Boss MD2 Mega Distortion MODS

A forum devoted to mod, tips and suggestions for upgrading and rehousing your VERY CHEAP commercial stompbox to near boutique excellence.
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Gila_Crisis
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Post by 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.
Attachments
Boss_MD-2.png
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stolen
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Post by 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

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Gila_Crisis
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Post by 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).
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Post by 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.
Last edited by Gila_Crisis on 25 Mar 2021, 08:16, edited 1 time in total.
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Post by stolen »

Glad we could provide some useful input!
Gila_Crisis wrote: 21 Mar 2021, 12:54 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 :D. We can't think of any other mass produced circuit featuring a parallel distortion stage like this!

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Gila_Crisis
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Post by 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! [smilie=a_jester.gif]
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Post by stolen »

Gila_Crisis wrote: 21 Mar 2021, 16:18 (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?

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Post by 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.
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Post by 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.

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Post by 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!

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Post by Hump »

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

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Post by Gir »

Hi,

I'd like to mod my MD-2. I don't have anything in particular in mind, just want to see whats out there first.

First of all thanks to @Gila_Crisis and @stolen for sharing their ideas. I have searched the web for mods, but couldn't find anything, that isn't offline but this.

For anyone interested I have put what Gila did together in one summary:
- Remove fizz and add body (Gila_Crisis Mod):

C9 47n
C10 2n7
C7 3n3
R21 47k
R73 Jumper
C40 10n
R49 1k8
C18 1uF
R13 220 ohm *
R42 220 ohm *
C100 150nF **
C30 82nF **
R69 10k
C40 10n
R77 jumper ***

* Less Noise, gain and fizz (Stolen)

** Center bottom control around 80-90hz (Gila_Crisis)

*** Less Distortion at zero positon (Gila_Crisis)

Other Suggestions by Gila_Crisis:

- 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
Hump wrote: 06 May 2022, 03:08 I did the WAMPLER mods to this one.
Where can I fnd the Wampler mod?
:block: Leeeeet's fix it! :block:

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mauman
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Post by mauman »

Gir wrote: 06 Jul 2024, 10:37 Hi,

I'd like to mod my MD-2. I don't have anything in particular in mind, just want to see whats out there first.

<snip> </snip>

Where can I fnd the Wampler mod?
Here's a cross post that might answer your question.
Wampler mod.png

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Post by Gir »

mauman wrote: 06 Jul 2024, 12:44 Here's a cross post that might answer your question.
Wampler mod.png
Thanks! Yes, it looks like Brian Wampler wrote this post. I take it that this is the Wampler mod. This is great.
:block: Leeeeet's fix it! :block:

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Post by Gir »

I'm not sure I understand mister Wampler correctly. Without seeing the schematic it is hard for me to tell.

This is what I got from his post:
wampcat1 wrote:BOSS MD-2 Wampler mod suggestion (part1):

r62 - 100k trim pot (adjust to taste)
c12 - remove
c40 - .022uf
c100 - .1uf
So far so good. This is pretty straight forward. The next part is a bit confusing to me.
BOSS MD-2 Wampler mod suggestion (part2):

tlo72 (22k) in the feedback loop after the gain control - insert diodes in parallel
As far as I understand it mister Wampler suggests to put the parallel diodes between the tlo72 (22k) and what follows next or between it and the ground after the output of this particular section? It's hard to tell without the schematic...
BOSS MD-2 Wampler mod suggestion (part3):

.33uf at the same spot of the tlo72 in the previous sections feedback loop - ???uf

The 2.2k/.22uf in the section before - ???k/???uf
Now I think I am crafty enough to adjust the trim pot in part1. I am unsure what mister Wampler means in part2 and I think I am not able to figure out part3 by myself.

I'm thankful for any help with this mod or suggestions for other mods.
:block: Leeeeet's fix it! :block:

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