What exactly does a cap in parallel with LED do to clipping?

[GrantsV]

Hi,
Sorry for the obvious question.  I was just wondering what the hell a cap in parallel with an LED does???  I understand it rounds off the clipping (reduces highs?)  does it give compression also?  I understand this is a Keeley trick.

Can you recommend some cap values for me to play with on my LED diode sections?

Thanks for your help,
GrantsV

[petemoore]

I understand it rounds off the clipping (reduces highs?)
 Smaller cap cuts less, bigger caps gets rid of highs A/O high end hash...and should be carefully selected.
 Use schematics to decided on a 'base' value and work from there.

[R.G.]

When a diode (any kind) clips, it goes from effectively non-conducting to conducting sufficiently that the voltage across it doesn't change much any more.

Before the diode turns on, it has essentially no effect on the signal impressed across it. Once it turns on, it acts like a very low resistance.

A capacitor across a diode acts like... a capacitor. Capacitors eat current to keep their voltages from changing quickly. So with the diode not conducting, the capacitor is the only operational device. It slows the rise and fall of the signal in concert with the signal source impedance.

Once the diode starts conducting, it limits the voltage change of the signal as well, so the capacitor has less work to do, and participates less. When the signal declines below the diode conduction voltage, the capacitor sources current to keep the voltage from changing as fast.

The net effect is that the capacitor keeps the "corners" of the diode's clipping from being as sharp. This reduces the higher order harmonic production of the clipper.

I understand it rounds off the clipping (reduces highs?) does it give compression also? I understand this is a Keeley trick.

It reduces highs. It does not give any compression. It was used for softening clipping long before Keeley.

[GrantsV]

Thank you for your replies.  What type of cap values should I use for this application?  For example 100-150 pf or much higher like 0.022nf?

[TheBigMan]

Start with a small value and work your way up.  Tube Screamers use 51pF, a DOD 250/Distortion+ uses 0.001uF, Boss DS-1 and Ibanez SD-9 uses 0.01uF to give you some ballpark figures.