Led types

suspiciousminds

If you use leds for clipping in effects pedals is there a sound difference when using either 3mm or 5mm?
And why do different colours give different sounds?

ecc83

One of my jobs was to setup A/B tests of conventional LEDs and SM jobs to see if they altered the sound. The "golden ears" on the firm said not to any great degree.

LEDs are different colours because they use  different semiconductor materials and these have different forward voltages (LEDs are like Zeners, they have a fairly stable voltage across them)

The first LEDs were Gallium Arsenide and have a Vf of ~2.2V . IIRC greens are GAPhosphide and Vf is a wee bit higher...FKs what blue LEDs are but Vf is I think about 3volts?

Your bog S silicon diode is 0.7V and Geraniums 0.2V.

Dave.

frankus

Have a wisdom for an interesting question, here's what I found:

http://www.diystompboxes.com/smfforum/index.php?topic=85219.0

ChrisMusic

L.E.D.s ~ do they colour your sound ~ what's the 'current' thinking, can anyone shed some light on it     X_X

Rocker

ChrisMusic said:

L.E.D.s ~ do they colour your sound ~ what's the 'current' thinking, can anyone shed some light on it     X_X

Red if you want a hotter sound, blue if you are into SRV

ThorpyFX

its all about forward voltage.....

ThorpyFX

http://i335.photobucket.com/albums/m447/thorpy6/File 27-04-2015 14 26 32.jpeg

they do all sound different, and its because the voltage that they clip at is different. a blue led requires more voltage to clip than a red led. mixing less can be fun as it gives uneven clipping that can sound pleasant to the ear.

suspiciousminds

Hmm, time to experiment i think.

thermionic

Surely the voltage they clip at is only half the story - it depends on the peak-peak signal coming out of the opamp, which is set by the feedback loop. You could get the same effect by changing the gain of the opamp stage. But it's a convenient way of experimenting I suppose.

I remember opening up my Marshall Valvestate and seeing two LEDs on the PCB glow brighter the harder I played... reminiscent of glowing valves!

ICBM

I should probably just scope this myself, but does the colour affect the sharpness of the turn-on point as well as the forward voltage?

suspiciousminds

thermionic;626417" said:

Surely the voltage they clip at is only half the story - it depends on the peak-peak signal coming out of the opamp, which is set by the feedback loop. You could get the same effect by changing the gain of the opamp stage. But it's a convenient way of experimenting I suppose.

I remember opening up my Marshall Valvestate and seeing two LEDs on the PCB glow brighter the harder I played... reminiscent of glowing valves!

This is what prompted me to ask tbh as i've just bought a madbean boneyard clone kit off this forum and the leds glow like that - its really neat to the point that when i house the pedal up i may drill the casing so the leds can be visible.

ChrisMusic

ChrisMusic said:    L.E.D.s ~ do they colour your sound ~ what's the 'current' thinking, can anyone shed some light on it     X_X

@thorpy6 said:

http://i335.photobucket.com/albums/m447/thorpy6/File 27-04-2015 14 26 32.jpeg

they do all sound different...

OK I think I've got it sussed

Chris'   ~    "Colour Sound" L.E.D. stomp box decoder: 

Infrared =  Warm up those Pot Noodles

Red =  Obviously for really hot tones, just like your red channel.

Orange =  Perfect for pushing an amp of the same name, can enhance any guitarists image by use of the "excessive fake tan" control

Yellow =  For getting the elusive driving Yello sound - see samples below -

The Race from 1988 

Duration 3.42

Oh Yeah

Duration 3:02

Yello - Touch Yello - Virtual concert

Duration

Green =  The environmental solution & the secret ingredient in the Peter Green sound, use with care, you know where dropping too many greens may get you !

Blue =  Da Bluze, obvious, innit ?

White =  Perfect for White Stripes mayhem, and a Meg-a bottle of Jack...

Ultraviolet =  Only because they couldn't spell Ultra Vox properly, also see ingredients, Indium Galium Nitride, definitely a recipe for a pedal for Indie noizez.

Hope that clarifies the technicalities    :o3

imalone

thermionic said:

Surely the voltage they clip at is only half the story - it depends on the peak-peak signal coming out of the opamp, which is set by the feedback loop. You could get the same effect by changing the gain of the opamp stage. But it's a convenient way of experimenting I suppose.

I remember opening up my Marshall Valvestate and seeing two LEDs on the PCB glow brighter the harder I played... reminiscent of glowing valves!

Yes, but I guess that relates to ICBM's question of the sharpness of the turn on, as you change the op-amp gain that will change. If I remember the circuits correctly, clipping depends on the reverse bias though, which isn't necessarily the same as the forward voltage.

The valvestate thing I'd guess is a current supply to part of the amp, there's a trick where you can use the voltage drop you get across a diode (including LED) to set the bias of a transistor, allowing you to provide quite a stable current source, which is useful for various amplification circuits. Blue and green LEDs having a larger forward voltage than red ones makes them better for that application (and LEDs in general better than normal diodes), though there's a better circuit that involves two transistors. LEDs used this way carry varying current according to what's happening in the bit of the circuit being driven.

ecc83

I has just discovered that an LED typically has a reversed biased capacitance of 50pf.

Not a lot but enough to modify the HF response of an op amp in a high impedance circuit. Then, once the LED conducts the capacitance disappears. Could this be a mechanism to explain the different sounds claimed for different LEDs?

Dave.

thermionic

imalone said:

The valvestate thing I'd guess is a current supply to part of the amp, there's a trick where you can use the voltage drop you get across a diode (including LED) to set the bias of a transistor, allowing you to provide quite a stable current source, which is useful for various amplification circuits.

No, it's just clipping diodes to ground on the output of an opamp stage. I'm guessing it was very similar to the Guv'nor or Bluesbreaker pedals of the same era, just built into the drive channel of the amp.