Filters are a necessary evil in typical sound systems where the speakers are multidriver systems. As for the speaker's well being it matters not whether the XO filter is directly before the drivers as is most common or before dedicated amplifiers. As for the whole system's well being, great enhancments can be made by putting filters before the amps and let them drive single units without power-robbing XOs.

Even if multi-amp systems in the mid/tweeter area is not conceivable, as is the case with me, since that requires great deal more money and time building the extra amps, big improvements can be had by at least get a decent sub and drive the main speakers from around 70-80Hz. I'd rather have one high quality fullrange power amp than many low quality amps just so I can multi-amp. A well built sub really doesn't need mega watts to play loud and deep, so even the smaller Rotel and NAD power amps work great, and don't cost a fortune.

So that brings me to the subject of filters. If you haven't guessed it yet I don't like ICs or complex circuits for audio. I know ICs can sound good, but they're not for me, enuff on that. I prefer passive filters where ever possible, and if your system is set up right, this is possible. One of the rules in high quality music systems, is matching the components to build a system that fits your taste and need.

Usually filters are placed between preamp and power amp, the preamp driving several filters, in turn driving the amps. Well, if you use passive filters in this setup, it's real hard to get anything higher than a first order filter since several filter sections are needed for higher order, and each section needs to drive the next. This means that the first filter section needs to have low input impedance, and with several filters for multi-amps, this will likely overload the preamp. In the higher frequencies I prefer low order filters anyways, so usually not a problem. But to the sub I prefer at least second order cutoff. So one way of driving high order passive filters is to have one highpass section, first order with a single cap in the signal path, feeding the main amp. Then put the low pass filter after the main amp and drive the sub amp from it. Now the first filter section of the lowpass filter can have a real low input impedance, perhaps just 10ohms, and several filter sections ca be cascaded for high order filter as the final result. The loss in signal due to the passive filtering is compensated more than enuff in the sub-amps voltage gain which typically is around 30.

Here I show the typical setup, version 1, followed by the other setup, version 2:

Of course, some times we want active filters, and many good solutions exist. My favorite though is the FDNR, or gyrator. It has the opamps out of the direct signal path. (The opamps are not out of the signal path at stopband frequencies though). This single section shown here is a third order, remove R5 and C3 and it's second order. Adjust C3, (make smaller), and you get peaks at the cutoff, sometimes wanted. Supposedly it has less phase shift than other filters, and introduces no noise or sound from the opamps in the passband. More on this filter can be found at BurrBrown's site, www.burr-brown.com, look under application notes, it's titled: 'A low noise, low distortion design for antialiasing and anti-imaging filters', which doesn't mean it can't be used at low frequencies.

Look here: http://www.burr-brown.com/download/ABs/AB-026.pdf

Even though R1 is shown with a 50kohm value it can be any value, found like this:

First deside on cutoff frequency, then capacitor values, all caps are equal, then use the usual formula: 1/(2pi*f*C) to find Z. The resistor values are then found like so:

R1 = (3/2)Z, R2 = Z, R3 = Z, R4 = 1.333Z, R5 = (1/2)Z.

You can of course manipulate the equation to find C when you've desided on a value for R1(which sets the input impedance). I leave that exercise to you:)

As with all active filters, the opamps must have high bandwith for proper stopband attenuation. Even tho you make a lowpass filter with cutoff at 100Hz or below, the active filter will stop attenuating as the frequency nears the opamps upper frequency limit. Since the opamps openloop gain drops with -20dB/decade, for a -60dB attenuation through the audioband, you need a minimum bandwith of 20MHz.