Impedance Matching & Crossover network query

[Redhatter]

Hi All,
Firstly, I'm new to these forums, and do-it-yourself audio in general. I know a little physics, ohms law, etc, but I'm yet to start First Year Engineering, (I'm switching to a dual IT/Eng degree), so there are some big gaps in my knowledge here.

I recently bought a pair of Sound Barrier AW-100 10" subwoofers (Dick Smith Electronics, Brisbane City were flogging them off at $10 a speaker), and was looking at the option of hooking these up to my amplefier.

I've also got a pair of Digitor 40W Indoor/Outdoor speakers, which I've had for some years. These work fine for the upper frequencies, but the woofers in them aren't able to cope with the lower frequencies. They can be configured to run as either 4 ohm impedance, or 8 ohm.

Here's the catch, the subwoofers are 4 ohm (presumably for car audio). The amplefier, a Sherwood AD220B (30W RMS, based on the STK4853 power amp IC), is designed with 8 ohm speakers.

Currently, this is the wiring I have on each speaker -- this is based on the circuit used in an old pair of PYE audio speakers...


This performs reasonably well, but the impedance of the entire circuit works out to just over 4 ohms, which I feel is a little low. (I've already had to replace the power amp chip once after blowing it up, I don't want to waste another $30 on a repeat performance).

Also, I'm not sure of the cause, but the subwoofers cut out occasionally -- sometimes for a few seconds. I suspect the capacitors are at fault, but I'm not sure. There are no loose connections (everything is soldered properly), and the actual circuit above is mounted on a circuit board.

So firstly, what sort of crossover network would I be looking at? Anyone got some schematics?

Ideally, I'd be looking for one which has a crossover frequency of around 500Hz~1kHz and a total AC impedance of around 8 ohms (+- 2 ohms).

Once I've got this going, my next project is to put the subwoofers in suitable boxes -- as they are loose at the moment. (Far from ideal, but it'll do for the present)

Any help/advice/pointers to other threads greatly appreciated...