I've built one of these before, but it was impractically heavy. This one is much smaller; I can cheerfully let it dangle from the headphone socket of a radio without fear of it tearing the cable or damaging the socket. Because it is added inline with the headphones, it can be used on any radio, but it will only match the impedance of a pair of iPod or Walkman-style 32-ohm headphones. It's a constant-k T-section low-pass filter. It's design ed for a termination of 64 ohms, when the 4.7uF capacitor is used. The 1uF capacitor gives a miss-match, but this is not apparent in use.It's delightfully simple, and inexpensive. Most homebrewers will find the bits in their junk box. There are just eight components:
An old headphone lead with plug
A headphone socket
Two 10mH power chokes
A 1uF capacitor
A 4.7uF capacitor
A small SPST switch (anything would do)
I used RS 233-5487 chokes, but any reasonably large choke will work. The main feature is the DC resistance; at under 4 ohms, these are ideal. Little wire-ended chokes will have a resistance of 80 ohms or more, and will prove too lossy. Current price is GBP1.49 each.
Electrolytic capacitors were used, but polyester caps would also be good. The switch and one of the caps could be omitted if you are happy to have just one cutoff frequency, or you could add another. I chose the values given because they give cutoffs of 1000Hz and 3000Hz, ideal for telegraphy and phone respectively.
The biggest benefit to users of simple receivers is that this filter will remove much of the hiss produced by an LM386 or similar simple (but noisy) AF amplifier. The box I used measures 50x50x35mm, but it could be squeezed into a smaller volume. It would be best to separate the two chokes as far as practicable, to reduce mutual coupling. It is light and small enough to be pocketed, and makes my homebrew receivers sound great.