Week 5 Distortion/Overdrive Pedal Build

Not having many materials, by Week 5 I decided to purchase something to construct.

After doing a bit of research on which pedal to buy, in part guided by the folks at Acidvoice.com’s “TB-303 Pedal Distortion Test“, I decided that I liked the resulting sound of the Fulltone OCD the best (near the bottom of the page). It seemed like a good-sounding circuit that didn’t rely on germanium parts to add to the tone, it does it quite well with ICs and modern transistors. It features an LP/HP switch, which stands for “High Peak/Low Peak”, as well as a “Tone” knob, an input level knob, and a distortion/drive knob.

Having bought the kit and chassis from Mammoth Electronics, I was able to not fuss around with locating parts and having them all come in separately, I could just open the box up and start soldering, however…

The kit, the website I ordered from, nor any of the materials I was sent in the box came with a proper Bill of Materials. Sure, everything was there, but no actual link between the parts and the numbers on the board (i.e. what value is C5?). After that I realized I had ordered a kit from Mammoth, but the design was somebody else’s, who preferred that everyone building his pedals had to sign in to a forum and hunt around for a proper bill of materials.

Why? Not sure. Either way, many things COULD have been avoided had Mammoth had permission to distribute their kits with a BOM. I emailed Mammoth to find out exactly why they didn’t indicate that it wasn’t going to ship with a BOM, and they basically said that the owner of the designs at GuitarPCB.com preferred to not have anyone distribute ‘his’ circuit designs. I think, more than protecting his property, he wants an ego trip. Either way…

The BOM I managed to find was, as I would later learn, for a different revision of the pedal than the one Mammoth had sold me. So, I ended up “not having” a capacitor I needed, while having a capacitor I “didn’t need”. Fortunately, so I thought, I had a capacitor lying around that was of the value the BOM asked for, so I soldered that in and switched it on.

The LED lit, but no signal was passing through the pedal when the SPDT switch was “ON”. So, I went through a very laborious and painstaking process of desoldering a small capacitor I had inserted from a fully populated board and trying out the one I was supplied. Seriously, the board has so much flux from desoldering braid on it right now that it looks like I baked snot on the board. Because of this I have purchased a desolder pump and some 99% Isopropyl alcohol for cleaning boards.

I think this goes to show that no matter what preparations you might do, how much research you’ve done in advance, there is always something that’s out of your hands you won’t think will end up fucked up, but will be fucked up somewhere down the line. Had I had a proper Bill of Materials of the version I was constructing (and there is no indication anywhere of the version I had bought, nor any links to any other than the most current version on the GuitarPCB.com forums), I would not have had to spend an hour desoldering one component from the board. I am baffled that Mammoth, nor GuitarPCB.com have received hundreds of emails a year about how they distribute their information. It’s such a daft process it doesn’t even logically work out.

Regardless, I finished the pedal. Since I wanted to use 9v batteries instead of plugging the pedal into an outlet, I didn’t attach the barrel adaptor for the power supply. What I found was that, even with the pedal off, the battery drains anyway… Who thinks that’s a good idea? Seriously. The battery is draining because the circuit (I guess) needs to be active even when off to pass the signal through it (as it doesn’t do this when the battery is removed)… Probably one of the worst design choices I’ve ever heard of. Why even include a battery clip at all if you have to expose the board and open the entire chassis everytime you stop using the pedal? I can’t believe it! I have an Ibanez digital delay pedal that does the same thing… Just pass the signal through a pole of the SPDT switch that’s connected when the pedal is turned “off”. It’s not tough, or complicated, and I would do the modifications, but all of the SPDT terminals are already occupied, so that would mean adding another switch, or buying an SPDT switch with more terminals.

So instead my solution was to simply add a switch that disconnects the circuit from the battery…

Anyway, once I had finished the circuit and everything was plugged in, I put a square wave from the Rubicon oscillator into the distortion circuit and looked at the output through the oscilloscope.

The LP/HP switch basically changed the gain of the circuit, there was a noticeable difference in the tone as well. The lower the signal was (in Low Peak mode), the distortion was less clipped and more rounded, the square wave had a tiny peak at the rising edge and a rounded fall. In High Peak mode, the distortion was louder and more pronounced, and the waveform was slightly mangled. The ‘resonant’ peak of the square wave was higher, and while the fall was rounded, there was a slight dip in the waveform.

That slight dip on the falling edge of the waveform was adjustable by the “Tone” knob, it moved the dip’s position from about 25% of the cycle to about 50% and then back again.

Overall, all of the tests turned out well, the circuit sounds nice, and I might try swapping out a different op-amp to see what that changes. I tested it with a square wave oscillator, tested it with one of my synths at line level, and took it to a friends’ house and ran it through their TB-303.

I suppose that I’ve learned that while kits in general promise a lot less fuss, it actually invites quite a lot more fuss, because less of the project is in your hands. So there’s really no way to win, other than having a huge parts collection and having a lot of time to design circuits. That or just having a lot of time to wait for parts to come in the mail that you’ve specifically ordered.

Passive components Capacitors Active components Aptly titled pedal

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