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Adding a subwoofer to a stereo or multichannel home-theater system is one of the best I ways to improve its sound quality. It’ll provide the firm low-frequency foundation on which massive musical and sound effects are built if it is placed, hooked up, and adjusted properly. But there’s the rub: Subwoofer placement, connection, and level setting are miserably handled by most subwoofer manuals. Last January’s  “Subwoofer Secrets” explored the placement question, and this article on connection issues will take you one step closer to bass bliss.
Consider a typical “full-range” speaker response, as shown by the red trace in Figure 1A on the next page. It rolls off at the lowest frequencies, which is why a subwoofer is needed in the first place. A typical subwoofer frequency response is shown by the blue trace. It rolls off at higher frequencies. The ultimate objective in hooking up a subwoofer is to arrange the frequency responses of the subwoofer and the main speakers at and around the frequency where their outputs converge the “crossover” point — so that they combine to produce a flat overall response (the green trace in Figure 1A). If the rolloff points of the sub- woofer and main speakers are too widely separated, you’ll get a dip or hole in the combined response (Figure 1B), and if they are too close together or overlap, you’ll get a rise in response in the crossover region (Figure 1C).
In the real world, the humped response of the green trace in Figure 1 C is the most likely result if you simply throw a subwoofer into a system. In order to “force” the responses of the subwoofer and the main speakers into predictable behavior, a special crossover filter circuit is used. At the very least, a subwoofer crossover has a low-pass filter section (only the low frequencies pass through) that rolls off the subwoofer’s high-frequency response at a fixed rate but usually with a user-adjustable frequency. The low-pass output is fed to a power amplifier, which then drives the subwoofer proper (Figure 2).
Many subwoofer crossovers also have a high-pass filter (only the high frequencies pass through) that rolls off the lows fed into the main speakers. Whether the high-pass output should be “looped back” to the main system is a controversial point, since some people think that the main speakers should be run full-range, with no loop-back high-pass filtering. But such filtering produces several very important benefits:
• It reduces the power necessary to drive the main speakers to high levels. Looked at another way, it increases the overload margin of the main-speaker amplifiers by reducing the amount of low-frequency signal they must deliver.
• It reduces the main-speaker distortion produced by overdriving them with frequencies they cannot handle properly.
It increases the chances of getting a more even overall response in a room by reducing the interference effects produced by any subwoofer and main- speaker response overlap.
On the whole, the arguments in favor of utilizing a subwoofer crossover’s high-pass output make a very convincing case for trying your utmost to incorporate a loop-back connection into your subwoofer hookup. High-pass filtering is done as a matter of course by THX surround decoders and is available on other components as well. You’ll also find a high-pass filter in many powered subwoofers’ crossovers. High-pass filtering should be performed at line level, which complicates its use, as we’ll see. Some components supply speaker-level high- pass filtering, but that can cause substantial response irregularities in the main speakers from impedance inter actions unless the high-pass filter is designed to operate with that specific brand and model of main speaker.
Now for some specific subwoofer hookup recommendations. Keep in mind that whenever I say “receiver,” the remarks apply equally to an integrated amplifier and, usually, a preamp with the same available connections. All these components can be of stereo or A/V configuration. Also, my comments refer most specifically to hooking up a subwoofer’s crossover into a system, since that is where the confusion usually arises. With powered sub- woofers you don’t have to worry about much else in the hookup, and a passive subwoofer requires a power amplifier connected after the crossover, a straightforward matter.
Many low-cost receivers have no line-level out puts suitable for feeding a powered sub-woofer. Tape-recorder, equalizer, and similar external-processor connections, which are all at line level, won’t work because they occur in the signal path before the volume control. Connecting a subwoofer to these line-level outputs would make the bass volume uncontrollable by the master volume control. What’s needed is to tap into the signal path after the volume control, such as at a receiver’s speaker outputs. So, although it is not an ideal arrangement, many subwoofers must use speaker-level connections. One such hookup is shown in Figure 3. Note that here each of the receiver’s speaker terminals (shown as multiway binding posts, but they could also be spring or snap-grip connectors) gets two wires attached to it; one goes to a main speaker and the other to the corresponding speaker-level input terminal of the subwoofer. Note also that it is important to preserve the phase relationships among the connections plus terminals to plus terminals, minus to minus — but left-right channel relationships need be preserved only for the main speakers (unless you are using stereo subwoofers).
Unshielded speaker cable can be used with any speaker-level hookup. In Figure 3 the cables that run from the receiver to the subwoofer don’t have to be thick or expensive. That’s because the subwoofer’s speaker-level inputs draw very little current corn pared with a typical speaker. Besides, a subwoofer reproduces only low frequencies, and any potential cable-related signal-altering effects would occur at very much higher frequencies.
There are, however, variations on the speaker-level theme in which high-current cable should be used throughout.
For example, your subwoofer may have loop-through, not loop back, connections: a set of speaker level "output" terminals hard-wired directly to the input terminals. You might want to use these instead of the speaker terminals on the back of the receiver in order to shorten or other wise simplify the cable runs around your listening room. The speaker signals would then run from the receiver to the subwoofer and from there to the main speakers. If you do use loop through connections, all of the speaker cables, including those between the receiver and the subwoofer, should be of a suitable heavier gauge.
Most mid- to high-priced A/V receivers have a single subwoofer output carrying a sum of the left and right channels. Although this is not ideal either it makes no use of the crossover's high-pass output - it is the simplest line-level connection to use and may prove more convenient than a speaker-level hookup. Connect the receiver's subwoofer output to the sub woofer crossover through a shielded stereo cable by using a Y connector to feed both crossover inputs with the same subwoofer signal (Figure 4), because the subwoofer will probably be operating in mono (stereo subwoofer outputs being rare), you might think that connecting just one of its inputs would suffice. But using the Y connector to feed both inputs eliminates the possibility of picking up noise and interference from an unterminated input.
The best way to hook up a subwoofer - a configuration that will finally take advantage of a crossover's high pass output is shown in Figure 5.
Even though it is also simple, needing only a pair of common stereo cables, this connection requires your equipment to have a set of pre-out (preamplifier line-level output) and main-in (power-amplifier line-level input) connections. The signal goes from the receiver to the subwoofer crossover, where it is split into high-pass and low-pass portions. The low-pass signal is amplified and sent to the subwoofer; the high-pass signal returns through the receiver's main-in connections to be amplified and sent to the main speakers.
Unfortunately, pre-out/main-in connections seem to be rare on A/V receivers and integrated amplifiers, though not on two-channel stereo equipment. The line-level surround decoder outputs of A/V components are not suitable. But systems having a separate preamp and power amp can use a variation of the previous hookup too another good reason for prefer ring separates to receivers and integrated amplifiers. In this case, the crossover's line-level high-pass output feeds the separate power amplifier (Figure 6).
Those are the main subwoofer hookup options. Although only single subwoofer hookups are shown here, stereo subwoofer hookups follow the same general principles. But there is an additional subtlety to subwoofer operation that is, surprisingly, often overlooked in equipment manuals, and while it is not directly related to sub-woofer hookup, it deserves mention.
If you have a separate center-channel speaker in your subwoofer equipped home theater system, you must switch the surround-decoder center mode to Normal, not Wide or Wideband. Most low bass in sound tracks is steered by a surround-sound decoder into the center channel. Not using the Normal setting risks either losing center channel bass altogether or overdriving the center speaker with low frequencies it can't handle with out severe distortion. Using Normal mode will shunt any bass that is steered to the center by the surround sound decoder equally to the left and right front channels, from which they will eventually reach the subwoofer. If you don't have a separate center speaker and are using a surround de coder's "phantom" center-channel set ting, don’t worry - the center's bass will get out. But using a separate center speaker is usually preferable (unless it is of inferior quality) because it provides superior image stability from off-center listening positions.
Normal mode must be used in all of the subwoofer hookup schemes discussed above. The only case in which it is not required is in a Home THX system (one using at least a Home THX surround decoder and a set of Home THX speakers, including subwoofers) because of the specifically matched characteristics of Home THX speakers and the crossovers supplied in THX surround decoders.
Once you've finished placing and hooking up your subwoofer(s), you can join us in contemplating how to correctly set its level and adjust its crossover frequency relative to the main speakers. These related problems are very difficult to solve without the use of special test signals and a sound level meter. But when we come up with solutions, you'll be the first to know.
Source: Stereo Review (09-1995) BY DAVID RANADA
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