The prototype
The prototype
The speaker input also feeds the filter directly. The filter is incredibly simple, made from a transformer and two capacitors. I used a 600 ohm 1:1 isolating transformer meant for telephone applications. In UK/Australia/New Zealand etc you may be able to buy this from RS Components, part no. 217-826. It's a bandpass filter: the low end is set by the 4uF cap resonating with the magnetizing inductance of the transformer. The high end is set by the 0.44uF cap resonating with the leakage inductance.
If you can't find the same transformer I encourage you to experiment with whatever you can find. A telecom or audio transformer is probably best but others might be suitable: a little 120V:12V transformer from a wall wart, for instance. You would need to change the capacitor values to get a good sound. First, leave out the 4uF cap and hook the primary straight to the dummy load. Adjust the 0.44uF cap until you get the right treble sound, not too shrill or too muffled. Then try different values of cap in the 4uF position till you get the bass sounding good.
The line output is taken across the .44uF cap via a simple voltage divider. On higher powered amps you need the resistor R, to stop you frying your mixing desk/headphones/etc if you turn it up too high. If you use a step-down transformer instead of a 1:1 then R may not be needed. My prototype just used a 10K pot with no R. Due to the isolating action of the transformer, you can have a balanced line out.