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LED biasing in preamp - cap in parallel required?

I'm looking to implement LED biasing on a guitar amp I'm prototyping currently.

Is it necessary to install a small capacitor in parallel to ensure current is always flowing and avoid switching noise?

I've read advice to do this, but can only find schematics with the LED on its own... One source suggests a 100nF in parallel.

I'm looking to use LED biasing in two locations - on the 1st stage (Merlin suggests this as it potentially lowers noise) & on the 2nd stage which features local negative feedback (an LED will remove a low frequency roll-off pole from the equations).

Thanks for any advice.


Paid Member
2007-09-15 8:14 am
For very low noise you should shield the LED from extraneous ambient light... LED's also work as photo diodes albeit very very badly.

The true electrical noise from an LED is so low anyway that it hardly seems necessary to add capacitance across the device in an application like this.
Thanks guys. Is this true even if the stage is driven well in to cut-off?

I initially tried without caps and all was fine. I only installed after reading that the LED would switch off when the valve is in cut-off and take a bit of time to switch back on again (what I assume would be the cause of switching noise). Hence the cap to supply current during the cut-off period. That was my understanding, but I am no expert - only repeating what I read...

I'm using cheapie multipack LEDs from Maplin (RadioShack eqv) - a red LED giving 1.6V and an IR giving 1V.
It is detectable in a forward bias condition but in practice its a non issue. Post #73244


Isn't this similar to what happened with the Raspberry Pi? When people took photos of it with the flash on, it would power cycle due to a diode somewhere on the board picking this up and causing a voltage spike. The solution was to stick a bit of blutac over the offending diode. Or stop taking photos of it.


2009-03-13 9:09 pm
I built a preamp using LED's in the cathodes instead of resistor/cap combo. It works beautifully. Think of it as a constant voltage source. I also found that using clear, orange LED's have the best sound. I tried white and blue and they sounded kind of gritty.
Another important aspect is that the LED should have at least 5 ma. flowing through it so that it's on solidly. If you starve the current, the LED voltage will become rough and unstable. Thus damaging the sound.
In my case I needed 2.5v for the cathode, so I added a 1N4148 diode in series to the LED to increase the voltage.
...Think of it as a constant voltage source. I also found that using clear, orange LED's have the best sound. I tried white and blue and they sounded kind of gritty.

Those other color LEDs have different voltage drops, so trying to use them will misbias your preamp tube.

Another advantage :D is that orange LEDs will look like really tiny tubes (similar color to heaters)... :D
I've a fairly low B+ on the triodes where I want to use the LEDs, so only have the choice of IR and red to get a suitable bias... But it is nice to see the red one flash when you hit a chord.

Looking forward to getting Merlin's new book and see what is in there about diode biasing in general. I can see a downside in guitar amps as a lot of the tone shaping (on the bass side of things) has to be achieved by the coupling cap. But for my current build LEDs seems to work nicely.