boost distortion with diodes

[comfortably_numb]

Hi.  I'm new here.  I've been reading stompbox and electronics sites for about a month now and soaking up all I can.  I've made some modifications to my existing pedals with good results and I'm about ready to build my own.  I may be a bit of an over achiever, but I am wanting to use a combination of designs to get something at least somewhat unique for my first build.  What I was thinking was using Jack Orman's Mosfet Boost and adding a diode clipping stage at the end of it which I can switch on and off.  As I was considering this I realized that the clipping introduced by the diodes would also lower the volume when switched in, which I don't want.  I don't particularly want to add an entirely new gain stage afterwards to make up for it.  Does anyone have any advice concerning this? 

Also, is the input signal into the pedal great enough to put the diode stage BEFORE the boost stage?  I am not wanting a great amount of distortion, just a minimal "bite" when I switch in the diodes.  I plan on using quite a few of them so raise the clipping threshold. 

Another quick question:  Does anyone know how the Keeley Katana works?  I have read a small amount about it and the concept is what I'm trying to recreate - a flat clean boost with the option of switching in the slightest amount of overdrive.  I don't want the schematic per se...I could just buy the pedal, but if anyone could give me a block diagram that I could translate into circuitry myself, I would be greatly appreciative. 

Much thanks,

CJD

[Peter Snowberg]

Welcome to the forum. 

If you don't want the output level chopped down too much and you don't want to add a recovery stage to the end of the circuit you have a couple of options. The first is to use LEDs or strings of diodes so that the forward voltage is higher. The second is to add a resistor in series with the clippers.

You could also place a couple of diodes before the boost, but most pickups won't put out enough juice to cause regular silicon diodes to clip. You can however use Schottky diodes or germanium diodes to get clipping at pickup output levels. I would suggest a pair of grounded diodes in inverse-parallel which are then in series with a 250K pot connected to the guitar output. I replaced one of my tone controls with a pair of Ge diodes and I love what it does to the tone when the control is turned up part way.

[comfortably_numb]

Thanks for the response.  Can you explain to me, in electronic terms, why a resister would re-raise the output level?  I thought resistors "resisted" current, and that the maximum output of any pot would be equal to the absence of the resistor.  Also, is there a way to link this resister to the initial gain circuit so that any adjustment in the first gain stage would result in a coresponding adjustment in the output level following the clipping circuit?

[Ben N]

The resistor that Peter is suggesting is not in series with the signal, but rather in series with the clippers, which themselves shunt signal to ground.  By adding additional resistance, you limit the amount of signal that can be shunted to ground by the clippers, increasing the amount of signal remaining--but also reducing clipping.
Ben

[amz-fx]

I was thinking was using Jack Orman's Mosfet Boost and adding a diode clipping stage at the end of it which I can switch on and off.  As I was considering this I realized that the clipping introduced by the diodes would also lower the volume when switched in, which I don't want.

Use LEDs for clippers and it will still be quite loud when they are switched in, although there will be a volume drop..

You might also read this article:

http://www.muzique.com/lab/warp.htm

regards, Jack

[Mark Hammer]

The MosFET booster works very nicely and does what it does well.  Your call, ultimately, but my own advice would be to make a separate clipping unit that can be bypassed, and leave the clean booster as a clean booster.  No reason why you couldn't stick the two circuits in one 1590BB (with the wide side facing you to accommodate two stompswitches), and let each portion maintain its own distinct personality.