ProtoTypo PT5 – BUD666

While working on my ProtoTypo PT3 build I came to realize that actually just about any piece of Behringer audio gear would likely make a pretty neat candidate for hacking. I suppose that Behringer brands mostly as sort of entry-level hardware for the budget conscious. That is, anyone looking to get into audio production will often choose the least expensive entry point and grow up from there.

This reflects on Behringer resale value too, as people looking to “step up” want to off-load their (older generation) hardware to a market that’s constantly flooded by Behringer with the next generation gear (often carrying similar price tag with older gen). Why this is great for hacking is, it puts a ton of ‘source materials’ up for grabs at a very low price. Hacking a second-hand Behringer, say, to even see if it works usually doesn’t set you back much.. Even if you end up destroying the device in the process. Just remember to sort & recycle proper!

Take something like the 5€ DJ mixer (VMX100) that I hacked PT3 from: You’d be happy to get even half of new knobs for the same price (VMX100 has 13), add to that everything else like jacks, PSU etc! Looking at it from the electronics side, say, a basic opamp summing mixer is a basic opamp summing mixer regardless of whether it rolls off a Behringer or Focusrite production line. Using a basic functional block like this in a some other context then simply boils down to identifying it with the help of schematics or a some reverse-engineering.

Anyway TL;DR, I’m almost getting carried away so let’s cut to the chase!

Some months ago I spotted this second-hand Behringer HM300 guitar distortion pedal selling for 15€. I couldn’t help thinking “Well that’s a bargain, wonder if this would make a neat Eurorack module?”, and bought the pedal just to have a look. And so we end up with this build post!

I give you ProtoTypo PT5 aka BUD666 aka BUdget Distortion 666 :D

Why the 666? Well although I didn’t end up adding any extra features to the electronics, I still figured my hack has to be at least 2.22 times better than HM300!

A Look Inside

Best part about hacks like this is, you get to tear apart a gadget and see how it’s made. I find it very interesting to see what kind of design choices have been made to manufacture the device.

Though popping open this HM300, I was mostly distracted about the 9V battery terminal which had been somehow lodged under the foot actuator. No matter how much I tried I just couldn’t get get it loose. Totally stuck & unusable with batteries. I wonder if this was the reason why somebody had chosen to depart with the pedal..(?) If there was a battery compartment under the actuator I sure didn’t figure out how to access it!

As this is one cheapo pedal, the budget choices are visible all around. Forget about a solid metal box, or a heavy duty stomp box switch – for the latter this one had just the bog-standard micro switch and a small rubber actuator to take off some of the force when the pedal is stomped on.

HM300 circuit board (PCB) solder side.

The main circuit board (PCB) on HM300 measures about 104mm tall and 59mm wide. While the former dimension is still within the maximum suitable height for a eurorack module (< 110mm), there’s absolutely no way I’m going to build a 12hp wide (60mm / 5) panel for a simple module like this! So the board needs to go in sideways and pair up with a new, slimmer panel which will accommodate the user interface parts.

With the HM300 PCB cut loose from the pedal case, the obviously unnecessary parts had to come off:

HM300 circuit board with jacks, battery terminal and DC power jack removed..

..and followed by potentiometers.

The potentiometers didn’t have visible value markings on them when soldered to the board. The values were printed on the board, but under the parts. How convenient is that?!

The Panel

Rummaging through my parts bins, it turned out I don’t have matching panel mount versions to replace the stock potentiometers. So besides doing a new panel I also needed a PCB kludge to proceed with the stock ones (250k potentiometer = too weird a value!). These stand quite tall from the top of PCB with their knobs installed: The full distance from top of PCB to front surface of panel was around 23mm whereas the Thonkiconn jacks I chose for this build measure about 10mm tall.

So to height-match these I decided to put the jacks on separate ‘spacer’ breadboards. Also by now it was clear this isn’t going to be one of those “skiff friendly” modules. The height of everything stacking up just goes well beyond that.

Test-fitting the parts with some breadboard bits I found that they will fit a panel 4hp wide. Which means the breadboards can be 7 rows wide. And there will be three of these; one for potentiometers and two for jacks.

Test-fitting the PCBs. Input jacks to the left, output to the right.

The determining factor with the knob & jack layout was really that the stock knobs don’t have bushings (not panel mount), and so the jacks will be the only hardware to secure PCBs to the front panel. This means that (similar to my PT3) they need to be on the opposite ends of the front panel.

I marked the front panel holes for drilling by first enlarging select holes on the large breadboard to 2mm diameter, then taping the board over the aluminium panel. Unlike the stock potentiometer shaft, the plug holes of Thonkiconn jacks are slightly off-grid from the standard 100-mil (2.54mm) breadboard holes. As my guides were determined by this grid, I slightly offset the jack locations by hand when drilling. Same goes for the effect on/off indicator LED, but that can be installed with the feet slightly bent.

Panel hole marking in progress.. Black lines are where I cut the middle 3 traces.

..and the resulting front panel with 2mm guide holes drilled.

To be honest, regardless of the amount of drilling prep work I do my end results always seem to turn out a bit wonky. I do measure, mark, punch guides, start off with a smaller bit and finish off with the proper width bit. But regardless of this, some hole always ends up off-grid. Oh how I miss having a CNC router!

To assemble the panel hardware, I first tacked down potentiometers, soldered jacks & LED on their own boards and added some pins to connect the boards.

Potentiometers tacked down.

Output jack board all good for adjusting final height.

With the jack boards tacked down I could adjust their final installation height by checking that potentiometer knobs sit flush & neat with the front panel.

Test-fitting the panel. Looks like a Eurorack module already!

Knobs nice & flush with the panel.

It’s perhaps worth noting that at this point I was still very much planning to include a toggle input to switch the distortion effect on/off with external gate signals.. Hence the two input jacks at the other end of the panel. More on that in a bit.

Wiring and stuff

Not much to wiring side of things..

Wirey wirey

I decided to document the original input jack solder pins, so as not to mix up the on–board connections..:

Input jack pins


Looking at the above picture, pin 1 is GND and with plug inserted will bridge with pin 4. This is used when the pedal is run off a battery; inserting a plug will connect the GND to battery and power up the pedal. Pin 3 is for the audio signal and with no plug inserted it connects to pin 2 to ground the input.

It would’ve been actually great to use the audio input jack to turn the eurorack module on & off, but that would require a matching 3.5mm jack (with both normal open & normal closed switches). Which I didn’t have either. I’d very much like my eurorack modules to power up only as they are actually used, and to serve this purpose, a guitar pedal-ish input jack power switch would be great!

As for the power header, I soldered it on the foil side of small breadboard piece and added a 78L09 regulator which connects parallel to the stock reverse voltage protection diode. According to the HM300 user manual, the pedal draws 30mA from a external power supply and thus the smaller-sized, low-power regulator (78L version) is entirely fine.

The power header sits slightly off the breadboard surface, so that it could be soldered to place and the breadboard installed smoothly on top of HM300 PCB (header pins flush with the reverse side of breadboard).

Eurorack power header board with 78L09. I may add decoupling caps at some point.

Power header breadboard mouting points on the solder side.

Finally, about the toggle input for effect on/off which I briefly mentioned earlier. The way the HM300 is set up, the pedal micro switch seems to drive some sort of sequential bistable multivibrator / flip-flop circuit. This is to implement on/off latching as the switch itself is momentary.

The input of this circuit sits idle at about +8VDC, which the switch (when pressed) then pulls low (to ground). Effect on/off switching occurs when this happens while the high state transition is neglected. I really didn’t want to bother reverse-engineering the circuit. However just to give some kind of idea / example it might be something close to this one I found from Electronics Tutorials:

Sequential Switching Bistable Multivibrator. Schematic by

Now if this circuit would be driven by a external gate signal you would need two pulses to complete a on/off cycle, whereas for external control it would actually make more sense to have the on/off state follow high/low states on 1-to-1 ratio. To implement this kind of toggling, I figured it would be easiest to bypass the multivibrator circuit entirely and drive the subsequent circuit via input-protected opamp buffer. Panel control could be handled with a toggle switch.

So I spent some time trying to locate what exactly does the output of the multivibrator circuit control down the lane, but couldn’t figure it out. I Didn’t find any schematics for HM300 online either. By this point my build session had already turned to a long one, and I absolutely didn’t want to spend more than one day on the build. So I just replaced the input jack with a momentary push button and moved on to..

Panel Art (MEH!)

Had very little ideas here besides trying to match the deep magenta tone of HM300 and perhaps drawing a pentagram :P

I applied white paper stickers over the aluminium panel and colored them with a magenta highlighter marker. The color of this marker isn’t as deep, but the more ‘neon’ appearance looks hilariously fresh with all the other modules on my Monster Base. Must try other color highlighters in later projects!

Other than that, it’s black marker for texts and a piece of packing tape on top to protect the paper surface. About as quick as it gets!

All done!

Neon magenta looking freshhh!!

I’m still intrigued by adding the external on/off gating mod, so that’s something to perhaps revisit at a later date. CV inputs for the knobs might be fun too, but to be worth more time spent hacking this puppy really first needs to prove itself useful. Who knows, maybe I’ll even get lucky and the HM300 schematics will pop up in the net in the meanwhile. Would be a great shortcut :)

Also for eurorack use, the distortion output level is a bit low. Adding a +/-12v railed opamp gain stage or might be a nice addition too.. Though on my setup I can just crank up the input gain on the AD/DA converter.

Sound Demos


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