What does forward voltage do to distortion?

[knucklehead]

Pre-emptively trouble shooting; some have experienced oscillation when there is a combined LED-based boost and distortion circuit in some instances. Could this be caused by identical LEDs in both circuits? Could using a different color or forward voltage value side-step this?

 

[Cybercow]

Identical clipping devices simply deliver what is known as "symmetrical" clipping. Using unmatched clipping devices yields 'asymmetrical' clipping.

And the Fvd (Forward voltage drop) of a clipping device is what limits the audio signal by giving it a 'haircut' at the measured forward voltage drop.So if the signal, without any clipping devices, is 1.0 volts, using a clipping diode (say a 1N914 or 1N4148), the Fvd is ~0.7 volts. Meaning the clipping device will only allow 0.7 volts of the original 1.0 volt signal to pass. That is what creates the "distortion".

 

[knucklehead]

Identical clipping devices simply deliver what is known as "symmetrical" clipping. Using unmatched clipping devices yields 'asymmetrical' clipping.

And the Fvd (Forward voltage drop) of a clipping device is what limits the audio signal by giving it a 'haircut' at the measured forward voltage drop.So if the signal, without any clipping devices, is 1.0 volts, using a clipping diode (say a 1N914 or 1N4148), the Fvd is ~0.7 volts. Meaning the clipping device will only allow 0.7 volts of the original 1.0 volt signal to pass. That is what creates the "distortion".

Thanks for the info - do you think symmetrical clipping can result in audible oscillation?

 

[temol]

There's many possible explanations for the oscillation. And colour of the diodes or symmety/assymetry of the signal would be the last factor on my list. Or even off the list. First of all - boost into the distortion. Adding gain into something that has a lot of gain.

 

[knucklehead]

There's many possible explanations for the oscillation. And colour of the diodes or symmety/assymetry of the signal would be the last factor on my list. Or even off the list. First of all - boost into the distortion. Adding gain into something that has a lot of gain.

Thanks - must embrace the mantra "do or do not - there is no try" . . . . .

 

[Cybercow]

Thanks for the info - do you think symmetrical clipping can result in audible oscillation?

No. Symmetrical or symmetrical clipping is not responsible for oscillations. Oftentimes, a simple small valued cap (30pF to 100pF) in a feedback loop should squelch feedback oscillation. It all depends on the specific circuit. "LED based boost or distortion" is not a thing really. Boost comes from an active device, (like a transistor or opamp), properly setup to increase gain. Distortion can come from overdriving (saturating) an active device - OR - distortion can come from passive devices like diodes, LEDs, or P-N \ N-P junctions of BJTs or certain FETs that will "clip" the audio signal. "Clipping" is an un-natural distortion. Driving a device beyond it saturation point is a more natural approach to distortion. But know that it is the breaking of the "rules" of electronics that achieves some of the great dirt pedals.

What specific circuit is giving you unwanted oscillations?

 

[knucklehead]

This is a pre-emptive query - I am basing my concern on what I've read on the forums specifically as it has rarely occurred in Parenthesis and Rat-based dirt boxes.

I am just completing a Mini, and may never get as far as a full Parenthesis build (despite having all the components) if it does what I need it to do. I am modifying the signal path a bit, changing it to Octave > Boost > Distortion from its original Octave > Distortion > Boost, and want to consider or prevent problems at the build stage should I do the second build.