Line out audio over phone cable?

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Michael Burr - 19 Mar 2008 06:52 GMT

Hi all,

I'm working on a whole house FM transmitter project and would like to
mount the xmitter in my cupola to get it good and high. So I need to get
the audio line out signal from my wiring closet up there.

I'm considering pulling pairs 3&4 off a twisted 4-pair phone cable and
using on one line from each for left and right channel, and combining the
remaining leads from each pair for ground. The run total run distance
would be approx 35'.

Has anyone done something similar? Any thoughts would be greatly
appreciated.

Thanks,

Michael

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Robert L Bass - 19 Mar 2008 13:28 GMT

> Hi all,
>
[quoted text clipped - 9 lines]
> Has anyone done something similar? Any thoughts would be greatly
> appreciated.

Michael,

There are several ways to accomplish what you want. One simple methods is to
send speaker level audio up to the cupola and reduce it there to line level
using an audio transformer. Another option is to use baluns at each end.
Third, you might be able to use the twisted pair cable as is for line level,
depending on how much and what kind of noise is present.

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Regards,
Robert L Bass

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Michael Burr - 19 Mar 2008 17:28 GMT

>> Hi all,
>>
[quoted text clipped - 17 lines]
> Third, you might be able to use the twisted pair cable as is for line level,
> depending on how much and what kind of noise is present.

Thanks for your thoughts Robert. I thought I'd start with line-level and
work my way up to speaker level if signal quality becomes an issue.

My biggest concern right now is wanting a "sanity check" on the idea
regarding form and current - don't want to fry wires in the walls. As I
understand it - typical current between line-level devices is in the
very low ma range and my 24G solid copper wire can take at least 500 ma.
One end will be line-out from a PC sound card and the other the FM
transmitter. I haven't found specs on it input impedance, but it has
typical line-in and will accept even MP3 players as input.

Michael

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Robert L Bass - 20 Mar 2008 01:19 GMT

> Thanks for your thoughts Robert. I thought I'd start with line-level and
> work my way up to speaker level if signal quality becomes an issue.
>
> My biggest concern right now is wanting a "sanity check" on the idea
> regarding form and current - don't want to fry wires in the walls.

OK, not to worry, Michael. You won't fry the wires with line level audio.

> As I understand it - typical current between line-level devices is in the
> very low ma range and my 24G solid copper wire can take at least 500 ma.
> One end will be line-out from a PC sound card and the other the FM
> transmitter. I haven't found specs on it input impedance, but it has
> typical line-in and will accept even MP3 players as input.

There shouldn't be a power issue with your plan. The only question is whether
you'll pick up noise. I'd try it using just the wire and see. Then if you
still need baluns you've lost nothing.

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Ryan - 18 Apr 2008 15:53 GMT

Hello All,

I can attest that you should have absolutely no problems. I am using a
75ft run of twisted pair to run line-level audio from a wireless video
receiver to a computer line input .. and it works perfectly fine .. it
is directly wired to the connectors on either end.

In my case , the bottle neck for quality is the video
receiver/transmitter setup , not the twisted pair- so keep that in mind.

If you do get noise .. maybe try attaching one each of the pair together
in different configurations (I did this without thinking). It is
possible that one of the configurations may help to attenuate the noise
while it travels across the wire.

>> Thanks for your thoughts Robert. I thought I'd start with line-level
>> and work my way up to speaker level if signal quality becomes an issue.
[quoted text clipped - 13 lines]
> whether you'll pick up noise. I'd try it using just the wire and see.
> Then if you still need baluns you've lost nothing.

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Homeowner Forum / Home Automation / April 2008

Line out audio over phone cable?