What is the worst possible mistake that I could make when installing my new self-powered subwoofers?
Well, your subs probably have both "high level" and "line level" inputs, and the provided instructions will doubtless say that you can simply connect your subs' high level inputs across your main loudspeakers' input terminals. (I.e., in parallel with your main power amplifier outputs.) Yes, that's very easy to do; but is it truly advisable? Well, here's what legendary subwoofer expert Barry Ober, a.k.a. "The Soundoctor," had to say…
"Simply connecting a sub to existing mains speaker (or amp) terminals is the WORST POSSIBLE WAY to do this. EVERYTHING scientific and acoustic about this method is wrong, from the additive delay issues to the back EMF of the mains affecting the LF signal. However there are plenty of people who simply do not understand correctly integrated bass, and they will be reasonably happy simply sticking another box on to their system without regard to timing, phase and frequency issues, and they will think it sounds 'ok' or 'good' and for those people it doesn't really matter."
Take Barry's advice. Don't use those noxious high level subwoofer inputs! They're provided purely to accommodate the rank novice. Don't use the sub's high level inputs even if you aspire to behave as a duly certified incompetent jerk. Always utilize the line level inputs.
Can't I just buy one subwoofer and use it in blended "shared bass" mode for both stereo channels?
A single subwoofer setup is appropriate for home theater applications, where the intent is to reproduce the monaural low frequency effects (LFE) signal that's running on the program's "boom channel". However, two channel stereo presents a bigger challenge. The low bass content in stereo music is an integral part of the performance; it's not a random interruption, as with gunshots, explosions, and monster footfalls. Instead, real directional cues are conveyed. In addition, paired (or more) subwoofers can deliver vital acoustical advantages (refer "Acoustics" paragraph… https://classicalcandor.blogspot.com/2020/05/on-other-ways-to-woof.html) in the control of low frequency room resonance, as well as provide better midrange response (see below). These latter assets just won't be apparent unless there are two or more separate subwoofers.
|"The $64,000 Question"|
Outside of more bass, no further benefit will accrue until you… (a) finish the upgrade by adding an external active crossover network control unit; (b) adjust it properly; and (c) optimize the subs' phase angle and input level control settings. Adding subwoofers is a classic "do it right or don't do it" proposition, and persistence is essential.
What will adding an external active crossover network really do for me?
It will allow you attain the other major benefit that subwoofer installations can bestow: Render a more accurate, more realistic midrange. By inserting an active crossover controller at a point in the signal path that's immediately before your power amplifier stages, you can then dictate what feeds to the self-powered subs and what feeds to the main power amplifier and main speakers. Be aware that it's always those big fat bottom bass notes that perpetually smear midrange clarity. When you divert the low bass and send it to the subwoofers (where it belongs), the main speaker system's mid-woofer drivers are then free to handle the upper bass and midrange frequencies independently, without any of that disruptive low bass modulation. You'll get a clearer and more articulate midrange. The improvement will be especially apparent when listening at high sound pressure levels. This midrange benefit is one of the most significant assets that a properly managed subwoofer setup can bestow, and many experienced listeners feel that it's even more rewarding than gaining deeper bass response.
My subwoofers already have their own built-in crossovers. Why do I need an external crossover?
Well, first check on that purported "crossover." Do your subwoofers really have a crossover network, or is it merely a low pass filter? Here's the difference…
With a low pass filter, the signal sees a selective gate that allows only the intended low frequencies to enter. Content above the gate's setting is blocked, so higher frequencies can't progress beyond the low pass gateway. But the full frequency signal is still free to enter any other open portal that might be connected, e.g., the portal to the main power amplifier. The consequence is that the sub's low pass filter feeds only low frequencies to the subwoofer amplifiers, but the full spectrum signal—all lows and highs together—flows into the main power amplifier and then to the main speakers.
With a full crossover network there are complementary low pass and high pass filter gates. The low pass gateway works as described, while the high pass gateway provides inverse screening. It passes the high frequencies through to the main power amplifier while blocking the lower frequencies. In effect, the full crossover network splits the incoming signal into two separate paths, with the lower frequencies diverted to the subwoofers' power amplifiers, and the higher frequencies diverted to the main stereo power amplifier.
The extent of any filtering that's provided inside your self-powered subwoofers will vary. Most subs offer only some simple low pass filtering, but others have self-contained (full or partial) crossover networks. Know what you've bought. Regardless, if you elect to use an external active crossover network, it's then vital that you understand how to bypass (deactivate) all of your subwoofers' internal filtering. You never want to apply both the internal and external filtering networks simultaneously, so bypass the subs' internal filter/crossover circuitry when using an active external crossover controller.
But my new subwoofers really do have their own internal crossover networks. Is using an external active crossover network control going to work better?
In essence, you need an external active crossover controller—one that's placed in a convenient and accessible location—because you intend to use it. If you confined your use to each of the respective subwoofers' internal filters (if/when they're truly provided), you'd then have to crawl to each separate subwoofer to enter every such adjustment. That's not just awkward and impractical, it's unacceptable. Adjustments often call for simultaneous left/right channel changes. A fully integrated central controller is essential.
Do some crossover networks pass or block the signal at different rates?
Yes, and those rates can vary. There are different means of implementing the crossover filtering, with different advantages and limitations; it can get complicated. The modern consensus favors cascading two Butterworth filters to form a Linkwitz-Riley type crossover network operating at a full -24dB per octave attenuation slope. This is also known as a "4th order" filter. This design results in relatively steep attenuation, and maintains closely coherent phase response* with no loss of the summed amplitude when both passbands combine at a coincident -6dB down crossover juncture**.
What crossover frequency should I set?
It's generally best to set the crossover at some frequency between 80Hz and 98Hz. This decision primarily depends on the bass capability of your main speakers. If they're small mini-monitors with little 5 inch diameter woofer cones, then pick a crossover in the 94 to 98Hz range. If you've got main speakers with 6 inch or bigger woofer cones, then you can go lower, but don't push below 80Hz (and never < 70Hz) or you'll invite other messy problems (bass range too compressed, latency offset too long). The high end is mostly a matter of your subwoofers' capabilities. Some subs are capable of relatively flat output to 130Hz, whereas others droop abruptly beyond 100Hz. In most cases, and assuming main speakers with 6 inch or bigger woofer cone Ø, a crossover of 84 to 86Hz is ideal.
My room layout requires that my subwoofers be spaced pretty far away from my active external crossover control unit. Will I need any special cable to make the runs to my subwoofers' line-level inputs?
That route is really benign. You can use any ordinary RCA-type unbalanced coaxial (shielded) cable. Consider: The output is at low impedance (assuming the use of any respectable active crossover controller), and the signal is from the controller's low-pass output, so it's exclusively low frequency information. You can use a fancy XLR cable hookup if you like, but there would be zero benefit.
What are the essential control options that my self-powered subwoofers should offer?
Your subwoofers should provide…
(a) Automatic sound-sensing with remote turn on/off capability, and a manual on/off override switch. Also some visual means (e.g., LED lights) of indicating the current state (fully off, stand by, or active).
(b) A bypass switch to permit the defeat of all internal filtration.
(c) An input gain level control.
(d) A polarity inversion switch.
(e) A fully variable phase angle control (encompassing 0° to 280° [as referred to 80Hz]) to optimize the sub-to-mains phase synchronization setting.
BG (June 9, 2020)
*Both sections emerge closely matched in phase, although the low pass section will lag the high pass section by approximately one full wavelength due to group delay encountered in processing. One wavelength at a typical crossover frequency entails a lag time of ~ 12 to 16 milliseconds. A delay of that duration still remains well within the documented "fusion zone" limit of 30 milliseconds max., hence will not be perceived as a separate sound. (Refer 7.6.4 of Floyd Toole's Sound Reproduction, 3rd edition [Routledge, 2018, ISBN 978-1-138-92136-8]).
**Refer graphic, https://en.wikipedia.org/wiki/Linkwitz–Riley_filter.