Transmitting audio over power line

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In my quest for "audio everywhere" I thought that it would be convenient to be able to have a remote speaker that just plugs into any AC socket. I am not looking for ultra fidelity here, just a backyard speaker that I can plug in when needed.

One idea is in an old National app note:

This scheme uses fairly simple circuitry (although obtaining the specified transformers may be a problem). I would use a different output amp to get more power to the speaker (probably LM1875 or the like).

Any other ideas on how one might do this, or on possible improvements to the National circuit? I considered modifying a "wireless" intercom but the fidelity would probably be terrible.

I have also considered a wireless solution but that's a story for another day (and another thread).

Thanks in advance for your ideas...
I thought about this a year or so back but decided against it as being too expensive (you have to duplicate everything for stereo) and too much hassle. Instead I went for a small VCO based FM transmitter.

I bought a kit from and modified it slightly so that I could control the voltage to the output power stage. That means I was able to scale back the power output so that it covered the house and garden nicely but didn't intrude into the neighbours property too much. I think I am transmitting with something of the order of 20 mW or less.

The kit arrived promptly and was complete. It was very well made and really easy to assemble (one of my kids did most of it). The only thing that caused me any hassle was the antenna, but a little bit of surfing sorted that out pretty quickly.

Another great advantage of this is that I don't need special equipment to listen to what I'm broadcasting. I can pick up the signal on every ordinary FM radio in the house, from the 1950's valve monster to the alarm-clock-radio to the old car radio I have in the garrage.

If you want the fun of doing something different, go for the power line system. If you want something versatile that works really well and is easy to set up, go for an FM transmitter.

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