Clipping LED's

[gritz]

There was a thread a few weeks ago regarding this and the upshot (including some tests by me) was that some LEDs have significant capacitance that can affect the higher frequency output of a typical clipping circuit at low signal levels (i.e. before clipping sets in). This can be a good or a bad thing depending on what you want your circuit to sound like.

With the limited number of LEDs I had to test I found that some greens tended to exhibit highest capacitance and in a Tubescreamer (for instance) stage they might at least double the 51pF capacitance that sits in the opamp feedback path. This might give rise to a noticeable loss of treble at low guitar levels. In contrast, low current red LEDs (similar to the ones used in Boss pedals as status indicators) exhibited only a very small (~15pF) capacitance and had very little effect. Bear in mind that as soon as the clipping LEDs start to conduct this capacitance is effectively shorted.

It's worth listening out for when experimenting though!

This is interesting - but I have a newbie question - is that "flat" capacitance that is present due to the LED being used or is it related and variable to the state/operation of the LED, does it shift as the led is driven to clip I suppose I'm asking?

The capacitance is inherent in all PN semiconductor junctions and it varies with the (reverse) voltage across the junction. In things like signal diodes it's designed out as far as possible to preserve high frequency operation, but it can make itself felt in the reverse biassed base - collector junction of an ordinary bipolar transistor (see miller effect) and is used in special diodes called varicap diodes - which are effectively voltage controlled capacitors, albeit with a rather small capacitance when compared to what we tend to use in audioland.

Capacitors are (very basically) two conducting "plates" held apart by an insulator called a dielectric and they store charge as static electricity (keepin' it simple alert!). Plates with greater surface area give more capacitance (which is easy to understand), but making the dielectric thinner so that the plates are closer together also gives more capacitance, as the charged plates have more influence upon each other.

In our semiconductor junction, the P and N type lumps of semiconductor form the plates and the junction where they join forms the dielectric - where they join there is a very narrow insulating layer called the depletion layer where there are no "spare" conductive elements - either electrons or holes (which are  shortages of electrons, rather than mysterious particles in their own right). This depletion layer is the reason that we have to apply forward voltage to a semiconductor junction to make it conduct.

So... if we apply a reverse voltage to a PN junction the depletion layer gets wider (and the capacitance falls) and if we decrease the reverse voltage and apply forward voltage the depletion layer gets narrower and the capacitance increases, right up to the point when the junction stars to conduct and the "capacitor" is shorted.

The capacitance is (if I remember right) inversely proportional to the square root of the applied voltage, so as you increase the reverse voltage, the rate at which the junction capacitance reduces will itself reduce. In practical terms, if you have two identical diodes wired back to back in a clipping circuit then the variations in capacitance will cancel out to a fair degree. However, if you had a high capacitance LED opposite a low capacitance diode there would be much more variation.

"Yeah, but how would that sound?"

For the vast majority of the time in a drive pedal one of the diodes is conducting and the (small) capacitance of the other diode is effectively shorted. If there is an audible effect due to modulating the capacitance I guess it would only be apparent when the signal decays below the level at which the diodes are conducting for an appreciable proportion of the time. Smart folks like PRR and R.G. would be able to make a much more sensible call than me on any effect it might have though.

[mistahead]

Thanks for the explanation there - makes good sense - will explore deeper into the underlying theory and leave it in the back of my mind for a rainy day...

There have been some "alternative" or anticlipping threads I will need to re-read in tandem to see if my aching head is trying to link something to something else or if its just grouping LED clipping facts.