Clean Boost Circuit Breakdown

[phillip]

Here's the schematic of a clean boost pedal that I found at "Homebrew Tweaks" pedal page:



It looks like a very quick and easy build, but I have a couple of questions.

First, what is the purpose of the two diodes?  It looks like D1 is going to conduct only if the battery is reversed, since it's reverse biased to the power supply.

Second, could the circuit benefit from a Zener diode across the Gate-Source junction to protect it?

TIA!
Phillip

[Peter Snowberg]

The diodes are for clamping. They will absorb anything that is greater than Vdd+Vf or anything less than Vss-Vf so they end up doing the same thing as a source-gate zener. The source-drain zener keeps things protected on the output side.

That's a real textbook MOSFET stage. Very nice. Try two or three of them in series.  :twisted:

Take care,
-Peter

[phillip]

Thanks for your help, Peter!  The nice thing about the BS170, 2N7000 and other similar MOSFETS is that the Drain-Source Zener diode is built right in to the package...very nice of them

I was thinking about running two of those together...I bet that would really start to clip     Maybe a blend control between the two?

Phillip

[Tim Escobedo]

The gate-source Zener can be used in lieu of D1, D2.

One thing, the 1M pulldown resistor on the input really limits input impedance to less than 1M. The two 10M biasing transistors are probably unecessarily large in this instance.

[Peter Snowberg]

Good call Tim. You could swap those three resistors for all 3.3M or 4.7M units and maybe get a tiny bit more high end and a tiny bit less noise. (this is assuming that thermal noise isn't being swampped by all kinds of other noise and your transistors are as pure as the driven snow)

Take care,
-Peter

[RDV]

Gotta say it. That's taken from the schematic that's out there claiming to be the SHO.

I've built it and use it in a dual configuration and have a layout that I have not distributed out of respect for Z.Vex.

Sorry Z

RDV

[phillip]

How does it work in the dual config?

BTW RDV...I'm Spartacus

Phillip

[jrc4558]

So where's the schem?

[will]

Hi,

Does this configuration self-bias when adjusting the 5K gain source pot for high gain? I would normaly have put a cap from the pot wiper to ground as not to mess up the biasing.

Regards,
Will

[csj]

Peter,
No offense intended...

I think you've fallen asleep at the wheel.

Clay

[mattv]

Is this the one from the initial post?
http://www.kilback.net/homebrewtweaks/pedals/cleanboost.htm

Here's another mosfet boost:
http://www.muzique.com/schem/mosfet.htm

There's a great article at Geofex, and more schems elsewhere I'm sure.

Not that it would give mosfets any kind of credibility, but I've heard some commercial designs are simple mosfet boosts. It's no surprise really.