Hi - its not impedance matching, it is level matching.  If the fm 
transmitter is expecting a 2.5V pp max signal, but the lappy only puts 
out 1Vpp, it is going to be low output from the radio.

Does the lappy line out as loud as other sources when hooked up to a 
stereo?  (if it has a line out)

Does the fm transmitter output full volume when hooked to a stereo line out?

Maybe a cheapo headphone amplifier would do the trick, to get more gain 
to get full volume.

Good luck...
J





Mario Mendes wrote:

> Hi guys,
> 
> Recently I boutght a second mp3player/walkman/laptop fm transmitter to
> be able to transmit music from my laptop in the house to a radio in the
> backyard. Just like the first one it does not sound good (this one is
> being returned).  Both transmit like they should, but the sound coming
> out of the radio speakers is really low, which leads me to believe that
> there is an impedance mismatch between the laptop's output and the
> transmitters input.
> 
> The laptop's specs from the manufacturer do not list the output
> impedance anywhere and the manufacturer's web site was of no use for
> this either.  Googled for the same information but I did not find
> anything reliable as far as this info is concerned.
> 
> Does anyone know what is the typical output impedance of a laptop's
> headphone jack?  The only usefull info I have found is that the majority
> of these jacks will put out 1v rms.  Also, not being knowledgeable in
> analog electronics (I do fine in with digital electronics though), can
> anyone help me out with coming up with an opamp circuit that would take
> care of this impedance matching?  Since this is for home use, I thought
> that a couple of opamps (1 left and 1 right) could take care of it with
> either a 9v battery or a simple power supply.  This would plug into the
> pc headphone jack and the fm transmitter would plug into it.
> 
> Thanks in advance.
> 
> 
> -Mario
> 
> 
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