MXR Blue Box Red PCB Volume beyond "C11 Mod"

[tokyoburns]

So I was at band practice and we were writing a song about a giant robot and I found myself wishing I had a MXR Blue Box when all of a sudden I notice one in my guitar players pile of pedals he doesn't use. I plug it in and sure enough the volume drop makes the pedal almost useless. So I get to looking it up and I see the "C11 Mod". sounds easy enough. So I try that mod and it helps ever so slightly but definitely does not fix the problem. It did make the pedal a little brighter with a little more headroom. Over all a better sound. So I put the Blue Box in front of my behringer eq pedal (the cheap one), flattened the EQ raised the gain to gauge how much boost I really needed to make that pedal up to par. Turns out that it only needs about 3db. With that little bit of gain and the "C11 mod" it sounds great. Exactly what you expect out of the pedal without any disappointment. However I'm not gonna dedicate my EQ pedal to make one effect work during one song and have to adjust it. So I thought how about I build a small buffer at the output of the pedal and if it is to much I'll always be able to dial it back with the output knob of the pedal. Searching through buffer schematics on this site I found a great simple JFET guitar preamp.     http://www.montagar.com/~patj/preamps.gif       It claims to add exactly 3db! perfect! So I built the preamp with a few mods of my own. here is my schem.

and tested it side by side with my EQ to see if it gave me what I wanted. It does! Perfect! IT's small enough to fit in the battery compartment too! yay! 
So here is where the problem comes in... I was gonna use the battery clip leads to provide power my preamp but there is no power to the clip when the pedal is plugged in to the wall. WTF? Is this because of an input switching jack or something? I assume that the battery is just run in parallel with power input so then why no voltage to the clip when the pedal is plugged in and on? and the pedal is on. I already checked that. I can't find a definitive schematic for this pedal. Every schematic I find is a sort of "this is good enough you can figure out the rest" sort of schematic. which doesn't help on this particular problem. Help?

Some more information about the build to finish up the post. I'm going to cut the middle lug of the output Pot so that part is still soldered to the board and the other half is still connected to the pot. the two new leads will respectively become the input and output of my preamp. The negative lead of my preamp will be soldered in to the old negative solder hole of the ye' ol' C11. I haven't tested this part yet but it seems right to me. If you spot any stupid mistakes in that line of thought be sure to point them out. Thanks guys!

[theehman]

So here is where the problem comes in... I was gonna use the battery clip leads to provide power my preamp but there is no power to the clip when the pedal is plugged in to the wall. WTF? Is this because of an input switching jack or something?

Exactly.  When the DC jack is used it disconnects the red battery lead from the circuit.  Otherwise it would try to charge the battery.
Connect the preamp to the power and ground wires going to the circuit board instead.

[tokyoburns]

I soldered the power leads to the preamp to the power in leads from the dc jack and I don't know why i thought i needed a ground lead to the c11 i just grounded to the power lead. also i took the battery leads out all together since they had a quick connector but i plugged it in and... nothing! haha but i quickly found a bad solder joint fixed it and it worked! it looks and sounds great thanks for the help i appreciate it. I absolutely recommend this mod to anyone with a blue box. the volume is about unity with the bypass when the output knob is at 3 o'clock. after that it starts to feel like i am overloading the preamp but i haven't really checked it and that doesn't seem to make sense to me in theory but over all i am super stoked with it. It's not that hard of a build and really brings the blue box to it's full potential.  only other mods i can think of would maybe be to put a switch on the preamp so i can use it as a buffer when the effect is bypassed and also maybe try to fit the battery somewhere in there and drop 18v across the preamp instead of 9 and give it some more headroom. but im happy with it and i dont think ill be doing either of those things.

[pegsusIII]

Hi there,
First time on these forums, just stumbled over this thread looking for an alternative to the C11 mod. From what I understand while the C11 mod can fix the volume drop it can also reduce the glitchyness of the pedal, which is what i love about the pedal!
What a fantastic idea! Putting a mini preamp into the circuit.
Just wondering if there is any chance you could post some more photos? Front/back of board and also where you connect the input/output leads to the pedals PCB?

[tokyoburns]

Well i gave the pedal back to my guitar player so I wont be able to post anymore pictures. sorry. but in my experience the c11 mod did not take away from the "glitchiness". it does seem to make the pedal a little brighter with more headroom and it is a very good start to making the pedal sound usable on a live pedal board. The c11 mod was included in this pedal. the input and output are attached to the middle lug of the output pot. I simply cut the lug in half with a pair of snips and bent part of it inwards so that the two halves no longer make contact. One side connected to the board and the other connected to the pot. this will sever the connection to the output and allow you to slip the preamp between the output pot and the output jack. the half of the cut lug connected to the pot connects to the input of the preamp. the half still connected to the board is connected to output of your preamp. hope that helps.

[tokyoburns]

also note that the reason i put this preamp in was because the c11 mod was simply not enough to fix the volume drop. the preamp with the c11 mod was exactly the way i wanted this pedal to sound. i recommend doing both since both mods are easily reversible. good luck!

[Seljer]

I did the same thing with mine. I used a simple noninverting opamp stage with I think +6dB of gain always strapped to the end of the bluebox pcb. Its a jumbles of wires inside the box but it now shakes of the room very heartily

[tokyoburns]

@seljer

cool. when i was looking for a good mod i didn't actually find any confirmed "post pedal amp installs" (thats what im calling 'em) so thats what prompted this whole mod. im surprised that this is not more popular with the mxr blue box. you should post your schematic and some sound clips on this thread if you can. i no longer have this box in my possession so i can not do the same. but i would love to eventually see a comparison between different amps. this kind of mod should be more popular.

[pegsusIII]

Just thought I'd upload some more pictures of this mod for anyone else wanting to try it.
I ended up using a 2n5484 because that's all I could get my hands on. It worked really well, it boosts the pedal to a much more usable level, works especially well in conjunction with the c11 mod.
I felt the c11 mod on its own altered the tone of the pedal (sounded like it cut out some bass or something), but with this pre-amp mod the pedal sounds awesome.

This was my first time using expressPCB and SCH, hope it is all clear and makes sense.
I definatley recommend this mod to anyone who owns a Blue Box!
I really like the tone with both output and blend at 3 o'clock, or for even louder craziness push the output to full.

Will try upload some sound clips/videos sometime in the future if i get around to it.

[Leepyringa]

Can anyone tell me the value of c11 cap ? I would be so happy!

[Leepyringa]

Btw ! I built this preamp and it works BUT I used 1Mohm instead of 10Mohm in R1 THOUGH It pops really loud when engaging/disenaging ...

Can this be because of 1Mohm resistor in R1 I read somewhere at geofex that values of pulldown resistors are not so important

[Dr. Satan]

Btw ! I built this preamp and it works BUT I used 1Mohm instead of 10Mohm in R1 THOUGH It pops really loud when engaging/disenaging ...

Can this be because of 1Mohm resistor in R1 I read somewhere at geofex that values of pulldown resistors are not so important

Move the 1M to where you have the 51K and the popping should stop.  That is, take the 51K out and replace it with the 1M.  You shouldn't need the 10M on the input at all.  I just built a stripped down version of this with 3 resistors, 1 cap, and a J201 and it works great. Gets about 7db of boost. Unity at about 1~2 o'clock on the dial.

[dromilious]

you put the mini booster in parallel with the mxr or you cut the in and out jacks ?

[kevinrontel]

Great thread.  I ended up using TWO pre-amps.  I opted for the Till design because it uses fewer parts.



I put one amp going into the blue box, which increases sustain and ends some of that gating problem, but which doesn't add much gain to the output due to the ridiculous compression the blue box imposes on the signal, and another coming out, which successfully boosts the volume.

And now I have a usable pedal.  Thanks, y'all.


Keep in mind that the pin-out of an FET transistor is different from yer typical transistor and it doesn't match the schematic.  For a typical tranny, the input comes in the middle leg.  With an FET, it's one of the side legs.  Someone please correct me if I have this wrong. 

[poroderick]

Hi there! Can you guys help me out please. Can I use something like this for this mod: https://www.parts-express.com/lf353-ic-dual-low-noise-jfet-opamp-3-mhz-8-pin-dip--lf353

Total electronic noob. Any advice will be welcome!

[duck_arse]

hello and welcome, poroderick.

what is the mod you want to use the LF353 for? that IC is the NatSemi equiv for the TI TL072, and natsemi has been gone for a while now. use it if you have it, but you might as well look for the TL0x2 (062, 072, 082) instead, to be sure of getting the real thing and cheap.