Electronics for Building Things · Lesson 31 · Bench Skills & Your First Build
Build-Along: The Distortion+ / DOD 250 Overdrive
A real, classic circuit — the simplest famous dirt pedal — on your breadboard.
This is the concrete realization of the blueprint from Lesson 29.
We're building the MXR Distortion+ (a.k.a. the DOD 250 — they're nearly
identical): one op-amp, a gain knob, two clipping diodes, a volume knob. It's the simplest classic dirt
circuit, which makes it the perfect first real build. Every part traces to a lesson you've already done.
The one win
A working overdrive on your breadboard that you built, measured, and understand — and that you can flip
between "overdrive" and "distortion" by moving two diodes.
Parts list
Values verified against ElectroSmash's analysis
and Wampler DIY's. The
resistor/diode/pot/op-amp values are exact; the coupling/EQ capacitors are standard values that shape tone
and are yours to tweak by ear.
guitar → input cap → op-amp gain (with drive pot) → 10k → clipping diodes → output cap →
volume → amp. The diodes clip around the 4.5 V bias (so they don't conduct on the DC).
Diodes to bias = "distortion"; move them into the op-amp feedback for soft "overdrive."
Pins 1, 5, 8 → leave unconnected. Find pin 1 by the notch (Lesson 12).
Build & measure, one stage at a time
Amp volume low. Multimeter ready (Lesson 30).
After each step, take the measurement before moving on — that's how we build together: you tell me
the readings, I tell you if they're right.
Power rails & bias. Run +9 V and ground to the rails. Make the 4.5 V virtual
ground with the two 1 MΩ resistors + 1 µF cap.
Measure: rail ≈ 9 V; bias node ≈ 4.5 V. (If bias isn't ~half
the rail, fix this before anything else.)
Op-amp powered. Place the chip (mind pin 1). Pin 7 → 9 V, pin 4 → ground, pin 3 → 4.5 V bias.
Measure: pin 7 ≈ 9 V, pin 4 ≈ 0 V, pin 3 ≈ 4.5 V.
Gain stage. Wire the feedback: 1 MΩ from pin 6 back to pin 2, and the 4.7 kΩ + gain
pot + 0.047 µF from pin 2 to ground. Add the input cap + 1 MΩ input bias to pin 3 side.
Measure: pin 6 (output) DC ≈ 4.5 V with no signal. Listen: guitar in →
output cap → volume → amp should give a clean boost. (Diodes not in yet.)
Add clipping. From the op-amp output through the 10 kΩ to a "clip" node, then two
diodes back-to-back from that node to the 4.5 V bias rail (not 0 V ground — referencing
them to the bias means they clip the AC swing and don't conduct on the DC). Listen: now it's
distortion — crank the gain pot and hear it dirty up.
(The original pedal instead uses a DC-blocking cap then
diodes to ground — electrically the same for your signal. Referencing to bias is simpler on a breadboard.)
Volume & out. Output cap → 10 kΩ volume pot → output jack → amp. Done!
Overdrive vs distortion — the one-move mod
As built (diodes from the clip node to the 4.5 V bias) you've got hard clipping = distortion. To
get soft clipping = overdrive, move the two diodes so they sit across the 1 MΩ feedback
resistor (pin 6 to pin 2). Both pins rest at 4.5 V, so again no DC issue — and the voice gets
smoother. A direct, audible demo of Lesson 26.
Then mod and listen
Swap clipping diodes: silicon ↔ germanium ↔ LEDs ↔ asymmetric (two on one side, one on the other).
Change the input or gain cap to shift the bass response.
Add a simple low-pass tone control after the diodes (Lesson 27).
No sound, or wrong readings?
Don't guess — go to Lesson 32 (Debugging). The most common
culprits: bias not at 4.5 V, op-amp in backwards (pin 1), a missing ground, or a coupling cap not
actually bridging two columns. Bring me your measured voltages and we'll find it fast.
I'm your teacher — let's build this together. Work through the steps and tell me the
voltages you measure at each stage (rail, bias, pin 6). If a reading's off or there's no sound, send me
what you've got and we'll troubleshoot it live.