I blogged a couple days ago about the outdoor speakers I mounted on the patio. I'd like to talk a little bit more about the problem I had in setting this up and how I solved it.
It's hard to find a standalone stereo audio amplifier anymore.
You can find amplified speakers, typically intended for use with computers. Those do take line-in, but typically have integrated speakers which typically are designed for a desktop installation. It's not really the right thing for a patio, and the speakers you get are certainly not weather resistant.
You can find home stereo receiver/amplifiers, but those typically have AM/FM radios built-in at the very least, and typically even in this day and age still include phono preamps as well. If all you want to do is plug in something like an iPod, it's just overkill.
No, what I want is an audio amplifier that takes a single stereo line level input and has a pair of speaker connections. For controls, a simple volume knob and power switch is fine. No need for anything fancier.
Well, I've found a solution, though it does come in kit form, so you need to be good with a soldering iron. It's the Velleman K4003 2x30 watt audio amplifier.
I actually bought one of these a while ago for use out by the hot tub, where there's another pair of speakers set up. I used to have another amplifier kit that I had built before that ran on 12 vdc. The best I could do was a 12 volt wall-wart power supply, but that was inadequate for the purpose, since it introduced a nasty 60 Hz hum in the speakers. The Velleman amp, by contrast, is designed to run with a 12 vac bipolar power input. That is, you hook up a 24 VAC center tapped transformer.
With an iPhone hooked up to the patio speakers, the output is quite loud. But even so, it's not at all distorted. The levels are high enough that you'd want to use a volume control in front of the input. A dual gang 20 kΩ logarithmic pot works perfectly. You hook the "counterclockwise" end of each pot up to ground, the opposite end to the audio input, and hook the sweep of the pot up to the amp's audio input.
The kit is available from Jameco, part # 129138. A suitable transformer can also be had from them, part # 99654.