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'[EE] Headphone out to RCA line-in converter'
2003\03\02@074220 by Chris Rocco

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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?  I can't seem to find any
circuits on the Internet, but I'm sure that they must be out there... :)

Thanks in advance,
Chris

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2003\03\02@080328 by Kyrre Aalerud

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I'd say this is due to either noisy power-source, or noise from the
hard-drive or similar.

Most likely, you can't filter it out either because it will be in the part
of the sound-spectrum you wish to hear.

   KreAture

{Original Message removed}

2003\03\02@080951 by Chris Rocco

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(thanks for the speedy reply)

My portable CD player has the same issue when I use its headphone jack
-- but it happens to also have a line-out jack that works perfectly
(meaning that there is no hiss when I use the line-out jack).  This
leads me to believe that there may be a [simple] solution out there...?

Chris


{Original Message removed}

2003\03\02@082112 by Kyrre Aalerud

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Then it's possibly a matter of signal-level...  A correct line-out is
supposed to have a signal level of 0.5 volts.  A headphone jack will have an
amplifier that gives a level of probably 2-3 volts to drive the headphones.

You probably need the volume on the stereo a lot lower when using the
headphone-output instead of the line-out, so you may try simply dampening
the signal in a voltage-divider to correct this.
I assume your system wants about 16 Ohm load, so use a 16-32 Ohm stereo
pot-meter and couple it between the signals and ground.  Now take the
variable outputs from the potmeter and wire to your stereo.  To avoid
DC-coupling between them you could also add a very large capacitor in series
with the signal.

Good luck.

   KreAture

----- Original Message -----
From: "Chris Rocco" <.....croccoKILLspamspam@spam@CORP.EARTHLINK.NET>
To: <PICLISTspamKILLspamMITVMA.MIT.EDU>
Sent: Sunday, March 02, 2003 2:06 PM
Subject: Re: [EE] Headphone out to RCA line-in converter


> (thanks for the speedy reply)
>
> My portable CD player has the same issue when I use its headphone jack
> -- but it happens to also have a line-out jack that works perfectly
> (meaning that there is no hiss when I use the line-out jack).  This
> leads me to believe that there may be a [simple] solution out there...?
>
> Chris
>
>
> {Original Message removed}

2003\03\02@125213 by Peter L. Peres

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On Sun, 2 Mar 2003, 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?  I can't seem to find any
*>circuits on the Internet, but I'm sure that they must be out there... :)

Obtain two audio transformers (600:600 ohms should work) and use them.
Either that or a stage 'direct box' which contains the same thing.

Peter

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2003\03\02@125627 by Peter L. Peres

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On Sun, 2 Mar 2003, Chris Rocco wrote:

*>(thanks for the speedy reply)
*>
*>My portable CD player has the same issue when I use its headphone jack
*>-- but it happens to also have a line-out jack that works perfectly
*>(meaning that there is no hiss when I use the line-out jack).  This
*>leads me to believe that there may be a [simple] solution out there...?

The headphone jack also outputs the pa noise besides the signal. Do not
use it.

Peter

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2003\03\02@141130 by Paul Hutchinson

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You probably need an impedance/level match and a ground isolator. Both are
available at Radio Shack and sometimes at other electronics stores like
Circuit City.

Paul

> {Original Message removed}

2003\03\02@145355 by Dwayne Reid

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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
re-appear.

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.
<http://www.trinity-electronics.com/pbc361a.htm>

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

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2003\03\02@162912 by Kyrre Aalerud

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Or, simply make sure you have a proper ground connecting the two devices :-)

But seriously, I believe the noise to be caused by the cheap amplifier in
the computer, in conjunction with running a 0.5v line-in with a higher
voltage speaker-out.

   KreAture


{Original Message removed}

2003\03\02@222336 by Anand Dhuru

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try putting in a 100K resistor in series with each signal (L,R). If the
volume is tto low, reduce it to probably 47K or 22K.

Regards,

Anand Dhuru


{Original Message removed}

2003\03\02@223818 by Brent Brown

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In my (limited) experience the headphone out level works OK into a
typical RCA line-in input (but do your own tests, keeping the volume
low).

For me a large amount of mains hum was present when the laptop was
run on it's switchmode mains power supply. Fine when the laptop is
running on battery, or when the laptop is grounded. In my case the
laptop became grounded when an external monitor was connected. Still
a little hum, but not too bad.

Pay attention to the whole audio circuit, ie. what is grounded and
what is not, eliminate ground loops, etc.



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2003\03\03@053332 by Kyrre Aalerud

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Running audio out on the comp at lov volume is exacxtly what is causing a
lot of the problems.  The amplifier in the singnal-source has least
distrotion and best signal to noise ratio at moderate to high output-levels.
By using a matched stereo potmeter to adjust the input level and giving a
correct load on the amplifier you eliminate a lot of the problems associated
with this.

Ofcource, you need the systems to share ground or the 50, 60, 100 or 120 Hz
hum will always be present.  The reason for the ling list of humms is
because the different frequencys on power grids around the world.  You will
get the hum of your power-grid, but it should instead be the double
frequency if a full-wave rectifier circuit is used.

Humm is always annoying so I simply use optical out and eliminate any
coupling problems.

   KreAture

{Original Message removed}

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