piclist 2003\03\02\145355a >
Thread: Headphone out to RCA line-in converter
face BY : Dwayne Reid email (remove spam text)

At 04:39 AM 3/2/03 -0800, Chris Rocco wrote:
>I would like to connect my laptop's headphone jack to my stereo's RCA
>input jack so that I can hear MP3's and video files on my stereo/TV --
>the problem is that I get a hum when I use a direct cable, even when
>there is nothing generating any sound on the laptop.  Is this due to an
>impedance mis-match?  How can I fix it?

Can you run your laptop from batteries only?  If so, disconnect the power
cord and see if the hum goes away.

Are you also connecting a video out from the laptop to the TV or anything
else?  If so, disconnect those lines as well and see if the hum goes away.

What you want to do is have ONLY the audio line from the laptop to the TV
connected - nothing else.

If the hum persists, you have to find out if it is coming from the laptop
or the TV. Disconnect the cable from the laptop and see what happens.  Does
the hum change as you move the cable and/or laptop around to different
locations?  Try hooking up a portable (battery operated) CD player to the
TV instead of the laptop.

On the other hand, if the audio is clean and quiet when the only cable
connected to the laptop is the audio cable, start connecting the other
cables back to the laptop one at a time to see what causes the hum to

My best guess is that you need ground isolation between the laptop audio
output connector and the TV.  The tests above just help to confirm this.

The quickest / cheapest ground isolation fix is a pair of 600:600 audio
line transformers - one for each channel.  The transformers are available
from a variety of sources - I can even supply you some if you need.

The transformers will most likely have 4 or 5 or 6 wires coming out.  You
need to use only 4 of the wires on each transformer: ignore the center tap
connections if they are present.

The easiest way to wire the transformers in is to cut the audio cable at a
location convenient for you - near the laptop end may be the best so as do
reduce magnetic field pickup from the picture tube (deflection & degauss
coils).  Cut the cable and strip the ends.  You should see a bare shield
covering the hot after you have removed the jacket.

Simply connect the incoming hot & shield to one side of the transformer,
connect the outgoing hot and shield to the other side of the
transformer.  Both channels have to be wired exactly the same to ensure you
maintain phase polarity.

That should do it!


Dwayne Reid   <dwayner@spam@spamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

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