LEDs for clipping section. What kind?

[gnugear]

I'm going to modify my BYOC Shredder (although I guess it applies to any device) and want to add a switch to flip between the diodes and LEDs. Does it matter if its a 3mm, 5mm or 8mm. Also, what differences do the colors make?

[Mark Hammer]

Red is what you want.  Other colours have forward voltages higher than red, and red's is equivalent to around three 1N914s in a row.

Size should not matter, though you want something that fits the available space.  To the best of my knowledge, the relative brightness rating of the LED should not matter, though I am willing to stand corrected if folks who have dickered with a range of millicandle ratings have anything to add.

[petemoore]

http://www.diystompboxes.com/wiki/index.php?title=Simplemods#Simple.2C_Easy_Mods.2FTips_.26_Techniques
 
 You can measure the threshold voltage.
 Also you can voltage boost the signal presented to the diode.
 Nearly the same thing..."lower the ceiling" or increase the wave amplitude = similar result.

[MikeH]

Red sounds good, but others (while not having as high of a forward voltage) can sound good too.  I have a big batch of leds that I've salvaged from all sorts of places and they all sound a little different.  The only ones I've tried that sounded terrible were the flat ones that look like plexiglass chicklets with legs coming out of them.

[earthtonesaudio]

One other interesting thing about LEDs is they can have really low reverse voltages.  I've seen some blue LEDs whose reverse voltage is lower than their forward voltage!

In practical terms, this means that you may sometimes be able to get away with one LED for clipping in "both directions," although the symmetry depends a lot on the LED itself and what's driving it.

[gnugear]

Thanks. Is there a reason you usually only see two LEDs used for the clipping section? I was thinking of doing three for asymetrical clipping (like you do with diodes) but I never really see it??

[Mark Hammer]

If a pedal is powered by +9v, and the op-amp is only able to swing from 1.5 to 7.5v (a 6v, or +/-3v range), and the input signal is, let's say +/-100mv, how much gain can you apply - without any diodes whatsoever - before the op-amp itself runs out of headroom and starts clipping?  Answer: 30x.  If you are using a diode combination that involves two LEDs in series (e.g., a 2+1 combo for "asymmetrical"), that poses a requirement for the half-wave of the signal those diodes are supposed to clip to be at least 3v amplitude.

So, the upshot is that if you are using a diode complement with a VERY high total forward voltage, AND you are using a 9v supply voltage, the chances are pretty good that what you think you are hearing from use of LEDs is actually the op-amp clipping itself....with maybe a small contribution from the LEDs.

And that is why you only see a pair of LEDs used in floor pedals.  If it is an amplifier running off +/-15v supply, then we'll talk, but for 9v floor pedals, there is simply no point in using more than one LED in either direction.

[PRR]

> Is there a reason you usually only see two LEDs used for the clipping section?

The most-common/cheapest Red LED is 1.5V one way and 5V the other way. This is too much asymmetry; also it is hard to get a 5V one-side swing from a 9V battery, also 5V clip is WAY more than you need to abuse a 0.5V amp input.

That's just logic. Feel free to do whatever you want; it's your effect.

Though I do think that three LEDs may be hard to saturate with a 9V supply, and after clipping you will need to cut-down a lot to get into a guitar-amp without more clipping in the amp input than in the LEDs.

> some blue LEDs whose reverse voltage is lower than their forward voltage!

Interesting. I knew the Blues had high forward voltage; I did not know they had low reverse voltage. The specs I am finding all show Blue is 3.5V-4V forward, and 5V reverse, though at 100uA which may be more than some pedals clip at.

[daveh222]

Another idea is to use a bi-color LED. They're two LEDs back to back in one case. They light one color in one direction, a second color with the opposite polarity applied, and a mix of the colors with a.c.  If nothing else, it would save space on a board.

dave

[Greg_G]

The bi-colour LEDs that I've seen are 3 pin devices..

Has anyone used smd LEDs ?
How do they sound ?

[daveh222]

I've used two lead bicolor LEDs for indicators in various projects.

Here is a link for two lead bicolor LEDs in lots of colors. They would make good asymmetric clippers.
http://www.elexp.com/opt_3362.htm

[Greg_G]

I've used two lead bicolor LEDs for indicators in various projects.

Here is a link for two lead bicolor LEDs in lots of colors. They would make good asymmetric clippers.
http://www.elexp.com/opt_3362.htm

Interesting.. thanks.
There's a good range to select from too.

[jbord39]

Different colors sound a little different.  I put in some of those quick connectors in Dr. Quack and messed around with different LED combinations.

[TheRiffGod]

What you want are TRI-Colour LEDs.

They have 3 legs. each tricolour is made up of 2 LEDs, so you have a possibility of the two original LED colours (say, red and green), or their combined colour (in this case, yellow).

The to LEDs are in an antiparallel formation already, so can save space on the PCB.

The pins are:

1 = LED #1 anode
2 = Shared cathode
3 = LED #2 anode

so, connecting pins 1 & 3 together at the anode side of a diode's wholes on a PCB, and pin 2 at the cathode side will give you exactly the the same effect as 2 separate diodes in antiparallel, but in half the space.

as to clipping threshholds, i am unsure. i would guess that red and green being different colours, and thus different materials, renders each side of the tricolour LED a different drop voltage. so essentially, (i guess) you can have a full LED asymmetrical setup in the space of 1 diode. seems pretty cool to me!

the other diode slot then could be used to accommodate something else then, maybe a resistor to control the diodes, of a low value (pF range) capacitor to smooth out the distortion, though, LEDs' tone is much rounder, softer, warmer than other diode types, so this may be an unnecessary addition to the circuit.

I think that what i have said here is all good and correct, i am tired though haha.

I hope i helped here, this being my 1st post and all

[wavley]

I have green LED's in something I built, of course I did a lot of tweaking in the circuit, but I think the pedal sounds fantastic.

[GGBB]

1 = LED #1 anode
2 = Shared cathode
3 = LED #2 anode

so, connecting pins 1 & 3 together at the anode side of a diode's wholes on a PCB, and pin 2 at the cathode side will give you exactly the the same effect as 2 separate diodes in antiparallel, but in half the space.

I don't follow.  The way I see it is that this would put the two diodes in parallel.

[TheRiffGod]

sorry yeah, i had a total brainfart there. 

you're correct.

[earthtonesaudio]

Check datasheets.  You can get them in nearly any configuration.  Parallel, anti-parallel, series, independent, etc.