Adding tone control to Piledriver Boost Pedal

[karlholz]

I managed to build this circuit from following the schematics, it was my first build so it took forever but it works!! I was curious about now modding the circuit to see all the possibilities I could get out of it, more specifically adding a tone control potentiometer.

Where in this circuit would you be able to add said tone control? From looking at other boost pedal schematics online I'm assuming it would go on the input side of the circuit leading into that .1uF cap that then feeds the "G" leg of the BS170? I could be totally wrong, I'm trying to self teach myself and it gets tricky!

Thanks so much for your help!

[GibsonGM]

Hi Karl, welcome!

That's a pretty common mosfet boost design; I like them, they're very useful.

If you put a tone control on the input, you lower your input signal (of course).  I know that's what a guitar does, but...This is 'frowned upon', generally...we want the max signal to hit the input unhindered, because adding 'junk' in the way introduces noise.    We want that mosfet to amplify a clean signal, and then we can do something with it.

My first thought would be to jam a basic tone control where the 100k is on the output; but that's kind of a hack.   A tone control will drop the output level a bit via "insertion loss", and your boost would then be less of a boost.  It would work, but maybe not that great.  You can try it if you want to experiment, see how it sounds.

If you want to go big, you could add a single control, or even a tone stack, at the output of the mosfet.  THEN, you'd build a second mosfet boost after it, as a gain recovery stage to make up for what the tone control eats.  It would also give you even more boost if you wanted.    There are many ways this could look...I'd build it, set it up how I like it, then replace the 2nd source pot with a fixed resistor so you don't have 2 gain controls.

That's just one way to do it, there are many, and I'm sure you'll have more suggestions soon!   A lot of designs cascade FET or MOSFET stages; it's the basis of a preamp.   

[karlholz]

My first thought would be to jam a basic tone control where the 100k is on the output; but that's kind of a hack.   A tone control will drop the output level a bit via "insertion loss", and your boost would then be less of a boost.  It would work, but maybe not that great.  You can try it if you want to experiment, see how it sounds.

If you want to go big, you could add a single control, or even a tone stack, at the output of the mosfet.  THEN, you'd build a second mosfet boost after it, as a gain recovery stage to make up for what the tone control eats.  It would also give you even more boost if you wanted.    There are many ways this could look...I'd build it, set it up how I like it, then replace the 2nd source pot with a fixed resistor so you don't have 2 gain controls.

I'll try your first suggestion tonight, I can handle that!
Just to clarify, the 1st and 3rd leg on the pot would be replacing that 100k resistor and then the 2nd leg would be output (B) as seen on the schematic?

Your second suggestion I understand but I'm not sure how to go about that without a schematic to copy, it's still a bit over my beginners head to visualize mentally.

Thanks again, the knowledge you guys have on this forum is really inspiring!

[GibsonGM]

Hey Karl,

Check this out.   C4 is your cap, U2 is a pot, 10K here.  This is a 'rudimentary tone control'...it won't load your signal much, but it isn't magical, either...but it works, something to at least mess around with, cutting highs.   Try different values of cap, and the pot can be another value, too (altho you'll have to mess with other caps to get something you like). 

You can scale for the pot....10X pot value, divide cap value by 10...see?   I could come up with better values for both, but I just got done shoveling the snow off my roof         If you haven't gotten more advice by the morning, I will post something re. the 2nd stage.    Go play on the breadboard!   

PS - I'd keep the 100k, it appears to offer a stable load to the mosfet...

[GibsonGM]

On U1, that wiper should go to ground - and should be higher value probably, over 100k    Sorry, long day yesterday.

Ok, to add a basic "tone stack" to your boost, you could do the following.  Note that there are several ways to do this, like driving it from a buffer input stage rather than a gain stage.    The C and R values likely need to be tweaked here, too...I lifted this Voight stack from Merlin B.'s book, optimized for triode output impedances; the mosfet's will be a little different.  But it will work and get you going.     Disregard my Mosfet stages, they are a bit different than yours tho very similar.   

Everything from C4 to C6 is what you want to do, the rest would simply be your gain stage and then duplicating it after the stack, and you'd adjust the source resistor of the 2nd gain stage to taste (probably with a 10k pot).  It can be set, or you can add a volume pot at the output.
Hope this is helpful!

[GibsonGM]

A little bored this morning...so here is another way to do this, using a mosfet as a buffer, driving the tone stack, with the boos stage AFTER it.   This will drive the tone stack better.

Tweaking definitely required, but it's close.  BTW, the 2N7000 is very close to your BS170.

More info here: http://www.muzique.com/schem/mosfet.htm