Learning how to choose a car amplifier for your subs can be confusing. Have you ever wondered why car amplifier manufacturers give you two different types of power ratings; peak power and RMS power? What does it mean when the amplifier specs say it produces X amount of power at 2-Ohms or Y amount of power at 4-Ohms? Choosing the right amplifier can be a daunting task. But it doesn’t have to be. This post will outline all the basic information you will need to make a smart purchase.
If you don’t already have an idea of what kind of subwoofers you want or if you haven’t already read my post on How to Choose the Right Subwoofer, I would highly recommend you check that out first.
Car Amplifier Classes
If you have done any research on amplifiers already then you may have noticed that they are categorized into different lettered classes. There are dozens of different amplifier classes but only four of them are most commonly used in car audio applications. These are the class A, B, AB, and D amplifiers.
Each class has its own advantages and disadvantages and it’s important to know these in order to make an educated decision on what amp you should get.
Let’s explore these four amplifier classes and discuss the different applications they might work best in.
Class A amplifiers offer the highest quality and most accurate sound reproduction of the four. Now, I know what your thinking.
“Sweet! I want good quality sound so Class A it is.”
Not so fast. This sound quality comes at a price. And I don’t mean dollars.
The design of a class A amplifier is such that there is always current running through the output transistors which translates into a lot of heat generated. This makes the amplifier very inefficient. A lot of the power consumed by the amplifier is lost as heat, leaving less power available to be delivered to the speakers.
Class A amplifiers are typically reserved for very high end system designs and are mainly used for smaller midrange drivers and tweeters, not so much for subwoofers.
Class B amplifiers, in a sense, are opposite in all characteristics of the class A amplifiers. They use a push/pull configuration so that each output transistor is essentially “switched off” for half of the audio wave. This means they generate less heat which makes them considerably more efficient than class A amplifiers…more power available for the speakers.
However, this switching on and off between transistors causes significant distortion in the audio signal when the waveform switches from positive to negative and vice versa.
Because of this distortion, and the availability of better options, the class B amplifier is not very widely used.
Class AB amplifiers are a hybrid of the pure class A and class B amplifiers. They offer the high efficiency of class B amplifiers with the low signal distortion of class A amplifiers.
This is accomplished in a pretty ingenious way. They use a push/pull configuration like a class B amplifier but instead of completely “switching off” the transistors for half of the audio wave, the current running through them is just reduced. This greatly reduces the distortion seen in class B amplifiers and still makes it significantly more efficient than class A amplifiers.
Class AB is typically used for multi-channel amplifiers to power mid range drivers and tweeters.
Class D amplifiers are significantly more efficient than any of the other classes we’ve discussed so far. This efficiency means less heat dissipation allowing for more compact, high power amplifiers.
Class D amplifiers are a “switching” amplifier that use pulse width modulation to amplify the audio signal. The constant switching or pulsing of the output transistors is what makes these amplifiers so efficient, but it also causes some distortion at higher frequencies.
But this distortion is not a problem when there is a built in low pass filter. That makes class D amplifiers the ideal choice for subwoofer applications.
Now that that’s all out of the way, if you’re feeling overwhelmed…don’t be. You don’t have to know how an amplifier works to choose the right one. All you need to take away is that class D is the way to go for driving subwoofers.
Peak Power Vs. RMS Power
Manufacturers will give you two types of power ratings for their amplifiers, peak power and RMS power. Confusing right?
To add to the confusion, the number that is sometimes written in big bold text on the front of the packaging is typically the peak power. And that isn’t really a number that interests us at all.
Peak power, for an amplifier, is the maximum amount of power it can output in a single instant. Since real music doesn’t consist of just a single bass note, this number isn’t very useful to us.
What is useful is RMS power. RMS stands for root mean square and is a fancy statistical term that basically means the average over a period of time.
To get a realistic comparison between any type of car audio equipment whether it be amplifiers, subwoofers, full range speakers, etc., you should be looking at their RMS power ratings.
And to truly get an apples to apples comparison, make sure that the product you are looking at is CEA-Certified. All products that are CEA-Certified are tested using the same testing methods so the results can be trusted to be equivalent.
How Much Power Do I Need?
The amount of power you will need for your system depends on the subwoofers you choose. Hopefully by this point you either already have your subs picked out or you have an idea of how to pick the right subs for you. If not, check out my post on How to Choose the Right Subwoofer.
If you will be powering more than one subwoofer from a single amplifier, you want them to all be the same. If they aren’t then the power may not be distributed evenly and one subwoofer could end up being underpowered. We’ll talk about why that’s not good a little later.
To figure out how much power you will need out of your amplifier, take a look at your subwoofer’s RMS power rating. Now multiply that number by the number of subwoofers you will be connecting to the amp….that’s it!…kind of.
A lot of advice out there will tell you shoot for between 75% and 150% of the total power that you need when choosing an amplifier. The reason they say you can go as low as 75% of the required power is because the average listening volume in the car is not going to be super loud so your subs won’t require all the power they can handle to keep up.
Personally, if I’m designing a system from scratch, I like to shoot for a minimum of 100% of the subwoofer power ratings. I mean, why design a system that can’t possibly perform at its peak potential?
Why You Should Not Under Power Your Subwoofers
You might think that overpowering a subwoofer is bad but it’s actually just the opposite. Underpowering a subwoofer will cause real damage to the sub and eventually fry the voice coil.
When an amplifier does not have enough power to drive the audio signal to meet demands, you get what’s called “clipping” of the audio signal. Take a look at the image below to see what that looks like visually.
As the amplifier tries to drive the audio signal to the peaks and valleys of the sound wave, it runs out of power and the sound wave is clipped.
This clipping of the audio signal causes distortion. This distortion not only compromises sound quality, it also generates a lot of heat in the voice coils. Over time the voice coil will degrade and the subwoofer will eventually fail.
An amplifier’s specifications will tell you that it can deliver a certain amount of power into a specific load (in Ohms). Manufacturers will typically give you a few different power ratings at different loads. For example, it might say 1000 Watts RMS at 1-Ohm, 600 Watts RMS at 2-Ohms, and 300 Watts RMS at 4-Ohms.
The load on the amplifier will be the subwoofers that you connect to it. How much that load is depends on the impedance of the voice coil(s) (in Ohms) and how they are wired together. If you are unsure of how to determine total load when wiring multiple subwoofers together or subwoofers with multiple voice coils, then check out my Subwoofer Wiring Guide.
What you are looking for is an amplifier that can deliver the total RMS power you need into the total load of your subwoofers.
To choose the right amplifier for your sound system you need to know what you are powering. Having your subwoofers picked out first is the key to designing a well balanced system. This will tell you everything you need to know about how much power you need and how much load your amplifier will need to drive that power into. Watch out for the common pitfalls discussed in this post and you’ll be well on your way to designing the perfect sound system.