I'm in the process of adding a sub and amp to my system which has an Eclipse 5425 head unit.

Question I have is in regards to the LPF crossover, I've read that I need to be careful not to juice my amp (sony xm222w) by setting the crossovers wrong as they can cancel each other out and show zero ohms to an amp at certain frequencies, re next paragraph.

The amp has an 80hz LPF -12db/oct filter switch, the head unit has a set of pre outs dedicated to the sub with a selectable 80hz LPF at -18db/oct.

Which one do I use? Or both? I was planning to use the head unit one thinking that the only frequencies sent to the amp would be below 80hz thus I would not need to set the one on the amp. But I'm not really sure.

 

Got the answer.

 

You will want to run the greater decibel slope.

Try this from http://www.passivecrossovers.com/index.htm

SYNOPSIS: The greater the decibel slope, the more accurate the reduction of sounds outside the filters range. In your case though, you will have to decide which sounds better.

"To better understand, try imagining any filter. As we have already seen, it will allow the transit of a certain portion of frequencies only. This portion takes the name of wave band and to it is assigned the conventional value of 0dB. At the cut frequency point the signal will be subjected to an attenuation of -3dB while beyond this frequency it will be attenuated gradually as much as the filter order is higher.

A first-order filter produces a rolloff of 6dB/oct. beyond the cut point. For example, a first-order lowpass filter with cut frequency tuned to 500hz, will let pass intact the lower frequencies, will return the 500s attenuated by 3dB, after which it will attenuate by 6dB the first octave (1Khz), by 12dB the second (2Khz), by 18dB the third (4Khz), and so on, with a constant — you'll say asymptotic to make a good impression at a cocktail-party — slope of 6dB for every next octave.

A second-order filter produces a rolloff of 12dB/oct beyond the cut point. Referring to the previous example, we'll always have the 500s attenuated by 3dB, but already the 1000s will be 12dB down while the 4000s even 36dB below. A third-order filter produces a rolloff of 18dB/oct beyond the cut point. A fourth-order filter produces a rolloff of 24dB/oct and so on, with an increase ratio in attenuation slope equal to 6dB/oct for every next order."

 

Good link, thanks.