Hey! You must be here because you have a HungryRat PCB you want to assemble into a module. Great! Let's do it.

The HungryRat is a 2HP Eurorack distortion module with the core of the classic Proco Rat distortion pedal. Beyond that, it's got a voltage-controllable starve system, allowing a CV to adjust the voltage/current going to the DIP op amp. You can choose any standard-pinout dual op amp for this module, and according to datasheet specifications, the TLC2262 is the available dual amp that's closest to the specifications of the original.

Supplies

Let's see what we need for this build.

the HungryRat PCB
6 2x1 Dupont-style pin headers
1 3x2 pin header
1 5x2 pin header for the Eurorack power ribbon, preferably boxed
3 Thonkiconn-style 3.5mm plugs with nuts
2 100K linear Song-Huei "tall trimmer" potentiometers with position marker
2 100K to 1M linear Song-Huei "tall trimmer" potentiometers
1 10pF to 10nF ceramic capacitor
1 2.2uF 16V electrolytic capacitor
2 4.7uF 16V electrolytic capacitors
3 3mm LEDs, any brightness or color
1 DIP-8 socket
1 op amp, any standard-pinout dual op amp, op amp selection really makes a difference with this circuit, so I encourage experimentation! My personal favorite might be the NE5532, a very standard "hi-fi" amp, not known for sounding good in distortion circuits.

You'll also need a soldering iron, solder, wire cutters, pliers, a PCB holder, and all the rest of the gear you use to build DIY electronics.

Step 1: Break Apart the PCB! Then Add the Little 2x1 Pins

Like that. File the edges to make them nice and smooth.

Some of the pin header pads are very close or even electrically connected to other pads. This is fine -- they are indeed electrically connected. Just take care not to fill in other pad holes with solder while soldering the pins in place.

Step 2: Thonkiconn Jacks and Potentiometers

Get the jacks in place and solder them.

Then, it's time to give the potentiometers some attention. See where the pliers are gripping the potentiometer? Those "ears" need to be bent "in". The idea is that the thinner part of the potentiometer's mounting legs will wrap around the PCB edges, and the bent-in-parts of the potentiometers will rest on the top of the board, preventing them from sitting too low.

Make all four potentiometers look like that.

Step 3: Getting One Potentiometer in Place

Here's some pictures showing what I'm talking about.

See how the "ears" of the potentiometer mounting tab holds it there? Then you can squeeze the tabs around the PCB and sort of flatten the tabs to the pads on the PCB where they will eventually be soldered in place. Don't solder the tabs yet, because you'll have to adjust the position of the potentiometers in a later step.

Step 4: A Note About Potentiometer Selection!!!

Here's a couple views of all four potentiometers in place. Looks good so far!

The DISTORTION potentiometer in my builds is a 100K linear pot. You might want to use a 100K logarithmic, or "audio taper" pot, which will give better control of the lower range of the distortion knob. You could even try a 10K potentiometer, which will allow a lot of control of the distortion setting, at the cost of extreme distortion. A 50K (47K) potentiometer might be a good compromise if you're looking for more control than a 100K pot can offer.

The Baxandall tone control was designed with 100K potentiometers in mind. You can most certainly use 1M potentiometers if you want, but the amount of boost and cut of the treble and bass frequencies will be dramatic indeed. With the 100K potentiometers, the output signal is limited to about 10V peak-to-peak, making this nice and Eurorack-compliant. If you choose the 1M potentiometers instead, you will be able to boost the bass and treble of the output beyond 10V peak-to-peak. This won't damage anything, Eurorack inputs (and outputs) are designed to handle any voltage available at any patch point, but you will be "coloring outside the lines" as it were.

Additionally, with the 1M potentiometers, the response of the tone control isn't entirely predictable, and the controls interact. I enjoy my module with the 1M pots, and you don't have to use the extremes of the potentiometer ranges.

Step 5: And Now We're Putting the Faceplate On!

Put the faceplate on the front of the PCB we've built so far. It will take some fiddling to get it to go on. Put the nuts on the 3.5mm jacks and tighten them. You'll notice that the potentiometers don't turn smoothly. Use pliers to wiggle, adjust, slide, tweak the positions of all four potentiometers until the shafts are not touching the faceplate holes.

Once all four potentiometers turn nice and smoothly without too much friction, and are straight and perpendicular to the faceplate, solder the pins and mounting tabs, locking everything in place.

Step 6: The Active Board! and Also, What's the Deal With Those Six Pins!??

The potentiometers and jacks mount to the passive board. It doesn't have any chips or anything on it, just passive components.

This next board is the active board! It's got smart electronics on it! Exciting!!!

Find where the 4.7uF and 2.2uF capacitors go, and install them. Pay attention to the polarity of the capacitors, although it's unlikely in this circuit that it matters. The voltages and currents involved are tiny indeed but still, mind the polarity.

Lay them over like in the picture. Then let's add the 3x2 pin header. This will select the op amp that the tone control part of this module uses.

I guess here's a good place to discuss which op amp you want to use for the tone control.

If you choose "Full Voltage", and set the three jumpers on the pins as in the diagram on the PCB, you'll be using a TL072 op amp already on the PCB for the tone control. This is fine, works great, but affects how the circuit sounds, especially when being "starved". The full-voltage tone control will boost or cut the treble and bass coming off the starved distortion circuit, but the tone control itself will never clip (unless you choose the 1M pots for the tone control.) This has the effect of the "starve" knob (or CV) not having as much effect as it turns up and down, especially in the middle-high part of the knob range.

If you choose "Starved Voltage" and set the jumpers accordingly, you'll be using the op amp that's being starved for the tone control. The tone control circuit works with higher signal voltages than the distortion core of the circuit (the original Proco Rat ran on only 9V compared to the 24V that's available for Eurorack). So the "starve" knob (or CV) will have a more gradual, constant effect on the sound in this case. If you plan to use the "starve" setting as a source of modulation, I think "Starved Voltage" may be a better choice. But in any case, you can certainly experiment and see what you like best.

Step 7: Tiny Cap, DIP8 Socket

Another choice you have to make! Don't let choice paralysis get you because it's really not a big deal, I promise.

One of the important specifications op amps have is slew-rate. Older op amps had lower slew-rates, newer or "faster" amps have higher slew-rates, and the original Proco Rat featured a rather slow op amp. This capacitor functions as a slew-rate limiter -- basically a simple low-pass filter. If you want pure Proco Rat sound out of this module, adjusting the specification of the little ceramic disc capacitor there may get you what you're looking for.

The slew-rate adjustment capacitor can be any value from omit to 10nF. I tried a 100nF capacitor, and it sounded fine, just removed a lot of the very high frequency content of the signal. A good middle-of-the-road choice might be between 47pF and 220pF.

Then hurl a socket in there! A socket for this spot will allow you to change out the op amps you might want to use to select which sound you like best. I've never had an interesting experience swapping out op amps until playing with this circuit. Modern (well, "modern", like, designed after 1980) amps are all Very Very Good, and function admirably well in almost all situations. But this module takes that op amp and sets it well outside the realm of what the designers intended. You might be using this circuit as an audio-rate modulation effect, making that op amp experience rail voltages far far beyond what the datasheet can anticipate. I did take care in designing this circuit to keep the voltages at the inputs safe compared to the fluctuating power the chip will be getting, so I can't imagine how the op amp might be degraded or damaged, but they do certainly sound different from each other.

Step 8: Power Tripping

And by "tripping" I mean "soldering the header on"

Your 2x5 Eurorack power header goes on like this, straddling the edge of the PCB. Make sure the notch in your header shroud faces the NON-PARTS side of the PCB, instead facing the WORD SIDE, if that makes sense.

Solder those pins down! We're getting close, impudent mortals.

Step 9: Active, Meet Passive, Passive, This Is Active

Take the PCB assembly you already built, the one with potentiometers and jacks, and once you find it (probably under a pile of laundry or paperwork if you're like me) slip the active PCB edge between the pins sticking up from the passive PCB.

Solder them in place. You may want to solder the middle pin set first, then do the ends, pulling the assembly together to keep the front of the module as straight as possible. Once each end is soldered in place and everything is snug and tight, solder the rest of the pins to the pads on the PCB.

Step 10: LEDs!!!

As a total afterthought, I added LEDs to this build that you can use to shine a light through the middle of the jacks. I accidentally put the silkscreen guide on the wrong side of the PCB, so you have to just remember with your brain that the square pad is pin number 1 of the LED, the positive (longer) leg. If you get a later version of this PCB, the silkscreen will be on the correct side.

Put the LEDs into the parts side of the PCB, and bend them over so the lens of the LED points vaguely toward the hole under the Thonkiconn jacks. It will be difficult or impossible to get the LEDs pointed straight out the jack, but that's okay. Also, don't give in to the temptation to stick the LEDs all the way in the hole (they will fit, I checked), since the jacks might not go in all the way if the LED's fully inserted. Oh boy that was a rough paragraph.

Step 11: Select Your Op Amp!!! Now You Have a COMPLETED MODULE!!!

You get to choose which op amp you want to use! Remember to bend the op amp legs to more easily stick them in the socket. Pay attention to which direction the chip is facing, the notch or marking is the "north" end of the chip, and it goes toward the capacitors. Getting this wrong can make op amps explode sometimes, but in this particular circuit, that won't happen because of the starve circuit which tries to emulate the limited current capacity (and lower voltage) of a 9V battery

Once the op amp is fully inserted, congratulations, you've got a HungryRat module! I hope it works for you forever and is rewarding and pleasing to your ears.