The answer is complicated, and it depends on how much money you want to spend, how picky you are about sound quality, and a few other factors. There are ways to make an existing amp work with a sub, but the best results come from matching a subwoofer and an amplifier to achieve the harmony you desire.
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 have and the car audio system that you’re trying to build. Since everyone wants something a little different from their car audio system, there aren’t any wrong answers, but there is a best answer for your situation.
What Is Subwoofer and Amplifier Matching?
When you look at the specifications for a subwoofer, it has an impedance listed in “ohms.” This number is the load that the sub puts on an amplifier. Since amplifiers put out power depending on the attached load, you need 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:
Determine the watts RMS rating of the sub or subs.Determine the impedance of the sub or subs.Choose an amplifier that can put out between 75 to 150 percent of the watts RMS the subwoofer can handle at the appropriate impedance.
If you have an amplifier, the basic steps to find a matching sub are:
Powering a Sub: Multichannel Amps and Mono Subwoofer Amps
As a general rule, a subwoofer needs more power than other components or full-range speakers. Even a small sub often requires upward of 50 watts RMS, which is more than the built-in amplifiers in many head units can put out total. When you get into bigger subs that need upward of 200 watts RMS to sound good, it becomes clear that you’re not going to get away without an external amp of one kind or another. However, you have the option of going with either a multichannel amp or a dedicated mono subwoofer amp. Without knowing anything about the amp you have, it’s tough to say whether it’s going to do the trick for your new sub. If you’re using all the channels to drive speakers, then you’re out of luck. If you have a multichannel amp with two open channels, 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 Multichannel Subwoofer Amps
To use a multichannel 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 is that most amps are stable down to 2 ohms per channel. If you hook up a load that has less than 2 ohms of impedance, you’ll run into trouble. Since nearly all the full-range speakers you can get for your car will have an impedance of 4 ohms, this usually 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. Subs can have multiple voice coils, which can complicate matters further. For instance, a sub with two 4-ohm voice coils, wired in parallel, provides a 2-ohm load, but those same voice coils wired in a series provide an 8-ohm load. As a result, if you bridge two amp channels, you’ll typically be fine powering a parallel-wired 2-ohm sub, but it’s important to take a look at the numbers first.
Mono Subwoofer Amps
The easiest way to power a new sub is to pair it with a properly sized mono amp. Unlike multichannel amps, mono amps are designed with subs in mind. Instead of messing around with bridging two channels, 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 bet is the safest. To get the best sound out of your new sub, consider 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 will sound. If you can push it up to 100 percent, you’ll end up with even better results.
Adding a Subwoofer Amp in Multiamp Systems
If you have an amp for your full-range speakers and want to add a new mono amp to your system, your options depend on the head unit. Some head units have multiple preamp outputs, in which case you can plug a new set of RCA cables into an unused output and hook them up to the 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), you can daisy-chain the new subwoofer amp to the amplifier you already have. Otherwise, you may have to use a Y splitter cable.
