Choosing the Right Amp for Your Car or Truck

Pay attention to these tips when shopping for an amp

Custom speaker installed in a car door


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Every car sound system has an amplifier of some sort, but most are built into head units. If you’ve ever cranked up the volume on the stereo and noticed a lot of distortion, it is likely due the underpowered, built-in amp. The power-handling characteristics of your speakers also come into play, but a good amp can do wonders even in a stock setup.

If you’re looking to upgrade an existing amp or install a brand new one, pay attention to these three main factors:

  • The Number of Channels
  • Power
  • System compatibility

How Many Channels Do You Need?

Amplifiers are available in a number of different configurations, and the right number of channels depends on how many speakers you have in your sound system. In general, you need one channel for each speaker that you want to amplify. If you’re adding a subwoofer to an existing system, then a single channel amplifier will get the job done. There are also mono amplifiers with a class D rating that are specifically designed to use less power and put out less heat when amplifying subwoofers.

Units that have two, four, or six channels are more versatile. You can use a two-channel amp to power two woofers or two coaxial speakers, or you can bridge it to run a single sub or power two sets of coaxial speakers. If you want to add a subwoofer and provide more power to your rear full-range speakers, then a four-channel amp will do the job. In that case, you can run each full-range speaker off its own channel, and then bridge the other two to power the sub. Alternatively, you can power all four coaxial speakers off the same amp, and then install a separate mono amp for the subwoofer.

Component systems can be more complicated, and you may end up needing more than one amplifier, external crossovers, and other components.

Don't Skimp on Power

If you want to get the best sound out of your car stereo, it’s vital that you don’t underpower your speakers. That’s why a lot of people pick out speakers first and then find an amp with enough juice to power them. If you’re just working with your factory speakers, you still should find the RMS (root mean square) value and then choose an amp that’s capable of putting out at least 75 percent to 150 percent of that number.

Power is also a concern if you’re looking to run a sub off the same amp that you’re using to drive your speakers. Bridging two channels of a multichannel amp may provide enough power to run a sub, but it isn’t ideal in every situation. If the amp cannot match your specific subwoofer’s power needs, then you’re better off looking for a separate mono amplifier that is capable of doing the job.

Head Unit and Amplifier Compatibility

If you’re building a car audio system from the ground up, then there’s no question about it: Buy a head unit that has preamp outputs and an amplifier that has line-level inputs. By providing a raw, un-amplified signal to the amp, you’ll end up with the clearest sound possible.

Most factory head units and many aftermarket units don’t have preamp outputs. If you’re working with an existing head unit that falls into that category, then you need an amp that has speaker-level inputs. This will result in better sound than what you would get without the external amp, and it will save you from having to mess around with additional wiring or adapters.

Aftermarket Car-Amplifier Installation

Installing and wiring an amplifier isn’t rocket science, but you should think about the location and how you will route the wires while you’re shopping for a unit. Most vehicles don’t come with amps out of the factory, so you’re going to have to find a technician to install the new hardware. With that in mind, taking some measurements before you buy an amp can simplify matters.

Some popular installation locations include:

  • Under one of the seats
  • In the trunk
  • Against the passenger-side firewall

Checking the measurements of those spaces ahead of time can save a lot of grief down the line. The same goes for installing component speakers and subwoofers, which typically won’t have been accounted for in the design of your vehicle.

Also keep in mind that you have to provide power to your amplifier, which means you have to run additional wires. You also need to make sure your car's alternator can handle the increased power draw.