and making some suggestions!