Riddle Me THIS! (LED Popping)

[aziltz]

I built a BYOC Chorus, with some added goodies like, vibrato, width and a 2nd speed. 


The flashing LED would cause popping, with any LED I tried including the stock boring RED.

But it does NOT pop when I use a water clear super bright BLUE.


What's up with THAT?

[Gus]

popping from leds can be caused by the voltage step on the power supply that is caused by the turn on of the led and the current it needs.  If the circuit is sensitive to noise and ripple etc on the power supply you might hear a pop
If you are using the same dropping resistor the blue most likely has a larger voltage drop, this would reduce the current step on the power supply.
http://en.wikipedia.org/wiki/Light-emitting_diode

Is the led ramped up or is it a square wave drive with "sharp" edges?

[aziltz]

its seems like its squared.  It works fine with blue so I'm not gonna mess with it.

being a physicist, my first guess should have been, blue light, shorter wavelenght, higher energy, higher voltage drop, etc.

[composition4]

Hmmm that's strange.

A couple of thoughts, just speculation though

If a blue LED has a softer Vf knee than a red LED, this would mean the blue diode turns on and draws the initial surge current over a longer period of time than the red, hence the surge isn't so sharp hence less pop.

My second thought was that maybe it had something to do with forward voltage itself rather than the knee shape.  Blue LED has a forward voltage of over 3.5 volts, and red is only about 2 volts.  But when I thought about it, I came to the conclusion that it would mean that the average current drawn over the turn-on times of the LEDs would be different, and it wouldn't make a difference to the initial surge which is what causes the pop.  In fact, if anything because the blue LED has a higher Vf then assuming you have the same current limiting resistor, the initial current draw surge of the blue LED would be higher, hence MORE pop.

So I'm sticking with my first thought, about the knees.  I'm sure someone will come in with a better answer though!

If you do want to use a red LED, you might want to try Jack Orman's advice at http://www.muzique.com/lab/led.htm to slow down the LED turn-on

Thanks
Jonathan

[composition4]

Actually Gus replied whilst I was typing and I like Gus' answer better I think

[aziltz]

actually i wanted to use an Orange, as i was using a blue already for bypass, and the popping LED was for speed monitoring.  I'm happy with two blues.

[aziltz]

this is happening in my Reverb build as well, using a Super Bright Green LED.

[Cliff Schecht]

After your current limiting resistor on the LED, place in a capacitor to ground to do some lowpass filtering on the signal driving the LED. My guess is that the signal driving the LED is too square and the quick rise times (which can be in the nS range) are causing crap to go through your ground and get into the audio. Lowpass filtering will reduce the squaredness of these edges and keep the dv/dt from being so quick that you get that audible pop. The value of the cap would depend on the highest frequency you are interested in.

Also, LED's can spit out a non-trivial amount of return current that if not dealt with properly can cause problems in the audio path. Another trick is to only connect the high noise grounds together at one point, right at the ground of the decoupling capacitor that should be present on the VCC input. This "swamps out" the noise and keeps return currents from appearing in the audio path. We use this trick at PAiA with much success.

[aziltz]

thanks for that advice cliff.

after prodding further, this time it isn't the LED.  The switch makes a mechanical pop, not every time, maybe 1 in 4 activations.

Maybe It will go away with time.