Mario Mendes wrote:

> 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.

> Does anyone know what is the typical output impedance of a laptop's
> headphone jack?  

I may be way off, but I think they are typically max. in the 100s of ohms,
possibly lower.

> The only usefull info I have found is that the majority of these jacks
> will put out 1v rms.  

You don't have to guess. You can measure that. You can also measure the
impedance, by loading the output with a resistor and measuring the output
while varying the resistor.

> 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?  

IMO, your transmitter input impedance should be above the headphone output
impedance. Hard to imagine that not. And 1Vrms is quite a bit, for line
audio, so I'm not sure this is really the problem. Have you tried this
transmitter with a different signal?

> 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.

As a first try, you don't have to match impedances. Use a very simple opamp
circuit (either with a floating ground, or with capacitors in the input and
output and a bias on the signal). It will provide high input impedance and
low output impedance. If the volume is good but the sound is weird, then
you can investigate impedance matching problems.

Gerhard

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