@TRL
I'm not good enough with electronics to design my own new circuits, but it's pretty easy to find simple circuits in books or on the internet which you can use.

The most difficult parts of building a low frequency oscillator such as you require are the output transistors on the amplifier, which have to be pretty meaty to handle the powers required, and consequently the power supply. You're not going to get much out of a 12VDC supply - IIRC a single-ended amplifier will only provide about 4.5W rms into a 4Ω load. You can get up to about 25W rms using a Class B, FET based amplifier, but that is a little more expensive and complicated.

The easy way to do it is to buy yourself an old PA amplifier from a music shop, then build a little oscillator on stripboard. You can run the oscillator from batteries, and the components shouldn't cost very much. Plug it into the amplifier, and turn up the juice!
( K2k6 has a proper job these days, Tue 29 Apr 2008, 16:02, Reply)