Poinsettia MkII
Now in new Lo-Z Flavour!
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This circuit is an updated version of my first pedal design, the Poinsettia overdrive. Although the heart of the Poinsettia - the slew-rate limited CA3130 MOSFET op-amp combined with MOSFET soft clipping - is retained, the MkII version shown above adds a few improvements for tone-shaping versatility and setup compatibility. For example, one fourth of a TL074 (or other quad op-amp as you see fit!) handles the tilt equalizer in this version, instead of another 3130 like in the MkI. I've also adjusted the response of the EQ a little relative to the MkI, pushing the center frequency up to 457 Hz and giving it a touch more treble boost/cut (now ±3.5 dB vs. the 2.3 dB of the MkI).
The input of the effect has been buffered to play nicer with active instruments, and the entire signal chain has been converted to low-impedance for better noise performance. I've added a Blend control too, so you can mix your clean input signal with the driven signal. The inclusion of the blend also made me feel comfortable upping the gain a bit in this version. Also new in this version is the "Voice" switch, which adds a second 2N7000 (or BS170 or what have you) clipper for a MOSFET-flavoured symmetric clipping option to go with the standard asymmetric option.
Since the Poinsettia is a slew-rate limited overdrive (flowing in the vein of many BJF pedals, to which this design shares a bit of structural similarity), changing the value of the compensating capacitor in the clipping stage will modify the slew rate. Altering this capacitor changes the entire character of the drive; decreasing the value hardens up the drive tone and sharpens the treble of the tone control at all settings but flat, and increasing the value does the opposite. It's a very subtle effect but a potentially interesting one to play with nonetheless.
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Additionally, I have added a +7db output stage, as I had gotten some feedback that the MkI Poinsettia could do with a bit of an output boost. Possible mods for gain and to the EQ section described on the MkI page apply here as well, so be sure to check that out if you're building this circuit for guitar use.
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Although the tonal profile of the MkI and MkII Poinsettias is similar by design, having the slew-rate-limited 3130 in the equalizer stage adds a bit of something different to the way it sounds, so I wouldn't say that the MkII is an outright replacement for the MkI, but rather both deserve to stand alone. I would favour the MkI if you're primarily using it for guitar, and the MkII if you're primarily using it for bass. In the description of the MkI, I said that you could get some "sweet, fuzzy undertones" at max gain. The increased gain of the MkII Poinsettia takes this a step further, and in the asymmetric mode, you can actually start to cross into Velcro territory if your input signal is hot enough.
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Below is a VERIFIED stripboard/veroboard layout for the Poinsettia MkII.
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 '5-Knob Type 1" enclosure format (with a caveat - see the build document for more information). The Gerber files are also available below in case you want to take on fabrication yourself (but remember, with great power comes great responsibility!).
And here are some glamour shots of Poinsettia MkII #001. This pedal was built in a pre-drilled Tayda PedalPCB 5-Hole Type 1 enclosure in Matte Red Sand and UV printed using Tayda's UV print service.
