Another Dirt Pedal

Chuck D. Bones

Well-known member
"You can't have too many Dirt Pedals."
This one is loosely based on the Krank Distortus Maximus. The DM is basically an LPB driving an LM386 with a Marshall TMB stack at the end. The LM386 is largely responsible for the sound. The DM is a great sounding pedal in stock form, but I saw an opportunity for some improvements. The 1st stage is now a Sziklai pair, using a JFET & a Si PNP. It has a higher input impedance and more gain. I changed the way the GAIN control is implemented for easier adjustment and lower noise. I added a bass cut switch (FAT) up front and a treble cut switch (BRITE) between stages. I stuck a 3-way gain switch (CRUSH) on the LM386. Lastly, I rescaled the impedances in the TMB stack to take full advantage of the LM386's drive capabilities and switched it to a Fender-style tone stack to increase the available Bass boost. The higher input impedance and lower output impedance enables this pedal to stack well with other pedals.

Biggus Dickus v2.4 sch.png

I'm digging the sound. With CRUSH at max and the FAT and BRITE switches off, it sounds like a DM. Chords hold together pretty well. Turn the FAT switch on and it's a BEAST. Either way, there's plenty of sustain and it's great for feedback. Has a great chunk sound when I dig in with the pick. Lots of bass available at low to moderate GAIN settings. Bumping up C3 and C8 will extend the bass response at higher GAIN settings. There is a provision for driving a back-to-back pair of red LEDs for the magic-eye effect, that's the red wire at the bottom that goes nowhere. Tried 'em out on the breadboard, but have not yet installed them on the boxed-up unit.

As a nod to Krank, I retained a Roman-style name, but changed it to one of the characters in Monty Python's Life of Brian: Biggus Dickus.

Monty Python - Life of Brian - Biggus Dickus.jpg

Controls are (L-R):
Top row: Volume, Crush, Gain
Middle Row: Brite, Fat
Bottom Row: Treble, Mid, Bass

Biggus Dickus front 02.jpg

It's not nearly as pretty inside as one of HamishR's boxes. There's some clear heat-shrink on the power jack terminals and LED leads.

Biggus Dickus innards 02.jpg

Here's the fit check, before wiring it all up. Those three switches were a pretty tight squeeze.

Biggus Dickus fit check 02.jpg

Here's the board with enough wires for an aliveness check. I managed to not muck anything up this time and it worked right out of the chute. The final verision has a JRC NJM386 in place of the TI LM386.

Biggus Dickus board 02.jpg


Well-known member
Shoulda called it a lottus going onnus

Always like reading and bookmarking your posts

I think most would agree you're the Don Corleone of knowledge round these parts the Modfather

And although I appreciate you sharing your knowledge with us lieutenants there ain't no way I'm kissing your ring

Thankyou though capo di tutti i capi

I've seriously learnt a lot from yourself and many others since hanging around here waiting to get found out and flamed

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Chuck D. Bones

Well-known member
Here's my layout. Red X's represent cuts. IC1-8 is not connected on the board, but is connected inside the IC. That's why there is a cut under that pin. Use my board pic (above) as an assembly guide. R2 & C16 aren't used. R13 & C15 are for the Magic Eye LEDs: a back-to-back pair of red super-brights with the other end connected to ground. You can omit R13 & C15 if you don't hook up those two LEDs. GND is 2 columns wide above IC1; bend one of the leads on C6 over on the solder side to bridge the two columns. Polarized caps have the line at the - side. C7 & C17 are a snug fit, mine kinda sit on top of the parts next to them. C14 can be 100uF or larger; I ended up using 220uF. Size Rled for the desired brightness. Q1 can be any JFET with Vp around -1V; pinout is D-S-G. Pots and offboard wiring are per the schematic. There are five GROUND connections, use as many as you like.

Biggus Dickus v2.4 vero (top view).png

Chuck D. Bones

Well-known member
Yes, you can use the same values for Marshall TMB. I encourage you to use Duncan's Tone Stack Calculator to see the effect of changing component values and the difference between Fender & Marshall.