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Old 23rd January 2004, 08:09 AM   #1
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Default Converting unbalanced AF signal to balanced

Hello !

Anybody have some simple schematic for converting an unbalanced (RCA) audio signal (from PC sound card) to balanced and then back from balanced to unbalaced ? I need the schematic because I have to transport the audio signal from PC sound card to amplifier which is located 30m away from it. I'm searching for an simple solution because the quality of transmited audio is not so critical (it will be used for listening the music in an car repairing garage).

Another question is, can I use UTP (4 twisted pairs of wire inside) cable for the transport of balanced signal ? The UTP cable is not shilded ! I ask this because I have this cable installed already and if it is possible I would use it .

Thanx !
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Old 23rd January 2004, 08:51 AM   #2
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You can certainly convert the unbalanced line output of a computer sound card to balanced by using a transformer.
Once the signal is properly balanced it is perfectly okay to use unshielded cable. This is done regularly in broadcast stations.
Use transformers with a 1:1 turns ratio.
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Old 23rd January 2004, 09:52 AM   #3
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If you don't want to go to the expense of high-quality audio transformers, the SSM2141 / 2142 / 2143 chips from Analog Devices (www.analog.com) are widely used in the audio industry for driving and receiving from balanced lines. Although they won't tolerate the very high common-mode that a transformer will, I think you'll find they work well in the majority of circumstances.

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Old 23rd January 2004, 10:06 AM   #4
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UTP will work well at transporting balanced audio under most circumstances.
Don't use other pairs for other signals such as Ethernet or telephone
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Old 23rd January 2004, 10:10 AM   #5
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Allied Electronics sells a 1:1 audio transformer which should do the job. Their part number is 227-0862. Price is $13.16 USD.
While this is not "audiophile" quality, it should provide acceptable audio quality for use in an automobile repair shop.
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Old 23rd January 2004, 10:29 AM   #6
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Car audio shops/wholesalers sell little isolation transformer boxes complete with RCA in/out connectors and are cheap.
They have E-core audio transformers that will work well enough for your application.
They also have speaker level to RCA level boxes too - I have used one of these to transport audio 50 m and it worked just fine.

Eric.
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