Xotic - EP Booster

The Xotic EP Booster has been causing a fuss for a while now. Supposedly based around the legendary Echoplex EP-3 preamp it's been recognised as one of the best boosters currently available.

Here's the description from the Xotic website:

"What do the tones of renowned guitarist such as Page, EVH and Johnson have in common? They all used the legendary echo machine EP-3 as a pre-amp.We've captured that magic in a new Xotic Effects pedal, the EP Booster.

We've used the highest quality parts available with a discrete FET design and low impedance output, the EP Booster provides up to +20dB of unadulterated boost with multi-dimensional, shimmering highs and lows, and no ear fatigue. The internal DIP switches let you choose the boost frequencies, and EQ settings.

Housed in a small 3.5"x 1.5" x 1.5" case with blue LED and transparent knob, the EP Booster can be powered by 9-volt up to 18-volts.

A simple pure boost that pays reverence to the magical EP-3 and captures the celebrated tones of the world's most famous players.
The EP-Booster,great tone starts here!"

And, of course, a ProGuitarShop demo video for you:

Well, the freestompboxes crowd whipped one apart and took a look inside (Thanks to Greg for the initial tracing):

The original transistors are supposedly 2N5457 and 2SC1815. As you can see it doesn't greatly resemble the EchoPlex EP-3 preamp:

But hey, nevermind, because this thing sounds pretty good. Here's a PCB layout provided by Masca of Handmades.br: http://www.mediafire.com/?eb0byo1n9s524q2

Here's a vero layout by IVIark of tagboardeffects.blogspot.com: http://tagboardeffects.blogspot.com/2012/02/xotic-ep-booster.html

I would make a few modifications to this - firstly it's got to run on 18volts! Then I'd replace the drain resistor with a 25k trim so you could dial in a sweet spot of your choosing. For extra control I would swap out that low pass filter for a version of Mark Hammer's "Stupidly Wonderful Tone Control", that would give you a real handle over your top end. Finally - what's with that nasty BJT transistor output buffer? I'd use a Jfet or Mosfet follower - better fidelity and clipping characteristics!

Anyhow, I might draw a schematic up!

[EDIT] - I told you I'd draw a schematic up, here's the business:

As you can see I've added a master volume control, a tone control (treble roll off) - you could call this control "Sparkle" for added mojo. I replaced the nasty BJT transistor voltage follower with a nice JFET type. I ommited the "bass boost" switch, I didn't like it, it muddied up the boost, you could add it back in if you want. I also included a quick schematic for a voltage doubler to allow you to power this circuit at 18v (where it sounds best) with a single 9v battery. Well, what you waiting for? Go build one :-)

Here's a vero layout drawn by IVIark of tagboardeffects.blogspot.com: http://tagboardeffects.blogspot.com/2012/02/xotic-ep-booster-briggs-modded.html, check out his site for an awesome array of vero board layouts of just about everything!

[EDIT2] - If you want to actually reduce the low end present in the circuit I'd reduce the input cap to 33nF and change the bypass cap on the "Boost" control to 220nF ~ 470nF ~ 1uF depending on your preference. I'd also recommend one other small change - replace the drain resistor on the first FET with a 10k trim pot then use it to dial in a 5.5v bias on the Jfet's drain to make sure you've got it in the "sweet spot". Just thought I'd add that ;-)

[EDIT3] - One last thing, for those interested in booster circuits which add colouration to your tone check this out (schematic below), it's a prototype I was working on back in 2009! The values in the treble control need tweaking to find the best response but it sounds pretty good "as is". This thing also sounds best at 18 volts :-) Build it up and tell me what you think:

For reference here is the freestompboxes.org forum topic: http://www.freestompboxes.org/viewtopic.php?f=7&t=7881