On Sun, 21 Dec 2008 11:45:51 -0800, Tom, N3IJ wrote:

Can anyone suggest a source of a circuit or kit for a mono amp to
increase my laptop's headphone output level to line level?

As a starting point, it would be helpful just to learn what these levels
are. I want to drive a line-level transceiver input for digital hamming.

Any suggestions would be appreciated.

You have an impedance mis-match. Use a radioshack xformer with the 8ohm
side on the headphones jack and the 5k output to the line level. You will
need a pad or simply a pot after that.