SUBWOOFERS FOR 2-CHANNEL SYSTEMS

How To Add A Subwoofer If You Don't Have A Preamp/Subwoofer Out

Or, why you should add a subwoofer even if you don’t have a preamp/subwoofer out


Do subwoofers belong in music? We’re used to seeing subwoofers in home-cinema systems, and multichannel preamps and processors nearly always have a subwoofer out. In fact, home-cinema subwoofers are given their own channel, called the .1 or the LFE output (low-frequency effects). 


But what about stereo systems? How do you connect a subwoofer, and should you? Some stereo amps and preamps have a “subwoofer out” which is usually just a preamp output, meaning it’s a regular full-bandwidth line-level signal that relies on the subwoofer’s crossover to decide how much high-frequency information the sub will throw away, and how much low-frequency information it will keep. (This also means that you can connect a power amplifier to the subwoofer output, but don’t depend on this being true of your particular make and model.) 


In spite of the name, a ‘subwoofer out’ isn’t the best way to connect a subwoofer to a quality music system. The preferred method works even if a preamp doesn’t have special subwoofer or preamp outputs, however, you do need a subwoofer that can accept a “high level” (also called “speaker level”) input. With this method, the subwoofer connects to the back of your amp exactly where you plug in your speakers.Don’t worry, the subwoofer will not draw power from your amp. An active subwoofer presents a very high impedance load, so the amp doesn’t work any harder when you connect one, two, or even a line-array of six subs to it. The sub is just seeing a signal and amplifying it. 

Why would you want a high-level signal?

When your subwoofer is on the high-level input, it’s seeing exactly the same signal as your speakers, with no timing differences at all. The RCA subwoofer output on a preamp is going to be ever so slightly out of sync with the amp’s speaker outputs. Though it’s a tiny lag, it’s enough for subwoofer-speaker combo to be noticeably more coherent on the high-level input than on the RCA input. 


Also, because the subwoofers “see” the same tonal balance and damping factor as your amplifier (very simply, ‘damping factor’ is the control an amp has over the bass drivers on the speaker), the bass signal for the sub is textured just as it is on the speakers, and starts and stops at the same time across all the bass drivers.While a subwoofer on a home-cinema system is all about chest-thumping explosions and room-shaking rumbles, a subwoofer on a stereo system has to perform a far more subtle duty. A home-cinema sub can be all about sheer bass output, and music sub needs to be more about speed and articulation. 

So what subs should you get?

For high-fidelity music systems, we recommend subwoofers by REL Acoustics. Subwoofers are all this legendary British company make, they’re its raison d’etre. We love REL because they design their subs like any high-end loudspeaker--with a focus on speed and articulation, and the ability to cleanly reproduce bass down to the specified low-frequency extension. It’s easy to make a bass bin that farts at 20 Hz, it’s far harder to make a true sub-bass system that has linear extension down to 20 Hz. 


Speed is everything for a sub. REL invests a huge amount of time and money in developing driver cones that are light, but stiff. Most subs have big heavy cones that produce whopping bass but are really hard to stop once they get moving. As a result, their bass is powerful, but also bloated and sloppy. It’s hard to create a light cone for subs because light cones tend to flex, causing phasing and other errors. REL works with materials such as aluminum and carbon fiber to produce cones that can move massive amounts of air quickly and without flexing. 


Speed is also an important consideration for the sub’s circuitry. Remember, the signal from your amp to the speakers has a much more straightforward journey than the one from amp to subwoofer, which has to hit another amp before it can move the sub’s speaker cone. As a result, REL pours engineering money into designing the fastest possible filters to process the signal for the subwoofer amplifier. 


All of this means that REL subwoofers can integrate with the best loudspeakers in the world. In systems where any other sub would sound slow, or fuzzy, or muddy, or boomy, RELs are incredibly deep, but fast and accurate. Kick drums have snap and attack just like they do in real life. Double basses go way low without turning to sludge or simply vanishing in the mix. 

It’s not all about bass

The true benefit of a fast, well-integrated sub-bass system is not about bass, at least, not in the way we usually think of bass. When REL subwoofers are set up right, you shouldn’t even know they’re there, until they’re turned off. The soundstage then seems to just collapse on itself, making a relatively flat, harsh picture. John Hunter, the owner and design director of REL Acoustics, has many demonstration tracks that have no apparent bass--just a voice and guitar for example--but the subs offer low-frequency spatial cues that make listeners feel they are in the recording space, and a part of the performance. Turn the subs off, and that illusion vanishes. Participants become observers.

Does humor belong to a sub?

One of the Upscale crew fondly remembers a profound demonstration from a couple of years ago featuring REL subs. John Hunter of REL set up a system with a pair of subs, and played a track that began with some funny studio banter that depended on unintentional comic timing. Everyone present at the demo laughed. John turned off the subs and played the track again. It was still funny, but nobody laughed. The subs were turned back on, and track played again. Everyone laughed. 


How did this work? With the subs off, the track was the playback of an event that happened at some other place and time. With the subs on, it was as if watchers were inside that studio, breathing the same air as the musicians. It was as if the action was unfolding in the moment. That feeling of being present made the track funnier with subwoofers. 


What all of this means is that even the best systems can use extra amplification and driver square-footage in order to sound more “real”. Think of the amount of energy put out by even an all-acoustic band, and the attempt to replicate that over just a few small drivers. This is why even a pair, heck, even a six-pack of subs is not necessarily overkill.


Home audio systems at every level can do with a little help, and once you get used to the sound of a system with a pair of fast, articulate, carefully adjusted subwoofers, it’s very hard to go back to life before sub-bass reinforcement.