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How to Connect Subwoofers
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March 2014   Audio Reviews   HDTV   All Reviews

Why you need Stereo Subwoofers

Bass Management.

Heathkit AD-1702 Crossover Review.

 

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How to Connect Subwoofers

The crossover and how it's adjusted are critical to getting great sound. There are a million ways to do it, and only one will sound best for you.

External crossovers work best.

Built-in crossovers in the subwoofer, preamp or receiver can work well, but they tend to lack the flexibility of a dedicated external crossover. Flexibility gives us more ways to optimize a system, and thus external crossovers tend to give us a better chance of getting the best sound. Every installation is different; try your built-in crossovers first if you don't already have dedicated crossovers lying around.

Once you make the hook-ups described below, setting precise levels and crossover slopes and frequencies can take months to fine-tune.

I love Dirac software to help tune a room. Subwoofer placement is even more difficult. The sound will change dramatically for every few inches you move your seat or a subwoofer, and subwoofers are heavy and hard to move. Keep trying; it usually takes me months or even years of experimenting until I get to the "Aha!" spot that sounds great.

Hint: few people outside the professional world realize that a small subwoofer well-placed in a good room will work much better than an extraordinary subwoofer placed casually in the most convenient corner or used in a mediocre room. Moving speakers and listening positions easily can vary bass response by 20 to 30 dB, which is far more difference than there is between the smallest and largest subwoofers.

I'm presuming you're using a subwoofer with a built-in power amplifier. If you're using passive subwoofers (raw drivers in cabinets), you do the same things below, just that when I say "feed the subwoofer," of course I mean feed the power amplifier you're using to drive that subwoofer.

 

How to Connect a Mono Subwoofer

If all you have is one subwoofer, it's easy: feed it from the Subwoofer or LFE (low-frequency effects) output of your preamp or receiver.

Set your preamp or receiver to "Small" for your other speakers.

Avoid connecting the subwoofer with speaker wire. The subwoofer will work well, but those speaker-wire connections on the back of the subwoofer, if it has them, do a poor job of isolating bass from your main speakers.

It's OK to run your main speakers full-range and cross-in the subwoofers on their own.

That was easy. The hard part today is how to connect two subwoofers for real stereo using modern equipment that's designed to use only one (mono) subwoofer.

 

How to Connect Stereo Subwoofers

Why you need Stereo Subwoofers

 

A/V Receivers

A/V receivers are unlikely to be any help here, with their mono LFE outputs. If all you have is an A/V receiver, use its stereo preamp outputs and main amplifier inputs (if it has them) as if you had a component system and follow along below.

 

Separate Components (preamp and power amp)

Ideally you need to be running separate preamps and power amps.

You use a separate low-level crossover to split the preamp's output into main (high-frequency or HF) and sub (low-frequency or LF) signals.

Feed a stereo crossover from the outputs of your preamp (or player). The crossover's HF outputs are fed to your main amplifier and speakers. The crossover's LF outputs are fed either directly to powered subwoofers, or to your bass amplifier and passive subwoofers.

If playing from a computer, Pure Music software also works as a crossover. You will of course will another DAC for the extra two bass channels of output.

I've been using a Heathkit AD-1702 Crossover for decades. Today, look around for a low-level active crossover to suit your taste.

Selecting crossover frequencies, slopes and phases can take forever to get right. It's just like positioning speakers. The HF and LF sections may need different settings if you can set them separately, because your main speaker's LF roll off is part of the summed acoustic response of the HF speakers. In other words, if your main speakers roll off at 40 Hz, you've already got the effect of a free 40 Hz crossover for the main speakers.

The crossovers you'll most often find for sale today are for concert and PA sound, not for home stereo. Also look at this one. JPGs sub harmonic synthesizers also have stereo subwoofer crossovers built in. To get the quality you probably want in a fine home system, you still may need to be an engineer and have to build your own to get the best sound, just like the 1950s.

 

The Subwoofer's Own Crossovers

If you want to skip a dedicated crossover, most subwoofers have built-in crossovers. Even if the crossovers included in a subwoofer are great, actually using them is a different story with the cabling required. Built-in crossovers are less optimum, but easy and free.

Your subwoofer's instructions will cover the details available to you for your particular subwoofers.

The best and most practical way to use the subwoofers' own crossovers is to run your regular speakers full-range, and have the subwoofers cross-in where your main speakers drop off. Use a Y-cable or other way to send the same preamp signal to your power amp for your main speakers as well as to the subwoofers. You also could feed the subwoofers from speaker level if they have that input.

This way we don't need to run the main signal over to the subwoofers and back again. Set the subwoofers' crossover frequencies to the -6 dB rating of your speakers, and set the slopes (if offered) to 12 dB/octave if your main speakers are sealed-box, and 24 dB/octave if your main speakers are vented or have passive radiators.

Another easy way to use a subwoofer's built-in crossover with any system is to use the speaker-level outputs with subwoofers that have speaker connectors. This is also the worst way, as the passive high-level crossover in the subwoofer is only a series capacitor.

What seems like a good way except that it involves running your main signal all over your house is to feed the outputs from your preamplifier over to your subwoofers, and then connect the subwoofers' HF crossover outputs back to your main amplifier. This isn't optimum because you'll be running low-level RCA cables to your subwoofers and back again to your main amplifiers, and you'll probably pick up hum.

Subwoofer signals don't lose anything running 25 feet, but you're asking for trouble running low-level, full-range unbalanced RCA cords 25 feet to your subwoofers and 25 feet back to your main amplifiers. This is why professionals use low-impedance balanced cables; they run long distances without problems. If you must do this, hope that your preamp and subwoofer crossover outputs have low source impedances.

 

Choosing a Crossover Frequency

Most people crossover at 80 Hz, which is the default for A/V receivers, but here's a hint: set the subwoofer lower, to say 40 Hz, and turn up the subwoofer level.

Now you'll get the same mid-bass, and you'll get more of the deep bass for which you got a subwoofer in the first place.

The easiest way to replace boomy, unclear bass with deep, tight bass is to set the subwoofer's crossover to it's lowest setting, usually 30 to 50 Hz, and then turn up the subwoofer's level.

Unless you're trying to design a concert PA system for maximum power handling, I set the crossover frequency for the main speakers based on actual listening. As you change their crossover frequency, if it's adjustable separately from the subwoofers, the bass will change. This can take a long time to fine tune; listen and experiment for the best sound.

 

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