LED Behavior in Soft Clipping

[psb962]

I built a Splendiferous (Royal Blue) OD and, with the back off, I noticed that the red LEDs in the feedback loop of the op amp are always clipping, in that they flash to some degree on every note, no matter how soft, no matter how low the gain. Obviously, the more gain, the harder I play, the brighter they flash. Given that red LEDs don't do anything until they have 1.7V across them, how is that possible on this circuit?

 

[falco_femoralis]

it's because of the way the feedback loop is set up. A fundamental function of an op amp is that it wants its pos and neg inputs to be equal. In the feedback loop are LEDs and resistors consume voltage, which means that the voltage going back into the neg input becomes less than the pos, so the op amp then outputs more to establish equilibrium. Much of the output current is contained in the feedback loop, and the capacitor C5 blocks any extra DC voltage from the op amp's voltage source from mixing with AC and continuing down the signal path. What's left over continues down the signal path. The LED's are powered using the +4.5v / -4.5v headroom the op amp is given from the 9v DC source.

 

[psb962]

it's because of the way the feedback loop is set up. A fundamental function of an op amp is that it wants its pos and neg inputs to be equal. In the feedback loop are LEDs and resistors consume voltage, which means that the voltage going back into the neg input becomes less than the pos, so the op amp then outputs more to establish equilibrium. Much of the output current is contained in the feedback loop, and the capacitor C5 blocks any extra DC voltage from the op amp's voltage source from mixing with AC and continuing down the signal path. What's left over continues down the signal path. The LED's are powered using the +4.5v / -4.5v headroom the op amp is given from the 9v DC source.

Understood, but let's do the math. If the minimum gain of the noninverting opamp is 1+ R7/(R2||R4), then we have a minimum gain of 1+ 5600/848 ie 7.6.

If the voltage across the feedback loop exceeds +/-1.7V then the input voltage with gain set to minimum must exceed +/-220mV. That's higher than I would expect for even the lightest pluck of a string.

It would be good to know if other soft clipping LED pedals exhibit same behavior. Or if anyone has a real Royal Blue OD, do your LEDs flash no matter how lightly you play at minimum gain?

 

[falco_femoralis]

You are thinking that the voltage thru the feedback loop is equal to your + input voltage, but it isn't. The voltage at the - input is equal to the + input. But there is a voltage drop across each LED and across the resistors. Therefore, the op amp is producing enough voltage to compensate for these drops while maintaining equal inputs.