Hi guys, newbie here

So I've always wanted to have LED lights around the contour of my guitar, so I did just that. I have an LED strip that goes around my guitar, the power comes from 2 9V batteries I have wired in series (so ~18V). When I turn the lights on using the provided colour controller (wireless), I get this phaser/space-age sounding hum coming from the guitar. Changing the intensity of the lights only changes the frequency of the hum, not much volume difference.

Im wondering what i can do to eliminate this hum, thanks in advance

 

Welcome to AAC!
The colour controller most likely works using PWM, which involves high frequency pulses of current through the LEDs. The wires linking the LEDs make an effective antenna and radiate interference which your pickup obviously senses. I think the only practical way of eliminating that interference will be to replace the controller with some alternative controller not involving PWM, unless large capacitors could be used to smoothe out the pulse trains feeding the R, G and B component LEDs in the string. Much would depend on the way the LED string is configured.

 

Is there any way you can put a little 'cage' or foil cover around your pickup and ground that cage so that no EMI gets to it? I don't know about guitars, I'm a drummer.

 

Welcome to AAC!
The colour controller most likely works using PWM, which involves high frequency pulses of current through the LEDs. The wires linking the LEDs make an effective antenna and radiate interference which your pickup obviously senses. I think the only practical way of eliminating that interference will be to replace the controller with some alternative controller not involving PWM, unless large capacitors could be used to smoothe out the pulse trains feeding the R, G and B component LEDs in the string. Much would depend on the way the LED string is configured.

Thanks for the answer alec
I just looked it up, yes the controller is a PMW. I have it setup in a way that I could put capacities between the led strip and the circuit board that houses the receiver antenna thing. Which value capacitor are we talking about? And I would wire the capacitor as a bridge bridge between the LED and the circuit board?

 

Is there any way you can put a little 'cage' or foil cover around your pickup and ground that cage so that no EMI gets to it? I don't know about guitars, I'm a drummer.

Thanks MOSFET
2 of my pickups are already shielded with copper tape, they still have the hum.

 

I could put capacities between the led strip and the circuit board that houses the receiver antenna

Without knowing a lot more about the LED strip (e.g. are there respective R,G B analogue buses going to series-parallel LED strings, or is serial data transferred to smart tri-colour LEDs) its unclear if/where capacitor placement might help.

 

Changing the intensity of the lights only changes the frequency of the hum, not much volume difference.

That's because PWM does not change the amplitude of the drive voltage.

I don’t know how fancy your RGB controller is, but putting caps on the output of a PWM controller will defeat its main purpose, that being to control the brightness and blend the colors.

You may find that by the time you have enough capacitance to kill the hum, you may also be at full brightness for the LEDs, or somewhere close. (if it works at all)

Once you narrow down the brightness control of the PWM controller, you may lose the ability to generate more than 6 colors and any color not requiring a 1 to 1 mix such as orange will not be produced. But this may not be a problem, depending on how fancy the controller is in the first place.

I also know that some PWM controllers do not like caps placed on their outputs and tend to scream like a banshee in protest. (not saying yours will, but I have tried this on some and got that exact result)

Just some things to be aware of.

 

There is an excellent chance that putting capacitors across the PWM output will kill the device creating the PWM signal. An LC low pass filter could probably turn the PWM into DC without hurting anything except the hum.



The component values will depend on the frequency of the PWM signal. A resistor can be used in place of the inductor but reduced "brightness" would result.

Since you can hear the PWM frequency and you are a musician you can probably tell us what the PWM frequencie(s) are.

 

You will want a controller that uses a high(er) frequency PWM. Some LED controllers will do this. For example this one supports "analog" dimming which uses a voltage level to determine PWM duty cycle. From the data sheet, it looks like the PWM frequency is 50 KHz which *should* be OK. It's not super convenient because you'll need a high enough voltage source to drive your LEDs configured in a string and you will need three for RGB LEDs. There may be better controllers that build in a boost converter as well.

 

Without knowing a lot more about the LED strip (e.g. are there respective R,G B analogue buses going to series-parallel LED strings, or is serial data transferred to smart tri-colour LEDs) its unclear if/where capacitor placement might help.

Attached is the setup, each RGB has its own output and input from the circuit to the led strip, as well as a V+ connection. Where would you throw in a capacitor in there?
Mod edit: remove duplicate pictures

 

Attached is the setup, each RGB has its own output and input from the circuit to the led strip, as well as a V+ connection. Where would you throw in a capacitor in there?View attachment 148448 View attachment 148449 View attachment 148448 View attachment 148449 View attachment 148449

Is just 2 pictures, I don't know why posted so many times

 

That's because PWM does not change the amplitude of the drive voltage.

I don’t know how fancy your RGB controller is, but putting caps on the output of a PWM controller will defeat its main purpose, that being to control the brightness and blend the colors.

You may find that by the time you have enough capacitance to kill the hum, you may also be at full brightness for the LEDs, or somewhere close. (if it works at all)

Once you narrow down the brightness control of the PWM controller, you may lose the ability to generate more than 6 colors and any color not requiring a 1 to 1 mix such as orange will not be produced. But this may not be a problem, depending on how fancy the controller is in the first place.

I also know that some PWM controllers do not like caps placed on their outputs and tend to scream like a banshee in protest. (not saying yours will, but I have tried this on some and got that exact result)

Just some things to be aware of.

 

You will want a controller that uses a high(er) frequency PWM. Some LED controllers will do this. For example this one supports "analog" dimming which uses a voltage level to determine PWM duty cycle. From the data sheet, it looks like the PWM frequency is 50 KHz which *should* be OK. It's not super convenient because you'll need a high enough voltage source to drive your LEDs configured in a string and you will need three for RGB LEDs. There may be better controllers that build in a boost converter as well.

So you're suggesting a different controller?

 

Probably way off base here, but wouldn't the filter idea work better on the pickups out put, instead of the led controller itself?

 

Probably way off base here, but wouldn't the filter idea work better on the pickups out put, instead of the led controller itself?

That would probably have an adverse affect on the tone.

But the real issue is wedging a digital switching source among next to sensitive, relative high impedance analog audio inputs. Lots of wiring inside a typical guitar is not shielded cable, relying instead on (usually) a conductive surround - copper tape, nickel spray paint etc. Inside the surround, it needs to be electrically quiet or as others suggest, running at a way-past-audio frequency.

 

Probably way off base here, but wouldn't the filter idea work better on the pickups out put, instead of the led controller itself?

I'm with John on that one, don't wanna mess with the tone

 

So you're suggesting a different controller?

Yes. I'm saying the problem is your controller. It's using a PWM frequency that is too low - in the audible range. If you filtered out the PWM frequency, you would have no LED control.

You might get away with just using a (much) higher PWM frequency. I doubt the controller you have could be easily mod'd to do that. The best frequency would be above the highest desirable overtone of the high E string.

You might try neopixels (WS281x based LED strings). The controlling signal is like 800 KHz but I think internally they use PWM which could be just as much a problem.

To do it "right" you'd probably need to go to current control of the LEDs which means replacing the LEDs and controller.

As to filtering at the pickup. Since the PWM frequency is in the audible range, that would filter out the frequencies the guitar strings produce. Maybe a really tight notch filter would be ok but still think it would adversely affect it.

 

Oh, and if you used a high frequency PWM, you might need to put a low pass filter on your pickup though your amp may already have a filter.

 

Where would you throw in a capacitor in there?

I wouldn't, having now had a glimpse of the setup.

 

That would probably have an adverse affect on the tone.

That's kind of what I thought,but haven't done much audio stuff. That's why I prefaced it with,"may be way off base". But I always learn something with my suggestions here.