Choose a subwoofer amplifier based on how well it can provide a signal to a large driver. This is the most important feature.
The rule of thumb when you set up your system is to send half the power to the subwoofer system. If you want to set up a 1,000-watt stereo in your car, 500 watts should go to the subwoofer. You have to move a lot of air to make bass that will keep up.
You should buy an amplifier that includes these three features:
These are three features that are nice to have but not needed:
Mono Design
Low frequencies have a very long wavelength. We cannot localize lower frequencies as well as we can localize higher frequencies. We can localize higher frequencies because high frequencies have a short wavelength. If you were hiking in the wilderness and you heard a bird singing, you would look in that direction. Our brains can use the audio signals from a bird song to triangulate the location of the bird. The wavelength is shorter than the distance between our ears. The phase of the waves at our ears will be different, as well as the sound pressure levels. Our brains take the differences between the bird song at our ears to tell our eyes where it comes from.
Low frequencies don't have this feature. If a footfall causes a rumble, we generally don't know where it's coming from. We'll get the thud. If the rumble source is moving, our brain can triangulate that position. Since we're in a fixed position in the car and the subwoofer doesn't move, we can use a mono amplifier. It's one less channel to fiddle with so it's easy to match to your subwoofer impedance and power output.
Low Pass Filter
We do not want high frequencies to play through our subwoofers. We need to filter the high frequencies out of the signal sent to the subwoofer. The filter should be adjustable to better match up to your main speakers.
Remote Bass Knob
There are several reasons to want a remote bass knob.
- You're not gonna go booming every time you get in the car. (It's true!)
- You want to listen to an audiobook or a podcast. A subwoofer tends to bring hard consonant sounds from P and B. Puh Buh, Buh Puh. It's distracting.
- You have small children in the car. Protect those young ears!
- You are listening to music where heavy, thumping bass takes away from the music. In my own experience, "Before you Accuse Me" on Eric Clapton's Unplugged CD includes a foot tap. The center frequency of this foot tap seems to be at most car interior signature frequencies. What should be a simple "pat pat pat" turns into a romping "thud thud thud" with a powerful car stereo. Using a bass knob can bring this down to a normal SPL.
You should use the knob to achieve balance from bass to treble.
Subsonic (or Infrasonic) Filter
If you have a ported enclosure, the subwoofer may not play much lower than the port tuning. You may cause the subwoofer to unload. A subsonic filter will cut the signal to the ported box for frequencies it should not try to reproduce. You won't need a subsonic filter for a sealed box. Like the low pass filter, you want to make sure the subsonic filter is adjustable.
Bass Boost
Bass boost has been a feature of car audio amplifiers since the early 1970s. Bass boost goes by many names. It boosts the signal around frequencies that make subwoofers fun. Most of these bass boost controls are adjustable in both frequency and intensity. You have to be careful with this feature. Bass boost can overdrive your amplifier. Overdriving the amp can introduce clipping and harmonic distortion. Both can damage your amplifier and your subwoofer's voice coils. It is a good idea to reset the amplifier gain after introducing some bass boost.
Clip Indicator
The clip indicator light could be on the amplifier chassis or on a remote knob. Some may include a light on both. The clip indicator is there to keep you out of trouble.
You can use a clip indicator light for setting gains. Play a test tone with the bass knob all the way up and the gain all the way down. Play a test tone at about 40hz. Turn the deck volume up until the clip indicator lights up. Turn the volume down until it turns off. Turn the gain up until the clip indicator goes on, then turn the gain down until it turns off.
The main selling point of a clip indicator is for demonstrations. When you have a large subwoofer system and you want to show it off to others, you want to make sure you take it to the limit. Taking it past the limit means equipment damage. You'll need a visual indicator to stay below that limit. The clip indicator is there to keep you out of trouble.
Final Thoughts
An amplifier was an amplifier in the past. It received a signal and then amplified that signal. Manufacturers included a gain control so you wouldn't go blowing things up. If one wanted filtering and signal manipulation, one would need equalizers and crossovers. Now we can get amplifiers with built-in BlueTooth and on-board Digital Signal Processors. This article narrowed down the list of features to three must haves for a subwoofer amplifier, and three features that are nice to have.
