Clipping LED question?

[blues4me]

Okay, I'm new here and I think this is my first post. And on top of that, I do not have a full understanding of these circuits (which is somewhat disheartening because between 1982 - 84, I studied electronics and learned a lot of stuff, but I don't remember a lot of it).

This is the deal. I bought an LDO clone already built at a great price (and I like it a lot), but my goal was to make it a CJOD clone. I was thinking that all I had to do was put in the Burr-Brown chip AND put in 5mm LED's instead of 3mm. Is that correct? Or did I pull that idea out of nowhere?

Anyway, I put in the bigger LED's and the pedal works in two modes, but not the LED clipping mode. The signal is making it through the switch to the LED's, but they are not clipping. And it sounds exactly like having the mode switch in the "no clipping mode".

What's the deal? Do the larger LED's require more voltage or something? I could have sworn that the two differences between LDO and CJOD was the chip and bigger LED's. I really am sorry to bring this up again.

Anyway, any help would be sincerely appreciated. Thanks.

[earthtonesaudio]

With LEDs, size truly does not matter.  Internally, the materials, and therefore the clipping threshold and "knee," are the same.  Color matters, and so does power handling, and whether or not the LEDs have built-in current limiting resistors (avoid that kind).

For clipping LEDs in a typical Tube Screamer type pedal, you want the "standard" low power LEDs, with the 20-ish mA maximum rating.  Avoid the super high-powered, automotive or industrial lighting types.  Most 3mm or 5mm LEDs are fine (not all, but 90% or more).


That said, it's possible you have everything installed correctly and just haven't cranked the gain enough.  LEDs have a higher clipping threshold than Si diodes (if that is what was in there before) and so you have to play louder to hear them clipping.  Or, your battery might be low, and so the op-amp starts "rail clipping" without ever making a voltage high enough to cause the LEDs to conduct.
On the other hand, you might have an error in your wiring.

[blues4me]

thanks, earthtones.

On the other hand, you might have an error in your wiring.

and, yes, you could be right about that (ha, ha).

Actually, I haven't had time to check this out further, but I should have tonight. I'm pretty sure I can get it working again (in all modes).

Very informative answer. Thanks again.

[earthtonesaudio]

Good luck!