high pass filter

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

how would i measure that Kal? thanks for the help

Let's make this easy, for a ~4 ohm impedence at or around 80hz, you will need the following:

6db - 500uF cap
12db - 350uF cap, 11mH shunt coil

For ~8 ohm impedence:
6db - 250uF cap
12db - 175uF cap, 22mH shunt coil

These values will get you in the ballpark and most likely be just fine and fine tuning the values in relation to the exact impedence at 80hz will yield better results but in most likelyhood, your room acoustics will swamp any differences an exact impedence match would reveal.

Use ~100 volt non-polorized caps in the main feed and build an external crossover box...

Cheers