Can a LED cause noise/hum in a circuit?

[culturejam]

I recently built a Big Muff from a GGG kit (built to the "GGG-tuned" specs), and I love the sound. I first built it without the LED because I wanted to test and tweak before I committed it to an enclosures, and I usually do the LED wiring last. After connecting the LED, there is now a very loud hum whenever the effect is engaged. The only thing different in the circuit is the addition of the LED, which I got from Smallbear ( LED T-1 3/4 5 mm Water-clear High-brightness, Orange ).

I've noticed that if I back off the Sustain knob about 10% of max, the hum gets much quieter. The "ggg-tuned" version of the circuit is quite hot, and there was some hum before I added the LED, but it was acceptable. Now, it's very distracting.

Is it possible that the LED is causing the hum?

Thanks.

[foxfire]

did it go in to an enclosure before or after you added the led? i'm thinking that it may be a matter of wires cross talking not the led. now that the wires are all packed into their new home they might be too close for comfort.

[R.G.]

It's possible that either (a) you accidentally messed up the wiring a bit when you added the LED or (b) your power supply can't handle the additional current loading of the LED for some reason. You don't mention how it's powered. Does it do the same when powered by battery?

[culturejam]

It's possible that either (a) you accidentally messed up the wiring a bit when you added the LED or (b) your power supply can't handle the additional current loading of the LED for some reason. You don't mention how it's powered. Does it do the same when powered by battery?

Bingo! I was powering it with a Boss 9v power supply and getting the hum. When I switched to battery, the hum went away. It had crossed my mind to try a battery, but for some reason, I didn't.

So can I put a resistor in between the LED and the board to cut the draw? If so, what should I use, a 47k? (that value seems popular with LEDs)

Thanks!

[Barcode80]

the resistor i think you are noticing in other builds is a current limiting resistor from the power supply to the LED, not from the board. if you didn't place a current limiting resistor between the power and LED, i think that could explain the excess current draw. although, if you are using a boss supply, it may be defective. no boss supply i've ever seen has introduced hum with a little LED, unless maybe you are running an excess of pedals off it.

[jakenold]

I would go with 2k-10K for water clear super bright leds. A trimmer with a small resistor in series (390 ohms or so) works well for fine tuning the value.

[culturejam]

Thanks for tips, everyone!

I'll play around with a few different resistors until I kill the hum.

[Paul Marossy]

Those wall warts often are a source of hum in guitar pedals, especially if they are not regulated and/or filtered. Craig Anderton suggests putting a a 100 ohm in series with and 1000uF cap and across the power supply to kill the hum. It works pretty well in my experience.

[culturejam]

Those wall warts often are a source of hum in guitar pedals, especially if they are not regulated and/or filtered. Craig Anderton suggests putting a a 100 ohm in series with and 1000uF cap and across the power supply to kill the hum. It works pretty well in my experience.

I think I'm going to build my own power supply from a Smallbear kit.

This is the only pedal I've had a problem with a power supply, but it's also probably the highest-gain circuit I've built. I'm going to do some testing and see if I can make the hum repeat on other pedals.

[PerroGrande]

This is the only pedal I've had a problem with a power supply, but it's also probably the highest-gain circuit I've built. I'm going to do some testing and see if I can make the hum repeat on other pedals.

This is probably not coincidental.  The higher the gain, the more likely "small" problems will become "big" (audible) problems.  (The same can be said for frequency, but we're not working with RF here...).  Noise can creep in from any number of sources, and the power supply is certainly one of them.  A good, clean regulated supply will certainly help, as will good shielding, ground loop avoidance, and construction technique.