following is an excerpt from a tutorial found on
http://www.beatmaka.com/v2/articles/...18-0410&page=1
pretty simple but gives an idea....
"Passive and Valve Equalisers
It's worth mentioning in passing here, the subject of passive equalisers.
Most equalisers use circuitry that actively boosts or cuts the sound in the various bands using electronic feedback techniques which can (and often does) introduce audible "ringing" in the circuitry. Passive equalisers on the other hand work by already cutting the sound in all frequency bands to begin with, using simple, unpowered, passive electrical components like resistors, capacitors and inductors. A single, simply-designed amplifier stage after the equaliser usually makes up for the loss in signal level, so that the level is flat when all controls are in their "centre" position. Therefore, on such an equaliser, when you "boost" a frequency, you're not really boosting it at all - you're just allowing it to seep through unhindered by the passive circuitry. Valve equalisers often work in exactly this way.
This gives a much smoother sound. In fact a lot of the "smoothness" attributed to valve equalisers, often has little to do with the fact that they have valves in - it is instead due to the fact that the equaliser circuitry is passive rather than active."