External crossovers for subs

One benefit of an external crossover is that the high pass filtering is done before the signal enters the power amp. Removing low frequencies from the signal sent to the power amp results in less amp distortion, and more power for the speakers.

By the way, the problem with the Harrison filters is that to achieve the crossover frequency you want, the input impedance of the power amp must be figured into the equation that determines the capacitor value required to achieve that crossover frequency.

What make/model is your sub? Many active subs have all the active crossovers you need built in. There are pros and cons with every type of crossover filter, whether active, passive, internal, or external. There’s not a set of absolutes that apply to all situations. Proper sub integration is a variable that’s unique to each situation - the room is significant factor, as are all the settings of the low pass frequency, gain settings, any high pass settings that might be applied to your mains, along with how all that works with the rest of the system in your room. Overcoming the particulars of your room with proper adjustments is more important than the type of filter or it’s proximity to the sub. There are DIY programs that can be used to help make adjustments, or you can do it by ear to suit your tastes. There’s really no wrong way to do it if you’re happy, and aren’t damaging anything.

My view is that a "sub" woofer should simply be reinforcement of the lowest frequencies that the main speakers might struggle with in a given room. Many, many subs are setup poorly IMO, and treated as a featured component with the gain set too high relative to the output of the mains, and low pass frequencies set too high where they affect the vocal range (above 70hz), and the subs end up dominating everything else in the blend. I prefer to set the low pass filter in the 40-50hz range so that the sub only augments the bass my main speakers produce, set the gain level so that it’s barely noticeable except for on bass heavy passages, and I run the main speakers full range because I prefer not to send the signal that goes to my main speakers through the filters of the sub, but that’s a matter preference based on the specifics of my situation.

Some speakers may respond well to having the low bass filtered from the mains via a high pass crossover filter (whether active or passive, or internal or external), and handled solely by the sub, but that’s also a variable, and there’s potential for degrading the main signal depending on how that filtering is done, quality of parts, etc. Many variables, many matters of preference, YMMV..

@jastralfu  1/2 the advantage of using a subwoofer comes from filtering out the low bass from the main speaker which lowers distortion in the main speaker. This requires a complete two way crossover. The simple low pass filter that comes with most commercial subwoofers is an unsatisfactory cheap solution.

Blending a subwoofer into a system requires more than just frequency management, it also requires time and phase management. This can only done accurately in the digital realm. The best solution is a digital preamp such as made by Anthem, Trinnov, MiniDSP and DEQX. MiniDSP also makes a crossover. 

For most people an analog solution is much less expensive and more practical. The easiest thing to do is put a series capacitors on the inputs of your main amp. This requires knowing the amps input impedance. The value of the capacitor needed to roll of the amp at any given frequency at 6 dB/oct can be easily calculated, the formula is online. Then you use the low pass filter in the subwoofer. 

• Jastraflu,

The subs internal is just a low pass. To better integrate and take full advantage of the sub a highpass is added to the speaker. The active crossovers are doing both highpass and low pass.

I have messed around with just using a low pass, thought it was pretty good then added a highpass and it was instantly better, not a little better but drastically better.

I recently went that route with a HPF from Merchand Electronics. It's 24db/oct at 80hz. Its a vast improvement in bass control and essentially turns my 3 way speakers into 4 way. Once I heard the result I added a 2nd Sub. I have a small room and chose the SVS SB1000 for their size and the great phone App for setup and tuning.