What are these diodes doing?

[DDad]

This is a snip of the Doxie Preamp output stage. What are the highlighted 2 reverse bias diodes doing?

 

[hamerfan]

I didn’t know it either, but sometimes its useful to use the search function. Read here:

Post in thread 'Gadget Fuzz Schematic Question'
https://forum.pedalpcb.com/threads/gadget-fuzz-schematic-question.8165/post-73393

 

[DDad]

That thread is talking about clipping on the input of a MOSFET. Is that the same as after an output stage?

 

[dot matrix madness]

D1 and D2 clamp the signal in between the respective voltages.
Typical use is to protect the input of digital chips from voltages below 0V and above +5V.
Often you see Schottky diodes (BAT42 e.g) instead of 1N4148 due to their lower voltage drop.

 

[spi]

Both diodes go to Vcc. I would expect this to keep the voltages between Vcc +- Vf and clip signals above or below that range.

On second look, one goes to -Vcc. So it'll keep the voltages between -Vcc-Vf and Vcc+Vf. It seems like that would have very little impact as the IC is also supplied with Vcc and -Vcc, I don't expect the signal to be outside the rails. No idea why it's done.

 

[DDad]

I think the bottom one is going to -Vcc not +Vcc.

 

[spi]

I think the bottom one is going to -Vcc not +Vcc.

thanks, I caught that before I saw your reply.

 

[Cybercow]

D1 and D2 clamp the signal in between the respective voltages.
Typical use is to protect the input of digital chips from voltages below 0V and above +5V.
Often you see Schottky diodes (BAT42 e.g) instead of 1N4148 due to their lower voltage drop.

This is the way . . .

 

[DDad]

Here is another output stage with those 'clamping diodes' from the Pumbaa Distortion. But please explain why they matter - the previous output stage (the FET) can't go higher than Vcc or lower than ground. What are these diodes really 'clamping'?

 

[DDad]

Never mind unless you come to a different conclusion than spi and I came to - it isn't doing anything. Just for added part count/complexity?

 

[lowpitch]

Possibly just foolproofing against someone doing something stupid, like plugging a tube preamp with much higher voltage swing than the TL072 can handle into the output of an effect.

 

[HamishR]

I may be wrong but I believe the Doxie preamp was designed by Thorpy FX. Thorpy is ex-army and in the UK and they design stuff to be electronically bulletproof. I would imagine this is part of that philosophy. You could probably leave the diodes out as most pedal manufacturers do and never notice the difference. Unless of course you are playing hard rock in a Blackhawk helicopter in a thunderstorm taking heavy enemy fire and experiencing enemy jamming (which is inevitably in the wrong key).

 

[Robert]

What are these diodes really 'clamping'?

ESD, and like @lowpitch said, user error.

Devices don't like their inputs and outputs to exceed their supply voltage, the diodes prevent the input/output signal from going outside of the VCC/GND range of the pedal to prevent any damage.

And like @HamishR said, this is pretty common to see in Thorpy designed circuits.

Under normal circumstances you can remove the diodes with no audible effect.