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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.
http://www.diystompboxes.com/smfforum/index.php?topic=85219.0
Red if you want a hotter sound, blue if you are into SRV
Nil Satis Nisi Optimum
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"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
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.
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.
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.