Some assistance please?

[Cardboard Tube Samurai]

I have just built a modified Grace Overdrive/Big Daddy from runoffgroove but it's got an issue. Using this schematic - http://www.runoffgroove.com/grace.html I added led clipping after the output cap (.1uf) but this seems to have given it a sort of gating/clicking sound on note decay. It's especially noticeable when adjusting the drive pot whilst a note is decaying. I'm not a complete noob to this so you don't have to dumb down your answer too much, but am I right in thinking that I need a cap somewhere in parallel to the leds to smooth this out? Or am I actually more of a noob than I think and is this completely off-track?

Oh, by the way, it's one hell of a nice sound for such a basic circuit and anyone new to this affliction should give it a crack.

[Cardboard Tube Samurai]

No? Anybody?

[Boogdish]

try your clipping diode before the output cap.

[gez]

If you haven't done so already, wire a resistor before, and in series, with the LED(s).  Take the signal from the junction of the resistor and LED(s) (many schematics will show you how this is done).  As things stand (assuming you've left out the resistor), when the output rises above the LED's threshold there's nothing (except the internal resistance of the LED) to limit current flow.  Not good for the amp and if it has short-cirucit protection on the output it'll probably shut down (this might account for the clicking you're hearing...who knows).

Don't put the LED before the output cap.  With no current limiting resistor, you're going to short the output if the bias point is above the LED's threshold (assuming the LED doesn't have a super-high threshold).  Even with a resistor, you're going to bias the LED so that it's always on.

[Cardboard Tube Samurai]

Awesome. Any suggested value?

[gez]

10k seems a popular choice.  Take a look at the MXR distortion +.

Edit:  just had a look at the distortion + schematic.  Making the volume pot larger is going to drop more voltage across it, thereby increasing distortion.  Something to bear in mind.

[Cardboard Tube Samurai]

Thanks, I will try it and report back later!

[gez]

I just edited my post, but you replied in the meantime.  Check out my comments.

[gez]

Not good for the amp and if it has short-cirucit protection on the output it'll probably shut down (this might account for the clicking you're hearing...who knows).

Actually, considering that the amp is a power amp it probably won't shut down; but the clicking might have something to do with recovery time?  Either way, a resistor is a good idea (you're wasting a lot of current without one).

[Cardboard Tube Samurai]

Hmmm... the pedal is rather loud as it is so I don't know that I want to increase the volume pot. Looks like I've got some experimenting to do. Might take it to the breadboard.

[gez]

Hmmm... the pedal is rather loud as it is so I don't know that I want to increase the volume pot. Looks like I've got some experimenting to do. Might take it to the breadboard.

You're not going to increase volume as the LED's clamp the signal.  With more voltage across the pot, more of the signal gets clipped so distortion (not output) is stronger.  My comments applied to the MXR circuit, anyway, which has a 10K pot.  The linked to schematic has a 100k.  I doubt that increading it is going to make much difference.

If the effect is too loud, simply stick a resistor in series from the junction of the LEDs to the pot (connecting to ground)

[Cardboard Tube Samurai]

Ok, so I've put a 10k resistor in series after the output cap and before the clipping diodes. Minimal change if any. Signal is still very gated on note decay. The only obvious change was that the LEDs now don't light up very much at all which I'd like to avoid becauce I've got them coming through the top of the enclosure in the eyes of a skull and I want them to light up as the guitar is played.

If you haven't done so already, wire a resistor before, and in series, with the LED(s)

Just to clarify, do you mean one resistor before AND one in series with them? I'm assuming you mean just one in series but before the LEDS.

[gez]

Signal is still very gated on note decay. The only obvious change was that the LEDs now don't light up very much at all which I'd like to avoid becauce I've got them coming through the top of the enclosure in the eyes of a skull and I want them to light up as the guitar is played.

LEDs have a large forward threshold, so if the gating you're describing is distortion going to totally clean as the note decays (with not much in the way of a transition), you could do with some more gain somewhere in the circuit.  If this isn't the case, what exactly do you mean by 'gating'?

Re the dimness of the LEDs.  Think of the current limiting resistor as...well, a current limiting resistor: adjust it accordingly to get brighter LEDs (lower resistance)

Just to clarify, do you mean one resistor before AND one in series with them? I'm assuming you mean just one in series but before the LEDS.

Keep the current limiting resistor from the output cap to the LEDs.  Disconnect your pot from this junction then reconnect it via a resistor.  All you're doing is dividing down the signal clampled by the LEDs (voltage divider with the pot).

[gez]

http://www.national.com/ds/LM/LM386.pdf

Pages three and five outline gain control (should you need to increase it).