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Topic: I think I've got a ground loop messing up my audio. Help! (Read 4 times) previous topic - next topic

scswift


OK, what you need is serious decoupling on the LED modules. On each one, connect a 100uF or greater electrolytic in parallel with 0.1uF or greater ceramic between the +ve feed to the LEDs and the ground pin of the TLC5947. Preferably, also connect an inductor between the incoming LED power feed and the junction of those capacitors with the +ve side of the LEDs. As an alternative to an inductor, you could use a low value resistor, if you can afford the voltage drop it will introduce. The idea is to get the LED switching current circulating around the capacitor, instead of feeding down the power and ground lines.


How would I go about selecting an inductor for this purpose?

Also, do you have any suggestions as to how I could isolate the audio portion of my circuit better?  The noise from the LED modules when running the amp on it's own power source is nothing compared to the noise generated when I connect servos to my circuit without powering them from their own battery.  My board is very small and I can't go attaching huge capacitors to every IO port.  Is there some combination of voltage regulator, capacitors, and inductors that I could put in the audio circuit that would minimize the noise from the servos, and presumably take cake of the noise from the LEDs as well? 

I don't think a voltage regulator alone would cut it because I've read they don't typically reject noise over 100hz, and the ground would still be connected and I suspect noise would get in there.

scswift

Btw, in case you're interested, here's a video of the board in action.  You won't hear the LED noise because in this demo the board was being powered by a battery and the amp from a wall socket.  It was only a few days ago I learned of this issue when powering them both from the same battery:

http://www.youtube.com/watch?v=mLGnVd03MxM

scswift

So today I had the bright idea of having my friend touch the battery ground to the collar around the RCA connector on the amp to see if that made the hum go away.  It did, but it also made the sound really quiet, so it didn't solve the problem. 

I'm wondering though, what caused this behavior?  I thought it might remove the hum because of a ground potential difference causing the ground loop, so I figured the battery ground would bring the ground at the RCA input up/down to where it should be, but I can't explain why this would make all the sound, rather than just the hum, go away.  Any ideas?

I also had the bright idea this evening to have him try a "ground lift", and simply disconnect the ground on the RCA cable.  Without a shield in place this might cause it to be more susceptible to RF but right now I'm just hoping to get something resembling normal audio out of the thing.  I'll have him try that in the morning.

And of course the ground loop isolator is in the mail so that will be getting here tuesday and we can try that. 

But if the ground lift works, that would be pretty sweet, since that's a cheap solution.  I am afraid it won't though given the lack of any success so far.  I had him try a "partial ground lift" before with a resistor on the negative RCA input and it did nothing.  I am also afraid that if that doesn't work, how can the ground loop isolator?

dc42

#13
Feb 25, 2013, 09:19 am Last Edit: Feb 25, 2013, 09:21 am by dc42 Reason: 1

How would I go about selecting an inductor for this purpose?


I've done some sums, and from the datasheet I think the PWM frequency of the TLC5947 should be around 1kHz (based on 4MHz nominal internal clock and 4096 clocks per PWM cycle). If you use a 100uF decoupling capacitor, that will give an impedance of 1.6 ohms to ground. To get a 10-fold reduction in the amplitude of the unwanted signal, the impedance of the inductor should be 16 ohms, which implies an inductance of about 3.3mH. The inductor needs to be rated at the maximum current drawn when all the LEDs are on. The resistance of the inductor needs to be small enough not to cause an unacceptable voltage drop.

You could also use a larger decoupling capacitor (say 1000uF), which would all9ow you to use a smaller inductor (e.g. 330uH for the same 10-fold reduction at 1KHz).

it is probably the high frequency harmonics that are causing most of the problem, so this L-C combination may be rather more effective than the calculation implies.
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scswift

Thanks for the assistance!

I really hope I don't actually need to put this information to use to solve my problem, because the boards have already been produced and I don't want to have to solder 300 inductors onto them.  Plus I'm not sure I could afford all those inductors at this point. 

But could you show me how you dd the math so that I can learn how to do this sort of calculation myself in the future?

Also, let's say I have my friend hook up his iPod to the amp with the Mighty drawing power from the same battery as the amp, but the RCA cable from the Mighty disconnected.  And let's say he doesn't hear any of this noise.  Would that then mean that the problem is not the power draw of the LED drivers?  I have not actually had him do this test yet, it's something I just thought of this evening.  I'll have him try it today. 

I'm going to have him try "lifting the ground" of the RCA cable as well.  Ie: disconnect it completely.  If that doesn't do something then it will seem unlikely at that point that a ground loop is the culprit. I'm really hoping the problem is a ground loop and not the issue with the power draw as you suggested because the ground loop is an easy fix, but adding inductors and capacitors to all my boards will neither be easy nor cheap. 

Even if I do ultimately solve the problem by eliminating a ground loop, your suggested changes will still come in handy if/when I'm able to redesign my LED modules, because even if a ground loop is at fault, I do still get a little noise when the amp is on a separate power source, and I doubt fixing a ground loop will fix that.  For now, that noise is something I can tolerate though if it means I don't have to do an expensive fix.

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