hambon---When a capacitor is attached to the input jack of a power amp (typically soldered to the back side of the jack, on the inside of the amp), it creates a high-pass filter which rolls off the signal the amp sees at a rate of 6dB per octave, starting at a corner frequency determined by 1- the value of the cap, and 2- the power amps input impedance. The formula for determining the cap value needed for a particular desired x/o frequency and with a particular amp input impedance can be found via a Google search. The value of the cap, the amps input impedance, and the desired x/o frequency are inter-related.
This method of high-pass filtering has a couple of sonic advantages over any and all active cross-overs, and one disadvantage. That disadvantage is that the slope of the filter is very shallow, only 6dB/octave. That is a 1st order filter, and active x/o’s commonly provide steeper slopes (2nd-3rd-4th order, 12dB-18dB-24dB/octave), removing more of the bass from the signal sent to the amp and then speakers.
The advantages are: 1- There are no active electronics added to the system; 2- An additional interconnect is not required (an active x/o does); and 3- It’s cheap! Just the cost of the caps (one per channel), plus the cost to have them installed if one can’t solder. Purists having been filtering this way for a long time.
