Poinsettia MkI
This is the properly-tuned iteration of the very first circuit I ever designed, way back in 2014. At the time, I was frustrated with the selection of bass overdrives available on the market, finding them to generally have way too much gain on tap - making them inarticulate or unimpressive, or otherwise turning one's tone into a muddy disaster/incoherent wall of bass. What I was looking for was a low-gain, well-controlled drive where:
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The gain range is useful and musical at all points (light grit at low settings and no mud at high settings)
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The tone control doesn't have useless settings (no blankets on the amp or ice-picking)
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The clipping characteristic is smooth and pleasant at all times.
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Since I had a difficult time finding that in a DIY-friendly package, I decided to make one myself. After a lot of breadboarding, the result is the Poinsettia MkI that you see above!
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At the core of the Poinsettia is a CA3130 MOSFET op-amp accompanied by asymmetric MOSFET soft-clipping courtesy of a single 2N7000 in the feedback loop. I set the minimum gain of the op-amp at a slight breakup (via the 15K resistor), however the builder may choose to modify this or omit it as they see fit. The Drive pot could be substituted for an A, B or C250K for more gain; you could even drop the Drive pot to 50K and substitute a bigger series resistor if you want more gain at the minimum setting. Following the op-amp stage is an active tilt equalizer built around another CA3130. I tuned the response to have an asymmetric sweep, offering ±6 dB in the bass and ±2.3 dB in the treble pivoting around a center frequency of 322 Hz. (The EQ is flat at the center of the pot's rotation.)
This yields an extremely dynamic and touch-sensitive overdrive (with apologies for the tired, clichéd description), responding differently to your attack. At gain ranges 0-30%, you get a little breakup when played fingerstyle, a bit more so with a pick. 30-60% gets you a nice bark when played fingerstyle with just a hint of sweet fuzzy undertones, and a solid clip with a pick. 60-100% is a sustaining drive that stays soft with fingerstyle playing and gaining a bit of edge with a pick. It remains articulate at the maximum setting, but still has enough push to achieve some hard-rock type tones. Dialed back just a bit lets you get just about as technical as you want without sullying note definition.
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Despite my limited guitar skills, I can say that this circuit does a great job with guitar as well - it's full of smooth, full-bodied lead tones, and works well as a rhythm drive as well. Guitarists may choose to raise the pivot frequency of the equalizer - this can be done by swapping out the 8.2 nF capacitors; for example, 5.6 nF moves it up to 472 Hz, 4.7 nF up to 563 Hz, and 2.7 nF up to 980 Hz, and so on. Choose what sounds best to you, or put a few options on switches if you want the flexibility. (Hint: using 33K for R5/R7 and 2.2nF for C4/C5 gives ±6 dB centered at 1085 Hz, which I've found works great for guitar!)
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Below is a VERIFIED stripboard/veroboard layout for the Poinsettia MkI, including topological improvements suggested to me by TagboardFX user 2liveis2die (thanks!)
If you're so inclined, you can purchase PCBs for this project at OSH Park, which have the added bonus of including Chuck D. Bones' "Celestial Bypass" soft-latch system onboard! Tayda Electronics is kind enough to offer enclosures that are pre-drilled to accept boards from PedalPCB, and to make things as convenient as possible for all manner of DIY builders, I have laid out my boards to fit inside these as well. This board uses the '3-Knob Type 1" enclosure format. The Gerber files are also available below in case you want to take on fabrication yourself (but remember, with great power comes great responsibility!).
When I first set out to "formalize" this project, I designed boards to fit in a 1590B-sized enclosure, but moved to the PedalPCB standard format later for convenience. As such, Poinsettias #001-003 are 1590B-sized, and just for fun, here are some glamour shots of Poinsettia MkI #001! Thanks to Scott from Obscura Manufacturing for fabricating the top-class enclosure. The lovely colour is Prismatic Powders PMB-6905 Illusion Cherry - photos really don't do it justice!
