Another interesting benefit of figuring this stuff out would be to determine how useful those fmod dealies would be. Since they are passive filters their cutoff frequencies depend on the resistance of the circuit they're in and this is almost entirely dependent on that pesky amplifier input impedance.
http://www.hlabs.com/technical/crossovers/page2.html
Say I've got ported box tuned to 28 Hz(I do actually :) ) and I need a subsonic filter and am looking at getting an fmod. Normally I'd go for a highpass filter with a cutoff frequency 1/3 an octave below tuning or about 22 Hz. Well the lowest fmod I could get would be a 20 Hz guy, but its actually 25 Hz with a 10 kohm load which is what the 2025 is, but 25 Hz? That fmod is beginning to roll off frequencies nearly at tuning. I know there's not a lot of material recorded this low, but I'm not sure this would help my low end extension much.
But wait, it gets worse, and a lot more complicated as well.
Say I'm using the A2002 in the same sub application and I'm running an old school Alpine 7909 w/o a line driver(read 500mv/.5v preouts) I'm gonna have to run the sensitivity pretty darn high to get enough output. This means the input impedance is going to be LOW, like less than 5 kohms, wait make that 2 kohms. I had the balanced input switch on, hehe. The A2002's input resistance is actually between 1.7 and 4.2 kohms depending on the "gain" with the normal setting. If this is indicative of the actual input impedance of this amp then how does that effect the "20 Hz" fmod's cutoff frequency? If the thing goes from 20 to 25 hz when you go from a 22 kohm impedance to 10k what happened when you drop the load down to 2 kohms? I'm guessing about a 25% increase every time the load is halved or about 40 Hz.
I hope I'm wrong, because a high pass filter with a cutoff frequency of 40 Hz and box tuned to 28 Hz ain't too sporty a combination.
And now the complicated part. If things are working like I believe then whenever I adjust the "gain" on the A2002 I change the input impedance which changes the subsonic cutoff frequency as well! I'm not going to even try to figure that mess out.
I know Harrison claims that "exact crossover frequencies are seldom noticed or necessary" and I'm not sure I've got this input impedance thing figuired right, but still, I'm not likin fmods a whole lot at this point, at least not for a car application.
-Fishy
me about it, do the same for him.
