How can I drive a pair of speakers and a subwoofer with a single car audio amplifier?

One of the keys to getting the most out of your audio investment is choosing gear that will go to work for you now, and won't become obsolete as your system grows. Most car audio amplifiers boast a design flexible enough to keep them in the game as your set-up expands.

If you go with a 4-channel amplifier, powering a pair of front speakers and a subwoofer is a breeze. You'll simply want to run your amp in what we call "true" 3-channel mode. To do this, bridge the rear channels to power your subwoofer, while the front channels drive the pair of regular stereo speakers. Bridging the rear channels means combining them in mono mode to create a single channel. Choose an amp that lets you engage a built-in, low-pass filter on this bridged channel. The crossover, along with the increased output from the mono channel, makes this an ideal way to power your sub.

As your system grows, you may dedicate a separate amplifier to your sub. At that time, you could add another pair of stereo speakers for rear fill, and run your 4-channel amp in stereo mode.

But what if you're starting out with just a simple two-channel amp and you still want to power a pair of stereo speakers and a sub? No problem. That's where a feature known as "Tri-way mode" (or "Tri-Mode") kicks in.

You can attach a Tri-way crossover to a pair of stereo amplifier channels to produce three channels — two stereo channels for front speakers, and one mixed-mono channel for your sub.

The Tri-way crossover is actually a special kind of passive crossover network. It applies a high-pass filter (typically at around 100 Hz) to each of your amplifier's stereo channels, sending the information above the crossover point straight to your stereo speakers. It also combines the low-frequency information from each of these channels into a single, mixed mono channel, which is sent to your sub. As is the case when you bridge an amplifier, the mono channel delivers more output than the stereo pair.

If you're about to purchase an amplifier, you'll get the most value by purchasing one that can run in true 3-channel mode. If your goal is to eventually own a high-performance "audiophile" car audio system, you'll want the performance and features that these amps typically provide. You'll be able to adjust the volume of the sub, and in most cases, the frequency of a built-in electronic crossover, as well. You give up these conveniences when you go Tri-way.

But while Tri-way mode doesn't give you the degree of control or performance that a true 3-channel system provides, it is an affordable, effective way to get the most out of a budget system built around a single amp. And because Tri-way crossovers are relatively inexpensive, if you decide to add more amplifiers and reconfigure your system later on, you won't be out a huge investment.