he already has his subwoofer crossed over at 85 hz
So adding the amp crossover at 65 hz is wrong
Either use the crossover on the deck or use the crossover on the amp - not BOTH
Yes! This is good information because by setting them both, you are only filtering the already filtered response, making the output even less sensitive. Great post crispin!
BUT, in regard to setting it lower than 250, that is acceptable in an LPF anywhere above 150. Bass frequencies, unless circulated through a pro audio driver with high sensitivity and higher Fs free-air, setting anywhere above 150 is ok. In most conventional designs for car audio use, especially for trunk use, a filter is already set by the enclosure, such as in a proper tline, that filters acoustically the higher frequencies of the design. Electrical filters are another story, which is what is being dealt with here. BUT, most designs exhibit less efficiency above 150Hz, and phase usually just goes crazy anyway when tuned low in the actual design. You cannot get efficiently more than an average 1.5 octaves from a design in subwoofer use. So, say you set it at 250, which is ok, then you are allowing OUT_OF_PHASE response to be played with the in-phase response, which is usually 30-90Hz respectively in most subsystems. So, setting it lower than 250, but equal to or above 150 is acceptable. And the more you filter the out of phase response, the cleaner it will sound. You cannot argue physics. From 150Hz down, the 1.5 octave range is 50-150Hz. In which I am sure the design is capable below 50Hz in phase. But again, you have to check the design capabilities on that, but generally, 30-90 is in phase in most designs for musical output. And when using bandpass designs, (even more acoustical filtering), the in phase response is less, which is why people say they can become less musical. There is a reason the LPF goes down to 35Hz. It is because the maker of the amplifier understands acoustical filtering as well.
You gotta understand this to give a good answer.
