Mod Boss Pedal to have 'Blend' control

[mwynwood]

Hi all,
I've just had a friend ask me if it's possible to add a blend knob to a Boss pedal.
I mentioned that a true bypass loop with a blend knob would do the trick, but they need it as part of the Boss pedal.
Sound like a cool thing to learn how to do.

My understanding of the anatomy of a Boss pedal is that they have an always-on buffer, and then the actual effect.
So, if I could tap in AFTER the buffer and BEFORE the effect, I could insert a blend circuit, maybe something like this: http://www.aronnelson.com/gallery/main.php/v/BarcodeVeroLayouts/Vero/BlendimumVero.gif.html
Does this sound right?

Has anyone done this?
Any examples, or direction would be greatly appreciated!

[mwynwood]

...or would it be easier to true bypass the pedal, and include the blend circuit then?


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[slacker]

Yeah that would work, you just need to find the right places in the circuit to splice the blender in, you want the return from the blender to before the output switching so that the blender is only in the effect part of the circuit and doesn't interfere with the bypass signal.
Using this SD-1 schematic as an example http://www.hobby-hour.com/electronics/s/schematics/boss-sd1-super-overdrive.gif you could connect "in" on the blender to the emitter of Q5. You would then break the connection between lug 2 of the volume pot and Q1. Lug 2 of the volume pot goes to "return" on the blender and blend lug 2 goes to Q1. 

[mwynwood]

Thanks Ian! That makes sense.
The pedal my friend wants modded is the famous... MT-2! 

Where might the blend circuit need to go on that beast?
Would it be Q11?
Here's the schematic: http://www.hobby-hour.com/electronics/s/boss-mt2-metal-zone.php

[bluebunny]

Q003 on the RHS switches the clean signal on or off (Q002 below correspondingly switches the dirt).  You want to bypass Q003 so that the clean is always on, but then combine the clean and the dirt (vertical line coming from Q002) with your blend control before sending it on to the output buffer Q001.

[mwynwood]

Sorry for this potentially completely incorrect interpretation of your reply, but did you mean something like this:

[bluebunny]

Nope.    I have no access to drawing progs atm... 

I don't know what you mean by "send" and "return", but whatever you did mean by "return", it's in the wrong place in your picture.  That connection is a control signal from the flip-flop turning Q002 on and off: it's part of the existing switching circuit.

[mwynwood]

Ah, ok - I got it wrong! haha
A diagram would be really appreciated, if you get the time.

The blend circuit I was thinking of including has an: "In", "Out", "Send", and "Return".

http://www.aronnelson.com/gallery/main.php/v/BarcodeVeroLayouts/Vero/BlendimumVero.gif.html

Thanks!

[bluebunny]

Ok, I kinda see what you were thinking.  Instead of "blend", say "mix".  You have the dry signal where your "input" label is, and you have the wet signal where your "send" label is.  You want to mix these where they meet, close to Q001.

[slacker]

For MT-2 you don't actually need the whole blender circuit you can do it with just a pot. Connect one end of a cap to the emitter of Q004, connect the other end of the cap to lug 1 of the blend pot, then disconnect lug 2 of the "Level" pot, connect lug 3 of the blend pot to lug 2 of the level pot and connect lug 2 of the blend pot to where lug 2 of the "level" pot used to go.

[mwynwood]

For MT-2 you don't actually need the whole blender circuit you can do it with just a pot. Connect one end of a cap to the emitter of Q004, connect the other end of the cap to lug 1 of the blend pot, then disconnect lug 2 of the "Level" pot, connect lug 3 of the blend pot to lug 2 of the level pot and connect lug 2 of the blend pot to where lug 2 of the "level" pot used to go.

Thanks for that info - so something like this?

A couple of questions:

• What value cap should be used on Lug 1 of the pot?
• What value pot would work best?
• Does the emitter of Q004 need to be removed from the board?

[slacker]

Yeah, like that. I would try the values from the blender layout 10uF cap and 50k pot. No the emitter of Q004 stays as it is, just solder the cap to it or to the resistor or cap that are connected to it if that's easier.

[aron]

Is there enough room on the MT-2 to add the jacks?

[mwynwood]

It looks like I'll only need to add one pot.
A little 9mm one should fit poking out the side.
I'll report back with how I went, when I do it

[strungout]

This might help: http://www.diystompboxes.com/smfforum/index.php?topic=114471.0

This is what I ended up with, which works very well, tho there is a bit of a drop in output volume, which I'm looking to fix.

I just put the Shaka Boost in parallel with the TS9, straight from the input to a blending knob between the two outputs. Works just as well with my guitar.