Boss MD2 Mega Distortion MODS

[Gila_Crisis]

Just found one of these for very cheap so I gave it a go, also to understand why everybody talks bad about this one.
Actually I like it, it has its own caracter and voicing (kind of a woody tone). But I understand why people may complain about the fizziness. Strange noboy yet opened a thread about this one, so here we go!

Here's what I did to mine, to remove the fix and add a bit more overall body to the pedal:
C9 47n
C10 2n7
C7 3n3
R21 47k
R73 Jumper
C40 10n
R49 1k8
C18 1uF

Note that C7, C18 and R49 are void pads in the pedal!

Other possible mods which I didn't try are (or I did only part of these):
- More Distortion
R37 1k
C19 470nF
C7 1k
R22 1K
R23 10k

- Overdrive Tones
R37 1k
R71 1k
R21 470k
C7 470pF
R30 47k
R75 2x 1N4148 clipping diodes in parallel, opposite direction
R73 Jumper
C40 8nF

- More Mids
R61 Install 4k7 trim pot to tune the knob frequency
C9 4n7
C7 4n7
R21 47k

Circuit Analysis
Ignoring all the switching, the signal enters through c9 into a 'discrete' opamp, which is
everything from Q11 — the 1uf cap before R21. R37 and c19 are the negative feedback portions
of this clipping circuit.
From there it goes into several standard opamp clipping circuits. The first is a non-inverting, and
then directly into an inverting opamp.
From there it goes into a ‘mu-amp‘ type circuit...sort of a transistor equivalent to a power tube
circuit in an amp. After this it goes into a buffer (q23), the signal is divided down via r69 and r75,
and r73 and c40 form a low pass filter of sorts, attempting to remove some of the fizziness.
After this, the signal goes through a basic eq circuit — an active bass boost and treble cut filter.

[stolen]

Hi,

thanks for sharing! This pedal and circuit flew under our radar so far, but it's quite an interesting mix, and cheap to get too! We might get one some day and mod around ourselves a little!

One thing we'd like to mention is that Q6/Q17 are not a mu-amp, but simply two standard common emitter amplifiers mixed together with fairly negligible electronic interaction - these would probably be responsible for quite some fizz. R13/R42 could be increased to reduce the gain at this stage, or a big muff style ac-coupled diode feedback could be employed, maybe just on one side! Lifting one of the mixing resistors would result in a more asymmetric clipping, a mix pot with different configurations for each side could add a good amount of sonic variety.

Exciting circuit :3

[Gila_Crisis]

Hi!

Thank you for the reply and hint!

Yes I was also amazed by this circuit, it is not a common design, and actually there are many good sounds in here!
I'm also start to think to take the boost part of it and connect it to a switch to that you can move it pre or post the main distortion circuit (as I have feeling this is what was about in a so called "Weed" mod a Japanese guy did to these, plus he wired also a switch to turn the overdrive part on-off).

[Gila_Crisis]

I did few more fine tuning to the pedal.
First as suggested by Stolen, I swapped R13 and R42 with 220 ohm resistors. It helps a lot! you have a bit less gain (well... if you max both Dist and Boost knob you still have tons of gain), but also less floor noise!
Added a 560pF capacitor (ala big muff) between collector and base of Q23 to tame a little bit the high end.
Then in the Bottom control I tuned to gyrator so that is centered around 80-90Hz (I thing I borrow always from the HM2), which means C100 150nF and C30 82nF.

Then:
R69 10k
C40 10n
C19 470n

I must admit with the mods you can fine tune this pedal a lot, and it's a very good pedal and interesting design. Odd that until now it received so less attention.

[stolen]

Glad we could provide some useful input!

I must admit with the mods you can fine tune this pedal a lot, and it's a very good pedal and interesting design. Odd that until now it received so less attention.

Yeah, same! It's really cool under the hood! We think we'll get one too next time we aren't broke, we're kinda hooked now . We can't think of any other mass produced circuit featuring a parallel distortion stage like this!

[Gila_Crisis]

I did some more tweaks (and now I think I'm done for sure): restored C19 back to 220n to have a more tight gain boost section (need more bass? crank the bottom/low knob!), and removed the 560p as the sounds was a bit too muffled.
What I did new was jumper R77 (it's on the pots board): this way with Dist on zero you have even less if none distortion and you can use the Gain Boost only to have some overdrive tones!

[stolen]

(and now I think I'm done for sure)

We couldn't help but stare at the circuit again (we think we'd bypass IC2 entirely) and there's an interesting thing we overlooked before: The antiparallel bjt distortion section is actually starved! By removing D6 and replacing it with a resistor (or a potentiometer with the other half replacing R48) you could make the supply voltage for that section adjustable - how does this circuit have so much cool stuff to play with and nobody is talking about it again?

[Gila_Crisis]

I tried to remove D6, just to see what happnes: The sound gets more sharp, as the circuit gets more volts in those spots. But in the end I soldered D& back, as I liked it better starved.

[Hump]

Is there anything that can be done to help the volume drop with the pedal engaged after the modifications? It loses a lot of volume.

[stolen]

Hi!

Not sure which mods you did but R69/R75 are a good place to start. You could just lift R75 and see if that's enough/too much. Since that changes signal impedance you might wanna take that 10k resistor and put it in parallel with R69 so that the frequency response doesn't change.

Good luck!

[Hump]

Thanks, I’ll try that.
I did the WAMPLER mods to this one.