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Old 9th July 2009, 04:46 AM   #1
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Default how to isolate USB/audio ground??

possibly a dumb newbie question here, but basically I just need to know how I can power an amp using a USB port to boost volume on the computer with using the same ground for the amp. It's a basic 2x LM386 audio amps Im building to make a little portable laptop sound boost. Sounds great when audio is pulled from another source (even plugged into another comp) but when the USB is powering it and the audio is from the same source (sharing ground) it sounds so horrible its worthless. Any suggestions? Many thanks in advance!
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Old 9th July 2009, 08:12 AM   #2
h_a is offline h_a  Europe
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I'm not sure I understand you fully here.

So it works with a different pc, but 'sounds horrible' with another? You mean you get tons of humm/buzz? Or is there a problem with the sound itself? Ground can only be an issue in the first case (humm/buzz) and that you can fix by using a good grounding scheme (search for star ground).

understanding star grounding

Have fun, Hannes
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Old 14th July 2009, 01:15 PM   #3
Telemin is offline Telemin  United Kingdom
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I think the problem is that you are forming a ground loop with two ground connections to your computer one either side of the amplifier chip.

I may be having a total brainfart but I think the best way of getting rid of a ground loop is simply to break the ground line connection between your computers audio out and the amplifier.

If that fails you can always use a pair of audio isolation transformers, one for each channel.

Hope this helps

Phil
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Old 14th July 2009, 03:22 PM   #4
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Default Re: how to isolate USB/audio ground??

Quote:
Originally posted by audiomodder
possibly a dumb newbie question here, but basically I just need to know how I can power an amp using a USB port to boost volume on the computer with using the same ground for the amp. It's a basic 2x LM386 audio amps Im building to make a little portable laptop sound boost. Sounds great when audio is pulled from another source (even plugged into another comp) but when the USB is powering it and the audio is from the same source (sharing ground) it sounds so horrible its worthless. Any suggestions? Many thanks in advance!

Depending on the configuration, the USB will supply +5V at either 100mA or 500mA. Can your amp work on that?

Jan Didden
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Old 15th July 2009, 07:53 AM   #5
Telemin is offline Telemin  United Kingdom
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@Janneman

Looking at the datasheet I would imagine as long as the USB bus can supply 500mA then it should be far more than enough even for a pair of LM386 at maximum output, and at normal levels even 100mA will be sufficient current.

Given the symptoms of the problem as well (ie. it works fine provided audio output and usb power come from different machines) I stand by my suggestion that a ground loop is being formed and some kind of filtering or ground interruption need to be used to prevent the problem.
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Old 15th July 2009, 12:31 PM   #6
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Default Re: how to isolate USB/audio ground??

Quote:
Originally posted by audiomodder
...Im building to make a little portable laptop sound boost...
If the USB source is a laptop, you may not have a ground loop because there is no ground connection for the laptop.

But then you say:

Quote:
Originally posted by audiomodder
... but when the USB is powering it and the audio is from the same source (sharing ground) it sounds so horrible its worthless...

So I am confused. What is the shared ground?

Dave
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Old 15th July 2009, 01:03 PM   #7
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USB Power can be pretty dirty -- you might try some ferrite beads, or a ferrite clamp on the USB power line. If you don't have any ferrite beads, try a tantalum cap -- 1 uF will do -- right adjacent to the LM386 Vcc pin.

fwiw -- you can get around this a bit by using a Class-D amp chip, not that I dislike the LM386.
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Old 15th July 2009, 02:33 PM   #8
Telemin is offline Telemin  United Kingdom
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@Dave Shared ground doesn't literally mean sharing an earthed connection, in truth the idea of a ground plane is completely arbitrary and electric fields only care about difference between two points not the absolute value of voltage. As such it is possible to have the ground line of the laptops audio output at a different potential to the level of the usb ground line, i.e a voltmeter will show a potential difference between them.

The upshot of this is that when you treat these two different ground lines as if they were at the same level and connect them together you cause a current to flow from one to the other. That is the essence of a ground loop.

Hence if you break the connection between the two grounds, i.e remove the audio ground connection to the rest of the circuit, you break the ground loop and get rid of the buzzing.

@audiomodder

Since all you need to do is limit the current flowing through the ground loop to take the buzzing below the noise floor you could try putting a 200R resistor in the ground line of the audio cable before it connects to the rest of your circuit.

Phil
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Old 29th July 2009, 07:05 AM   #9
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thanks for the help guys!
i solved the problem- turns out the audio inputs just needed 3k resistors to terminate to ground rather than 10k. also discovered the wire i was using for output was defective.

what is a ground loop though? this could help a lot in other audio projects im engaged in to reduce the interference of circuits sharing all the same grounds..
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Old 29th July 2009, 11:44 AM   #10
Telemin is offline Telemin  United Kingdom
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I gave a nutshell explanation in my post above, plus there is lots of information available on the internet about this already.

Here is a diagram just to hopefully make things a bit clearer:

Click the image to open in full size.
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