Gadget Fuzz Schematic Question

[BuddytheReow]

Here's an extract from the Gadget Fuzz schematic. What is the purpose of the1n4148 diodes here? Are they clipping diodes? Polarity protection is already covered in the power section.

 

[Feral Feline]

That's basically a SHO.

The diodes are protecting the input of the Mosfet, which is different from the polarity protection diode in the power section.

How does a zvex SHO work?

okay… I have always want to know. How does a SHO work? For example many boosters are double as buffers in bypass mode. My understanding of the MI...

www.tdpri.com

"The biggest downside to using a mosFET device is that the metal oxide layers in these transistors are thin and fragile. Hence the stacked silicon diodes or single Zener diode on the inputs of them." — 11 Gauge

He has an interesting build thread for the SHO, as well:

Quick And Easy DIY SHO

Hey all - I've heard of numerous attempts from folks building macguyverized SHO's with readily available parts (i.e. Rat Shack). I just had to...

www.tdpri.com

I'd like to learn more about how exactly this works. I'm guessing when the threshold of the MOSFET is reached, the diodes dump the excess signal, protecting the input.

 

[BuddytheReow]

I'm confused. What specifically are the diodes protecting the mosfet from? Just excessive input signal? Why is power going to them?

 

[Feral Feline]

That's why I say, "I'd like to learn more about how exactly this works."

 

[BuddytheReow]

That's why I say, "I'd like to learn more about how exactly this works."

Sorry. Still working on my morning coffee

 

[Feral Feline]

Well, they're connected in a straight line, doubling the forward voltage, which means the power not siphoned off to the Gate is going to ground; if the input signal exceeds the forward voltage of the lower diode, half its waveform gets clipped to gnd — but what about the other half of the waveform? Is it swimming up-stream towards VCC via the upper diode's FV?

?

Help me ChuckObiDBone-Keknowbe, you're my only hope.

Serious deja-vu here, right now...

 

[thesmokingman]

the diodes in a SHO were a function of necessity. early SHOs didn't have them and the expensive Zetex BS170P would get clobbered by ESD ... to the point there were spare transistors taped to the inside of the enclosures. since ESD can happen in either direction (too positive, too negative) you'd need one to ground and one to supply to fully protect the MOSFET

 

[Robert]

They're clipping diodes, but not in the traditional sense that we usually think of clipping diodes.

The diodes have no effect under normal circumstances, when the voltage on the gate (signal) is between 0V - 9V (VCC).


D1 prevents the voltage on the gate from going higher than VCC (plus the forward voltage drop of the diode, so around 9.7V).

D2 prevents the voltage on the gate from going lower than GND (minus the forward voltage drop of the diode, so around -0.7V).

 

[BuddytheReow]

So the purpose of the diodes is to limit the total voltage including both DC (power) and AC (signal) between 9.7v and -0.7v. This is done to protect the mosfet from being overloaded since it’s rather fragile to begin with. Got it.

stupid follow up question. How big can the signal voltage get from normal guitar input (boosters and buffers aside)? Let’s assume you’ve got really hot, passive humbuckers.