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Author Topic: Ground Loop Isolator Option  (Read 1950 times)
NS8Q
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Posts: 135




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« on: February 22, 2013, 01:57:35 PM »

I run a feed directly to LiveATC.net, which offers free live air traffic control audio over the internet.  I have a Uniden Bearcat scanner, with a single 3.5mm mono plug.  The scanner is connected directly to the computer sound card "Line-In" plug, which is also 3.5mm.  I am getting a terrible hum on the feed.  I have a temporary fix implemented within the software that sends the feed directly to the server.  However it was suggested to me by another ham that I use a ground loop isolator or ground the scanner directly to the computer case.  Ground directly to the cast makes sense, but I've never used an isolator before.  What is involved in implementing that into this situation?

73
Chris NS8Q
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AA4PB
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Posts: 12836




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« Reply #1 on: February 22, 2013, 03:03:12 PM »

An isolator is an audio transformer that keeps you from having a common connection between the scanner ground and the computer ground. Without the isolator you have a shield connected to the scanner on one end and the computer on the other. Any 60Hz leakage currents flow through the shield. The magnetic flux caused by the current in the shield couples into the center conductor and passes some 60Hz hum into the computer audio. The isolation transformer breaks that common path through the shield.

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G4IJE
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Posts: 245




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« Reply #2 on: February 23, 2013, 01:15:50 PM »

Sounds like you have a ground loop (or "earth loop" as we'd probably say on this side of the pond). As we are talking about an audio ground and not a safety ground here, you could try just disconnecting the shield on one end of your audio cable. It might seem counter-intuitive but it often works.
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AA4PB
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« Reply #3 on: February 23, 2013, 01:27:07 PM »

Disconnecting the shield on one end (telescoping) only works when you have a balanced line (two conductors and a shield). If you disconnect the shield on an unbalanced line then there will be no return path for the signal - the circuit will be incomplete.
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K1CJS
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Posts: 6034




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« Reply #4 on: February 24, 2013, 07:32:18 AM »

Disconnect that connection before you blow either your sound card of your scanner output!

The 'single' jack on the scanner is meant to feed a speaker, because it's an amplified output.  It's not meant as a 'line out' connection--which is what you're trying to use it as.  Most of the computer sound cards input connection is a 'line in' connection, very few (if any) have provisions for an amplified input.

The problem could be a common ground, but more than likely is simply because the signal into the sound card from the scanner is simply too powerful--and of the wrong impedance--for the sound card to take.  The solution is to get an audio transformer to both get rid of the direct connection from one to the other and to get the sound card input impedance near to something the sound card can handle.
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