Overview & Features
The OD-67 is a faithful re-implementation of the DOD 250 / MXR Distortion+ topology — a single-stage LM308 op-amp clipping amplifier with hard-clipping diodes to ground after the op-amp output. It is one of the simplest and most iconic distortion circuits in pedal history.
What makes the OD-67 distinctive is the set of switchable modifications, drawn from Brian Wampler's well-known Premier Guitar article on improving the MXR Dist+. Each modification is broken out to its own jumper on the PCB so you can audition them individually; the most useful one (your call) can be routed to the on-board SPDT toggle for footswitching.
- BASS — extend the low-end response of the gain stage
- COMP — fatter, less spitty input character
- SOFT — add a soft-clipping stage in the feedback loop (Tube-Screamer-style)
- HCLIP — three-way selector for the hard-clip diode voicing
- SWITCH — three-pin header that routes any of the above to SW1
Reference reading: Brian Wampler, "MXR Distortion+ Mods," Premier Guitar — premierguitar.com/articles/mxr-distortion-plus-mods-1.
Circuit Theory
Signal path
Input network. Signal enters through R1 = 2M2 to ground (pull-down, sets input impedance), then through the input coupling cap C1 = 1n into the LM308's non-inverting input. R2 = 10k ties the +IN to the gain network at the inverting input, R3 = 1M ties +IN to the VB bias rail. C9 = 33n sits in parallel with C1 through the COMP jumper — closing COMP fattens the input by lowering the high-pass corner.
Gain stage. The LM308N is configured as a non-inverting amplifier. The 1M reverse-audio GAIN pot acts as a variable feedback resistor between OUT (pin 6) and the inverting input (pin 2). The gain network at the inverting input is C2 = 220n in series with R4 = 1k to ground; this sets the AC gain. C3 = 25 pF between −IN and OUT is the LM308's external frequency-compensation cap — the LM308 is uncompensated by design, and this small cap dominates the high-frequency roll-off, giving the circuit its characteristic 'dark at high gain' voicing.
The small-signal gain ceiling is large — in practice the clipping diodes set the output ceiling long before the linear gain limit is reached. Above the clipping threshold the GAIN pot controls how hard the signal slams into the diodes, not the output level.
Soft clipping (feedback loop). Two anti-parallel diode pairs sit in the feedback path, between −IN and OUT, switchable via the SOFT jumper: D8 (Ge 1N34a) + D9 (Si 1N4148) one way, D10 (Si) + D11 (Ge) the other. Closing SOFT brings these into the feedback loop — the op-amp output clips against the diode stack at roughly ±0.9 V (Vf,Ge + Vf,Si) with a soft, rounded knee characteristic of in-the-loop clipping. The Ge/Si combination is intentionally asymmetric in knee shape even though the threshold magnitudes match.
Output coupling and tone. The LM308 output is DC-blocked by C4 = 4µ7 electrolytic, then R6 = 10k feeds the post-clipping node. C5 = 2n2 from the signal node goes to the wiper of the TONE pot (1M linear), whose two end pins are both grounded — so the TONE pot acts as a variable resistor from C5 to ground. Turning the pot down shunts treble to ground through C5; turning it up reduces the shunt and brings treble back. The hard-clipping stack also lives on this node (see §03).
Volume. The 100k audio-taper VOLUME pot is a simple output divider — top to signal, wiper to output, bottom to ground.
Power supply. +9V comes in through D5 (1N4001) for reverse-polarity protection, then R7 = 100Ω + C6 = 100µF to ground form a low-pass filter that produces the VA rail at roughly 8.3 V. R8 / R9 (22k each) form a divider from VA to ground, biasing the op-amp's +IN to VB ≈ 4.2 V; C7 = 10µF bypasses VB to ground for AC.
Why the LM308 matters
The LM308 is a slow, low-bandwidth, uncompensated op-amp that requires an external compensation cap (C3 here). Its limited slew rate and the way it interacts with C3 = 25 pF give the circuit a soft, smooth high-frequency roll-off that subjectively "compresses" the top end at high gain. A TL061 or TL081 will work in this socket (pin-compatible, internally compensated — C3 still required but acts differently) and will give a brighter, more aggressive voicing. Many builders socket the IC and audition both.
Mod Jumpers & Clipping Options
Each jumper alters the voice of the pedal. Audition them with simple solder-bridges or 2-pin / 3-pin headers; once you've picked your favourite, you can route it to the on-board SW1 toggle through the SWITCH header.
2-pin jumpers — BASS, COMP, SOFT
| Jumper | Open (factory) | Closed (modded) | What changes |
|---|---|---|---|
| BASS | C2 = 220n alone | C8 = 330n in parallel with C2 | Drops the gain-stage low-frequency corner from ≈ 723 Hz to ≈ 289 Hz. Fuller, fatter bottom end into the clipping stage. Closer to a Distortion+ voicing; open is more Rat-like and mid-forward. |
| COMP | C1 = 1n alone | C9 = 33n in parallel with C1 | Drops the input high-pass corner to ≈ 473 Hz. Lets more of the signal through to the clipping stage — sounds fatter, less spitty, with more sustain. |
| SOFT | no in-loop clipping | D8/D9 + D10/D11 active in feedback | Adds soft Ge/Si clipping in the op-amp feedback loop at ≈ ±0.9 V. Smooths the top end and tames the hard clip stage that follows — Tube-Screamer-style softening on top of the Rat-style hard clipping. |
3-pin jumper — HCLIP (hard-clip diode selector)
The hard-clip stack is on the post-op-amp signal node, clamping to ground. D6 (Ge) + D1 (Si) form a fixed forward stack on the positive half-cycle, always active — clamps positive peaks at ≈ 0.9 V. The HCLIP jumper selects which diode stack (if any) clamps the negative half-cycle.
Position 1 — D2 + D3
Ge 1N34a + Si 1N4148 stack, matched to the always-on D6/D1 pair. Symmetric ±0.9 V hard clip.
Position 2 — open (centre)
No negative-half clipper. D6/D1 still clamp positive peaks → strongly asymmetric clip, tube-like, more even-order harmonics.
Position 3 — D4 + D7
BAT41 Schottky + D7 (your choice). Lower threshold than the Si/Ge stack — louder output at the same clipping level. Asymmetric.
D7 — builder's choice
This slot is intentionally open: drop in another 1N4148, a 1N34a, an LED (≈ 1.8 V threshold = louder + cleaner), or another Schottky to taste.
SWITCH header — routing a mod to SW1
The 3-pin SWITCH header connects to SW1 (an on-off-on SPDT toggle on the PCB). Wire one of the mod jumpers to the SWITCH pins to make it footswitch- or panel-toggle-able. The on-off-on type lets you go between two modded states with a centre-off in between — handy for A/B'ing two voicings on the fly.
LM308 high-frequency behaviour
With C3 = 25 pF and Rfb ≈ 1M at full GAIN, the LM308 compensation pole sits around 6366 Hz. Above this, the small-signal gain rolls off at −6 dB/octave. This is what gives high-GAIN settings their dark, compressed top-end and is core to the Rat/Dist+ sonic signature. Substituting a TL061/TL081 shifts this character — different op-amps interact differently with the same C3.
Bill of Materials
| Ref | Qty | Value | Colour code | Notes |
|---|---|---|---|---|
| Resistors | ||||
| R1 | 1 | 2M2 | Red · Red · Black | Yellow · Brown | Metal film ¼ W |
| R2 | 1 | 10K | Brown · Black · Black | Red · Brown | Metal film ¼ W |
| R3 | 1 | 1M | Brown · Black · Black | Yellow · Brown | Metal film ¼ W |
| R4 | 1 | 1k | Brown · Black · Black | Brown · Brown | Metal film ¼ W |
| R6 | 1 | 10K | Brown · Black · Black | Red · Brown | Metal film ¼ W |
| R7 | 1 | 100R | Brown · Black · Black | Black · Brown | Metal film ¼ W |
| R8 | 1 | 22K | Red · Red · Black | Red · Brown | Metal film ¼ W |
| R9 | 1 | 22K | Red · Red · Black | Red · Brown | Metal film ¼ W |
| Capacitors | ||||
| C1 | 1 | 1n | Box film, MKT/MKS or similar | |
| C2 | 1 | 220n | Box film. BOM typo Docx BOM lists 47n; schematic and prototype confirm 220n. | |
| C3 | 1 | 25p | Ceramic disc — LM308 compensation cap. Critical: keep value close. | |
| C4 | 1 | 4µ7 | Electrolytic, ≥16V — observe polarity | |
| C5 | 1 | 2n2 | Box film | |
| C6 | 1 | 100µ | Electrolytic, ≥16V — power-supply filter, observe polarity | |
| C7 | 1 | 10µ | Electrolytic, ≥16V — VB bypass, observe polarity | |
| C8 | 1 | 330n | Box film — BASS mod cap | |
| C9 | 1 | 33n | Box film — COMP mod cap | |
| Diodes | ||||
| D1, D3, D9, D10 | 4 | 1N4148 | Silicon switching, Vf ≈ 0.6 V | |
| D2, D6, D8, D11 | 4 | 1N34a | Germanium, Vf ≈ 0.3 V — observe polarity (banded end = cathode) | |
| D4 | 1 | BAT41 | Schottky, Vf ≈ 0.4 V — HCLIP voicing | |
| D5 | 1 | 1N4001 | Reverse-polarity protection on power input | |
| D7 | 1 | builder's choice | HCLIP voicing slot — see §03. 1N4148 1N34a LED BAT46 | |
| Integrated circuits | ||||
| IC1 | 1 | LM308N | DIP-8, classic ProCo Rat op-amp. TL061 TL081 for cleaner, brighter voicing. Use an IC socket. | |
| Potentiometers | ||||
| VOLUME | 1 | 100k Audio (A) | 16 mm right-angle PCB-mount. Output level. | |
| GAIN | 1 | 1M Reverse-Audio (RA) | 16 mm right-angle PCB-mount. Drive amount. | |
| TONE | 1 | 1M Linear (B) | 16 mm right-angle PCB-mount. Treble cut. | |
| Switching, headers, hardware | ||||
| SW1 | 1 | SPDT On-Off-On | PCB-mount mini toggle for the SWITCH header routing | |
| BASS, COMP, SOFT | 3 | 2-pin pin-header | Mod jumpers — see §03 for function. 2.54 mm pitch; jumper or solder-bridge. | |
| HCLIP, SWITCH | 2 | 3-pin pin-header | HCLIP = hard-clip diode selector. SWITCH = routes one of the mod jumpers to SW1 (builder choice). | |
| IC socket | 1 | DIP-8 | Recommended for IC1 — allows op-amp swaps | |
Build Guide
The board is single-sided through-hole. Standard population order — lowest-profile parts first, tallest last — keeps the work flat and predictable.
Resistors
R1–R4, R6–R9. Bend leads, insert, flip the board onto a flat surface to seat them flush, solder, and clip the leads. No orientation, no polarity.
Small-signal diodes
D1–D4, D6–D11. The banded end of the diode is the cathode — match the band against the printed mark on the silkscreen. Mistakes here are easy to make and easy to spot once you know the trick: line on the part, line on the board, same end. Leave D7 open if you haven't decided on a value yet.
IC socket
Insert the DIP-8 socket — note the notch orientation on the silkscreen. Do not insert the LM308 yet.
Power-protection diode D5
1N4001 — significantly larger body than the signal diodes. Cathode band points away from the +9V pad, toward R7.
Ceramic and film capacitors
C1, C2, C3, C5, C8, C9. Non-polarised — no orientation. Box-film caps lie flat; the small ceramic disc C3 stands on its leads.
Electrolytic capacitors
C4, C6, C7. Polarised — the long lead is positive (+), the marked side of the cap is negative (−). Match the + symbol on the silkscreen. Reverse-mounted electrolytics can fail dramatically.
Pin headers (BASS, COMP, SOFT, HCLIP, SWITCH)
Standard 2.54 mm pitch — 2-pin headers for the simple jumpers, 3-pin for HCLIP and SWITCH. If you've already decided on a mod configuration, you can omit the headers and solder bridges directly across the pads.
Toggle switch SW1
PCB-mount mini SPDT, on-off-on. Mount before the pots so its body sits below the pot heights.
Pots (VOLUME, GAIN, TONE) — backside mount
The board-mounted pots go on the other side of the PCB. Use a strip of (double-sided) tape between the pot body and the PCB to make sure the metal pot case can't short any solder pins coming through. As you solder, tack the middle pin first, then pull the pot back about 1 mm before letting the solder harden — this aligns the pot horizontally and prevents the wide pot pins from shorting against the board. Clip the small bracket off each pot before mounting if it gets in the way of the enclosure.
Insert the IC
Once all soldering is done, push the LM308 into the socket. Notch on the IC matches the notch on the socket. If you're auditioning a TL061/TL081 first, this is the time to pick one.
Switch & Wiring
Off-board wiring is straightforward — input jack, output jack, +9V, ground, and an LED with current-limit resistor if you want one. The PCB has labelled solder pads for IN, OUT, GND, +9V and the GND1 secondary ground pad (use for the jack sleeves).
SWITCH header (routing a mod to SW1)
The 3-pin SWITCH header is wired into the SPDT toggle SW1 as follows:
| SWITCH pin | SW1 lug | Role |
|---|---|---|
| 1 | One outer lug | State A — one terminal of the chosen mod jumper |
| 2 | Pole (centre) | Common — the other terminal of that jumper |
| 3 | Other outer lug | State B — connect to ground or to a different mod, builder choice |
Cut the trace of the mod jumper you want to switch (or leave the jumper open), and wire the SWITCH header in place of it. The on-off-on toggle then gives you A / off / B selection.
True-bypass footswitch (off-board)
Wire a 3PDT footswitch in the standard true-bypass arrangement — input/output to the jacks, second pole tapping the input for muting, third pole driving the LED. The PCB itself is always-on; bypass is handled externally.
Enclosure & Drilling
The OD-67 PCB fits a standard 1590B-sized enclosure. A drilling template was not included with this revision of the documentation — check your PCB outline against the inside of the 1590B and mark hole positions from the board itself (the three pots, the SPDT toggle, and the three top-mounted holes between the pots).
Setup & Usage
Power with 9 V DC, centre-negative (Boss-standard polarity). Current draw is a few mA — any standard pedal supply or a 9 V battery works.
Suggested starting points
Stock Rat / Dist+ voice
All mod jumpers OPEN, HCLIP centre (no negative-half clipper). GAIN to taste, TONE around 11 o'clock. Asymmetric, bright, mid-forward — classic Rat character.
Fat saturation
BASS + COMP closed, SOFT closed, HCLIP position 1 (symmetric Si/Ge). More like a Dist+ on steroids — fat, compressed, sustaining.
Bright lead
BASS open, COMP closed, SOFT open, HCLIP position 3 (BAT41 + D7). Cuts through a mix without losing punch.
Tube-screamer-ish
BASS closed, COMP closed, SOFT closed, HCLIP centre (asymmetric, soft-only). Smooth top, mild compression, no harsh edge.
This is a basic but very good-sounding and versatile overdrive. Have fun.
Disclaimer & Licence
PCBs purchased from TH Custom Effects are intended for DIY and non-commercial use only. Redistribution of PCBs and artwork from this document is not permitted. You may use these instructions and PCBs to build and sell your own product based on PCBs ordered from TH Custom Effects.
© TH Custom Effects 2014–2026. Build documentation V1.0.