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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


No, that's how it's done.  A common mantra around here is "tie ALL grounds together" because everything needs to know what +5v is in relation to (for example.)  Same goes for audio. 


If I had a multimeter and I touched one probe the the RCA - and the other to the RCA +, it would be able to read the voltage even though the multimeter's ground is not connected to the circuit.  I see no reason an amp should need to share ground over an audio input to determine the voltage difference. And TI does have a document showing various "ground loop break circuits" for both input and output.  And of course the transformer worked. 

So that may be how it's done, but I question why it's done that way.  Perhaps it has something to do with reducing interference?  Or maybe it's just a cost reduction measure, keeping things simple?  I don't know.  I do know professional audio equipment circumvents this issue and the interference issue by using a differential setup though.

SirNickity

Single-ended (ground + signal) inputs always require the ground to be connected.  You can't have a circuit with one wire -- that defies the definition of a "circuit."  There has to be a return path.  It may not be immediately obvious -- like, with your setup, the amp's ground lead is derived from the same source as the audio ground.  It can also be floating, like you achieved with the isolation transformer.  That's the same result you'll get with another AC-powered device that has a separate PSU from your amp, or a battery-powered device.  But with either isolated device, if you don't connect the RCA shield, you'll lose the return path and get nothing (but maybe noise) on the input.

Differential inputs work a little differently, in that the potential measurement is between the hot and cold.  They reference each other.  The "ground" is only there as a shield, and thus, can be lifted on one side to receive RF noise and shunt it to ground.  (Acting like an antenna, as someone pointed out earlier.)

In your multimeter example, you're grounding the negative probe.  Try measuring the voltage at the RCA output with just the red probe, leaving the black probe lying about on your workbench.  That's what you get in your circuit if you don't ground the RCA terminal.  So, that's why it's always done that way.  :-)

scswift


In your multimeter example, you're grounding the negative probe.  Try measuring the voltage at the RCA output with just the red probe, leaving the black probe lying about on your workbench.  That's what you get in your circuit if you don't ground the RCA terminal.  So, that's why it's always done that way.  :-)



That's not what I get in my circuit if I don't ground the RCA terminal.

If my amp and my microcontroller are both connected to the negative terminal of my battery, and in my circuit, they are, then in theory, ground should be the same on both devices, and I should not need another ground wire in the RCA cable to perform a measurement on it.

If the amp and microcontroller are NOT connected to the same battery, then of course there is no shared ground, and a single wire for the audio connection is insufficient, and mimics your example.

Now going back to my first example, note that I said "in theory".  Obviously the reality is that the microcontroller's ground is at a different potential level than the amp.  But why should that mean we can't make a measurement of the input without having a ground loop?

A multimeter measures potential difference.  So in theory, if I touch my black probe to one wire with 1V, and my red probe to another with 5V, I should read 4V.  Note that neither of those is ground.  Yet I can still measure the difference between them.  And that difference is what matters.  The magnitude of that difference tells me enough to know where to position my speaker cone.  That's how differential inputs work.

My problem seems to have been caused by my grounds being a different potentials on my two devices, so current flowed from one to the other and carried noise along with it.  But if the amp only measured the potential difference between the + and - input, it wouldn't matter if the - input was at a different potential than it's ground.  All that would matter is that it is at the right potential level RELATIVE to the + input... just as with a differential input where + and - swap places.

scswift

I tried something new with the circuit today because I have an amp to test with again.  I placed ceramic capacitors on both + and - RCA lines.  But this did nothing to help with the noise.

The thing is, I don't understand why it didn't help.  I've been told this is a way to break a ground loop.  And it certainly should block any DC current from flowing, just as the transformer in a ground loop isolator does.  So why didn't it help with the noise like the transformer did?

A transformer is really just a couple inductors, right?  So might the inductance be doing something to filter the noise?  I'm not sure how this could be, since the noise is smack dab in the middle of the audio range and filtering out those frequencies would filter out audio frequencies as well I'd imagine and affect the sound of the output.  But I have not noticed any change in the audio with the isolator in place.

I intend to add some more filtering to my LED modules when I do a second run of them, but I would really like to be able to emulate what this transformer is doing without using a huge transformer on my board when I redesign it, in case my changes to the LED modules are inadequate.

SirNickity

I missed your last reply, so let me back the conversation up a couple days first.

I think you might be assuming "Ground" is somehow special.  It isn't, it's just a reference point.  You can probe any two points in a circuit with a multimeter and get a measurement.  Ground isn't any different in this regard, it's just that normally ground is the reference on which everything else is based.  So, when you say something is "+5v", that's 5v above ground.  Ground is only 0v because you're comparing it to itself, so the difference should very well be 0v.

Differential is a special case because it doesn't reference ground, it references another signal line.  Electrically, it's still a measurement between two points, it's just that neither of those points are (necessarily) ground.  Single-ended inputs imply that the reference is ground, and requires the system and input grounds to be tied together.

Again, in your case, the amp system and input grounds are tied together in two places:  The PSU and the RCA input jacks.  So you have a loop.  It doesn't mean there's any appreciable current flowing through that loop, it just means there is one.  That's not a problem in and of itself, and not something to be "fixed".  The problem is when there's a difference in potential between those two ground points.  The best way to fix that is to ensure the two ground points have low impedance paths back to the PSU ground.  (Easier said than done sometimes.)

If you take away the RCA shield, you're causing the amp to reference the PSU ground as the audio ground (since they're connected internally anyway).  The audio signal will then be referenced against a ground that is not at the exact same potential as the device creating the output (the Mighty), so there will be noise which is the difference between ground as the Mighty sees it and the (PSU) ground as the amp sees it.  Using a signal ground (RCA shield) will attempt to force them (the grounds) to the same potential, at the risk of causing current to flow down the path of least resistance, which may alternate as the load changes.

Ceramic caps across the input will shunt noise, but it will do so as a high-pass filter with a high cutoff frequency.  Typically, you would use, say, a 30pF cap.  This dumps RF noise straight to ground, but noise throughout (and even well above) the audio band will not be affected.  It can still be helpful if the audible noise is a lower harmonic, but that's a long shot.

The transformers help probably (not 100% sure -- comments welcomed) because the path through it becomes high impedance.  Noise is going to take the easiest path to the point of lower potential (Ground again), which will probably be elsewhere.

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