noise/hum with LED indicator added?

[Rodgre]

I've noticed that I'm getting hum on some pedals when I add the status indicator LED. Is there something that I should be doing to keep it from humming? Is it an issue with the size of resistor on the LED? Where I'm tapping power for the LED from? The kind of LED?

Roger

[brett]

Hi
are you using a battery or a wall plug?
If there is some ripple in the power supply, the LED might be "cycling" on(ish) and off(ish).
And supply the LED direct from the power source, not via the PCB/Vero.
cheers

[Rodgre]

It happens with a DC adapter. One is a Crybaby which I retrofitted with a 3PDT and a status LED and the other is a DIY ROG Matchbox.

Is there some sort of filtering I should be doing to the LED? I'm kind of baffled here, as I am not sure how the LEDs are inducing hum, but they are.

Come to think of it, this happened with my DIY Shure Leveloc (Sherlock) that I built a few years back. Without the power LED, it's nice and quiet. With it, it's humming.

Roger

[brett]

Hi
for good PS filtering, try a series 3.3 ohm resistor then a 2200uF cap to ground, followed by another 3.3 ohm resistor and a 2200uF cap to ground followed by a 0.1uF film cap to ground.  (ie 2 single pole low pass filters with fc = 22Hz)
I've built a couple of those for people opering from generators, near arc welders and with bad flourescent lights, and it seems to filter even the worst rubbish on the line.
cheers

[alarm]

Hi,

This is my first post

I have breadboarded a rangemaster circuit and when this is powered by a battery it works just fine with and without LED.
However when the circuit is powered by a power supply and an LED is added to the cicuit there is an annoying low hum. this is most evident when the gain pot at its lowest. The higher the gain the less hum there is! The circuit works without hum with the power supply but without the LED.

Does anyone have any ideas how to solve this annoying problem as I really want to use both a power supply and have an LED.

Thanks in anticipation

Jonathan

[Gurner]

This may or not help - but I'll put it forward anyway.

I'm presently breadboarding something up myself - I noticed an annoying noise whenever one of my LEDs was activated - I traced it to be a poor gnd connection at the LED itself - the gnd in this instance was from the LED cathode to the breadboard's common 'gnd'/supply rail. (I guess those 'common' rails on breadboard aren't that great!). Soon as I ran a jumper from the LED gnd side direct to the incoming PSU gnd connection the noise disappeared.

so check your grounding in or around the LED.

[amptramp]

If you check some of the wall warts (by breaking them open), you may find that the transformer feeds a diode bridge that goes to the output directly with an electrolytic capacitor in series with a resistor across the output.  With little current drain, the capacitor stays charged.  With more current drain, the capacitor voltage will sag a bit more and introduce more ripple.  Whether there is a discrete resistor in series with the capacitor or the designers count on the ESR (equivalent series resistance) to limit capacitor charge current, the single capacitor is not particularly good.  Add extra R-C filtering inside the unit and this should reduce the hum.  In many pedals, the LED takes more power than the rest of the circuit, so the effect is less noticeable when the LED is off.

As Gurner says, taking the ground to the correct point reduces the common ground impedance and reduces hum in that way.