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Old 21st September 2005, 03:08 PM   #1
nfway is offline nfway  United States
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Default LED Noise

Hi,

I'm trying to add some funky 4 color LEDs (auto change colors every second or so) to a small guitar amlifier that I built. The problem is that when I connect the LEDs (and required resistor) to my amp circuit the LEDs make a really nasty humming sound, even when the volume is all the way down. The humming increases and decreases when the color changes occur. Is there any way I could attach these LEDs and eliminate the noise?

Any help would be greatly appreciated!

Many thanks!
N
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Old 21st September 2005, 05:48 PM   #2
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what kind of amp have u got? a solid state or a tube-amp?
and to wich voltage supply have u connected the LED and R?
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Old 21st September 2005, 09:52 PM   #3
nfway is offline nfway  United States
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The amp I have is a 20W portable solid state amp with a rechargeable battery. The volume control is also the on/off switch which carries 12V DC to the circuit. I have tried connecting the LEDs and R to everywhere I can think of, but the bottom line is that I have to use the switching circuit to turn them on and off along with the amp.
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Old 21st September 2005, 10:22 PM   #4
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Do not put LEDs in your signal path!

Do you want the LEDs to blink in time with the music? Use a capacitor (to select frequency response) and a transistor. We used to build these "color organs" all the time in high school, except we used SCRs and 120VAC lights.

Wes
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Old 21st September 2005, 10:30 PM   #5
SY is offline SY  United States
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There's a strong possibility that the noise is from ambient light- LEDs make lousy photodetectors. Try shielding them from light (black tape or whatever) an see if the hum goes away. if it does, you may not have a fix, but at least you know the cause.
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Old 21st September 2005, 10:39 PM   #6
nfway is offline nfway  United States
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First off, I'm not putting the LEDs in the signal path. The volume control has a built in switch that has nothing to do with the signal. The switch is seperate from the connections for volume. The connections that I am using are solely for power to the circuit and have nothing to do with the signal path.

Secondly, I have tried blocking ambient light, and..... no dice. The hum is still there, and, as I said, it comes and goes with the fade in and out of the color changes. The LEDs switch from red-green-blue-yellow. This makes a very cool effect behind the clear control panel..... if I can get it to work.

Thanks,
N
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Old 21st September 2005, 10:43 PM   #7
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try running the LED, in it's intended physical location, but off of a seperate supply/battery with no connection to the guitar amp circuit and then see if it makes noise.
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Old 21st September 2005, 10:44 PM   #8
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Can you draw a quick sketch of what you've done here? It's possible that the LEDs are in the signal path but not in an obvious way (e.g., grounding).
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Old 21st September 2005, 11:21 PM   #9
nfway is offline nfway  United States
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Using a seperate battery to power the LEDs eliminates the noise completely, but this does not help me because I wanted to use the internal battery that also powers the amp to power the LEDs. Adding a seperate battery just for the LEDs would be way too much of a hassle just to get it to light up.

I've attached a very basic drawing of what I am doing to give you a better idea of what is going on here. The grounds are all tied together and terminate at the battery.

Thanks,
N
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Old 21st September 2005, 11:37 PM   #10
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The diagram does not show where the grounds go. You might be injecting noise from the LED (the mulicolor LEDs, the blinking ones, and the kind that don't need resistors (current source LEDs) have electronics inside them which might make noise, espeically when they "fade" from one color to annother, since LEDs work more efficiently when you pulse them on and off at full power rather than reducing the power level) into the ground. Try moving the ground of the LED to somewhere else, prefereably as far away from the input jack and preamp grounding circuitry as possible. Try to connect it to somewhere around the battery.
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