LM3886 in Differential Amp config to kill ground loops - diyAudio
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Old 17th May 2013, 11:13 AM   #1
minifly is offline minifly  South Africa
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Default LM3886 in Differential Amp config to kill ground loops

I was wondering if using the LM3886 in a differential (inverting) topology would be a good way to kill ground loops. I say inverting because that way the signal ground doesn't even get referenced to power ground except through the chip. Any thoughts on this? Am I wasting my time?
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Old 17th May 2013, 11:20 AM   #2
AndrewT is offline AndrewT  Scotland
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I don't understand.
Differential means two inputs to the chip.
Neither are grounded.
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Old 17th May 2013, 11:28 AM   #3
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Don't have a ground loop in the first place would be the answer or use a balanced line input with the ground lifted. In the PA world we call it a DI unit.
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Old 17th May 2013, 11:34 AM   #4
minifly is offline minifly  South Africa
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My concern is more from a safety aspect, since I want to earth my PS ground, but even doing so through a 5R6 gives me some ground loop noise at full volume when driven by a laptop with a cheap SMPS. If I set up the LM3886 in differential mode, as in :

Click the image to open in full size.

Signal ground connected to V1
signal + connected to V2
That way there wouldn't be any connection, except through the actual opamp itself, between the signal ground and the power ground, completely breaking the ground loop.

Last edited by minifly; 17th May 2013 at 11:46 AM. Reason: argh IMG layout...
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Old 17th May 2013, 11:59 AM   #5
minifly is offline minifly  South Africa
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On second thoughts, that isn't inverting. However, even if it was inverting (signal ground to V2, signal+ to V1) it would still benefit from the R2 and Rg being in the ground-loop path, and those would be several orders of magnitude larger than the 5R6 I am using to break the ground loop currently.
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Old 17th May 2013, 12:21 PM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by minifly View Post
Signal ground connected to V1
signal + connected to V2......
is conventional unbalanced non-inverting.
It is not differential, because you have some parts connected to ground.
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Old 17th May 2013, 12:38 PM   #7
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by minifly View Post
My concern is more from a safety aspect, since I want to earth my PS ground, but even doing so through a 5R6 gives me some ground loop noise at full volume when driven by a laptop with a cheap SMPS. If I set up the LM3886 in differential mode, as in :

Click the image to open in full size.

Signal ground connected to V1
signal + connected to V2
That way there wouldn't be any connection, except through the actual opamp itself, between the signal ground and the power ground, completely breaking the ground loop.
Mmmmmhhhh, that is a differential amplifier, which will reject common mode noise, useful if you have a weak signal through a long cable , as in balanced microphone preamp.

On this case , "balanced" is audio/oldspeak for techspeask "differential", the principle is the same.

Now, if you have *local* grounding problems at the LM3886, that's another problem, solved by better layout at the amp.
A different solution for a different problem.

And as Andrew pointed: if you have an already noisy and unbalanced signal coming from a notebook or whatever, this will not solve any of that.

In fact, this will *faithfully* reproduce your dirty signal
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Old 17th May 2013, 01:06 PM   #8
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by minifly View Post
If I set up the LM3886 in differential mode, as in :

Click the image to open in full size.

Signal ground connected to V1
signal + connected to V2
That will work well IF:
1) R1 = R2
2) Rf = Rg
3) Bottom of Rg is connected to power ground (or whatever the speaker return is connected to)
4) and finally the hard part: The source impedance seen by V1 and V2 must be equal.

e.g. If V1 is directly connected to signal ground, but V2 is connected to
signal + via a volume pot, it won't work. Similarly, if there's a coupling cap between the source and V2, there needs to be an equal coupling cap between V1 and signal ground.

The easiest way to stupid-proof it is to put a unity gain buffer between input signal and V2. That makes it immune to variations in source impedance.
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Old 17th May 2013, 02:12 PM   #9
minifly is offline minifly  South Africa
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Quote:
Originally Posted by JMFahey
And as Andrew pointed: if you have an already noisy and unbalanced signal coming from a notebook or whatever, this will not solve any of that.
The problem isn't the source, the problem is a ground loop. I can get rid of it by disconnecting my amp's PS ground from the mains earth, but that can be dangerous in fault conditions. I can also get rid of it by unplugging my laptop SMPS, but that only lasts until the battery dies. Because the voltage drop which causes the distortion is caused by the V=RI drop in the input cable due to the circulating currents on the ground line but not the signal line, I was thinking that by using a topology which doesn't connect the ground of the signal line to the ground of the amp power supply might fix the problem.
Quote:
Originally Posted by godfrey
That will work well IF:
1) R1 = R2
2) Rf = Rg
3) Bottom of Rg is connected to power ground (or whatever the speaker return is connected to)
Absolutely, that was what I was planning. I should have made that more clear.

Quote:
Originally Posted by godfrey
4) and finally the hard part: The source impedance seen by V1 and V2 must be equal.
Ah, I knew I was missing something, the moment I add a pot it changes the effective resistances. Good call. Any ideas on a nice opamp/buffer that would work on my +-25V rails so that I don't need another PS/regulator stage? Preferably something available from Mantech/Communica/RS?
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Old 18th May 2013, 05:34 PM   #10
minifly is offline minifly  South Africa
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Just thinking, if I need an opamp anyway why not put the opamp in differential config in front of a regular non-inverting LM3886. Then I can stick the decoupling cap and volume pot between the opamp and the LM3886. Of course, I don't get the high-impedance inputs that the buffer version would get, but as long as I keep the opamp resistors above 10k that shouldn't be an issue.
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