Subwoofer Amplifier Matching for Booming Bass

Unless your current amp can already handle a sub, you're going to need a dedicated subwoofer amp. Science Photo Library - VICTOR HABBICK VISIONS / Brand X Pictures / Getty

The only way to achieve truly great bass is with a subwoofer, but deciding to add a sub to your car audio setup is only the first step in a longer journey. All that power needed to feed your hungry sub has to come from somewhere, and that somewhere is an amplifier. The question is can you squeak by with the amp you already have, or do you really need to add a dedicated subwoofer amplifier at the same time you add your sub?

The answer is complicated, and it really depends on factors like how much money you want to spend and how picky you are about the way that the finished product sounds. There are definitely ways to make an existing amp work with a sub, but the best results always come from matching a subwoofer and an amplifier to make sure that they work together in beautiful harmony.

Who Needs a Subwoofer Amplifier?

The short answer is that everyone who wants a subwoofer in their car also needs a subwoofer amplifier. As to whether you need a separate amp for your subwoofer, that depends on the hardware you already have and the car audio system that you’re trying to build. Since everyone wants something a little different out of their car audio system, there really aren’t any wrong answers, but there probably is a best answer for your particular situation.

What is Subwoofer and Amplifier Matching?

When you look at the specifications for any given subwoofer, it will have an impedance listed in ohms.

This number is basically the load that the sub will put on an amplifier. Since amplifiers put out power depending on the load that's attached, it's important to make sure that these numbers line up.

The key figures are impedance, measured in ohms, and power output. In this case, power output is given as watts root-mean-square (RMS).

In terms of a subwoofer, watts RMS refers to how much power the sub can handle without producing distortion or becoming damaged. On the amplifier side, it refers to how much power the amp can put out.

The basic steps in matching an amplifier to a subwoofer are:

  1. Determine the watts RMS rating of your sub or subs.
  2. Determine the impedance of your sub or subs.
  3. Choose an amplifier that can put out between 75 to 150 percent of the watts RMS your subwoofer can handle at the appropriate impedance.

If you already have an amplifier, the basic steps to find a matching sub are:

  1. Determine the amp's power output, in watts RMS, at different impedance values.
  2. Divide the power output by the number of subs you want to add to get the optimal RMS value for each subwoofer. In practice, the subwoofers can be between 75 and 150 percent of this number.
  3. Make sure that the impedance also matches. Subs with multiple voice coils can usually be wired in multiple ways, which affects the impedance.
  4. Choose subwoofers that can handle the appropriate power output at the selected impedance.

Powering a Sub: Multichannel Amps and Mono Subwoofer Amps

As a general rule of thumb, your subwoofer will need more power than your other component or full range speakers.

Even a small sub will often require upwards of 50 watts RMS, which is more than the built-in amplifiers in many head units can put out total.

When you start getting into bigger subs that need upwards of 200 watts RMS to sound really good, it starts to become clear that you’re not going to get away without an external amp of one kind or another. However, you do have the option of going with either a multi-channel amp or a dedicated mono subwoofer amp.

Without knowing anything about the amp you already have, it’s tough to say whether it’s going to do the trick for your new sub. If you’re already using all of the channels to drive speakers, then you’re out of luck.

If you have a multi-channel amp with two open channels, then you may be able to use it to power two full range speakers and a sub, although the specifics of this type of setup can get a little tricky.

Bridging Multi-Channel Subwoofer Amps

In order to use a multi-channel amp to power a sub, you typically need to bridge two channels, and that doesn’t always work with every amp. The important thing to understand here is that most amps are stable down to 2 ohms per channel.

If you try to hook up a load that has less than 2 ohms of impedance, you’re going to run into trouble. Since nearly all of the full range speakers you can get for your car will have an impedance of 4 ohms, this typically isn’t a problem. However, it can be an issue when you throw subwoofers into the mix.

Unlike full range speakers, car subs don’t all provide 4 ohms of impedance. In fact, subs can even have multiple voice coils, which can complicate the matter even further. For instance, a sub with two 4 ohm voice coils, wired in parallel, will provide a 2 ohm load, but those same voice coils wired in series provide an 8 ohm load. That means if you bridge two amp channels together, you’ll typically be fine powering a parallel-wired 2 ohm sub, but it’s important to take a hard look at the numbers first.

Mono Subwoofer Amps

The easiest way to power a new sub is to just pair it with a properly-sized mono amp. Unlike multi-channel amps, mono amps are really designed with subs in mind. Instead of messing around with bridging two channels, you simply hook up the single channel of a mono amp to a corresponding subwoofer, and you’re good to go.

If you’re relatively new to the world of amps and subs, and you’re going the DIY route, this is definitely the safest bet.

In order to get the best sound out of your new sub, you’re going to want to go with a mono amp that has an RMS rating of at least 75 percent of the sub. The more power you use to drive the sub, the better it’s going to end up sounding, so if you can push it all the way up to 100 percent you’ll end up with even better results.

Adding a Subwoofer Amp in Multi-Amp Systems

If you already have an amp for your full range speakers, and you want to add a new mono amp to your system, your options are going to depend on your head unit. Some head units have multiple preamp outputs, in which case you can simply plug a new set of RCA cables into an unused output and hook them up to your new subwoofer amp.

Some head units only have one set of preamp outputs, in which case you’ll want to check your existing amp. If it has a pass-through (including a set of RCA preamp outputs) then you can essentially daisy-chain your new subwoofer amp to the amplifier you already have. Otherwise, you may have to use a Y splitter cable.

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