high pass filter

A capacitor.

As Kr4 said, a cap will do the trick BUT you will need to know the impedence of your bookshelf speakers at 80hz and whether you want a shallow rolloff(6db per octave) or a sharper rolloff(12 or 18db per octave).

Also, the quality of the main cap will make a BIG difference in mid and upper octave transparancy but a plain non polorized electrolytic will also do the job.

It would help if we knew the brand/model of your bookshelf speakers...

Cheers

they are cambridge audio s30's. i would like to try both 6db and 12db rolloffs. i have taken the speakers apart and i'm assuming that i could probably just put a cap on the midrange's + terminal to the lead feeding into the crossover?? i metered the midrange and it is showing 3.8ohms. would i use that reading to find what cap size i need?

1. You need to know the impedance, not the DC resistance, and you need to know it across the relevant range from about 200Hz down.

2. For 6dB rolloff, only a cap is needed. For 12dB, you will need a cap and a coil.

Kal