Analog / Audio — Personal Build — In Progress
Resonance Guitar Pedal
A build log for a modular analog pedal: a motherboard with swappable circuit cards, posted as it happens rather than written after the fact.
FIG. 0 — Motherboard schematic (KiCad). Input buffer + reference-voltage bias on the left, eight card-slot headers feeding a summing mixer, wet-blend control into the output stage, LED status indicator top left.
A motherboard, not a single circuit
Instead of one fixed signal path, the whole thing is built as a modular platform. The motherboard has eight 10-pin card slots, each one carrying +9V, a buffered VREF bias, an EXCITE_BUS line, and its own signal line back to the mixer. Every slot takes a swappable analog card, whether that's a filter, a resonance stage, or whatever I decide to build next, without touching the motherboard itself.
On the motherboard side, an input buffer and VREF divider set up a clean single-supply reference. All eight card signals get summed through 100k mixing resistors into a summing amp, and a wet-blend pot mixes that back into an output buffer before the jack. There's an LED off the reference stage too, just a simple power/status light.
Building the motherboard's core stages
Right now I'm breadboarding the motherboard side of the schematic: input buffer, VREF bias network, and the summing mixer, one stage at a time before wiring in the card slots. Testing each op-amp stage on its own has made it a lot easier to trace weird behavior back to the actual stage causing it instead of debugging the whole board at once.
The wet-blend stage has been the trickiest part so far. Too little and the mixed signal barely moves, too much and the summing amp starts to clip. Still tuning that one before it's worth plugging in a real card.
Designing the first plug-in card
While the motherboard's on the breadboard, the first card is done at the schematic and PCB level. I'm calling it the Notecard. It's a resonant bandpass filter: two summing/integrator stages built around a shared tuning node, with a pair of trim pots (RV_TUNE1, RV_TUNE2) setting the center frequency and a Q-limiting resistor keeping the feedback loop just short of runaway self-oscillation. The filtered signal comes back out as WET_OUT, straight into the motherboard's mixer bus.
It plugs in over the same 10-pin interface the motherboard exposes at every card slot: GND, +9V, VREF, EXCITE_IN, WET_OUT, plus pot wiper/ends and a couple of spare pins for whatever the next card needs. The board itself is tiny, 1.1" × 1.855", since it has to fit edge-to-edge with seven others.
Getting it off the breadboard
Once the circuit's stable, the plan is to fit it into something a guitarist can actually stomp on: panel-mount jacks, a footswitch, and pots positioned where a hand naturally lands. I'll model the enclosure in CAD around the final board layout before building anything.
Playing it, not just measuring it
Once there's an enclosed build, the real test is plugging into an amp: running through different guitars and playing styles to see where the resonance control feels musical and where it's just a dead zone. Bench testing on the breadboard can only tell me so much.
Where this stands right now
Motherboard's still on the breadboard, dialing in the buffer and mixer stages. The first card, the resonant filter Notecard, is designed and routed, so once the motherboard's stable it's next up to get fabricated and plugged in. I'll keep updating this as the build moves forward, so check back for progress or come back later for the finished writeup.