What Class Amp Is Best For Subs? [Ask The Expert]

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Typically, subwoofers are powered with Class D amplifiers.

Why is this done and what kinds of different amplifier classes are available and what is the difference between them?

There are many technical aspects to car audio that can be confusing and a bit intimidating to the non-audiophile.

You don’t have to be an audio engineer or an electronic engineer to understand the basic concepts to amp design and function although things tend to get convoluted pretty quickly and the more you research, the more terms are used which most people don’t understand and are hard to make sense of without an in-depth understanding of electronics.

Don’t worry if you set out to try and learn about amp design and end up more confused than when you started!

Some engineers tend to have a bad habit of gleaning enjoyment from making you feel stupid when they “talk shop” and the quicker they see that expression of WTF on your face, they happier they get.

Fear not though! Your friends here at Gear4Wheels are here to help you get a better understanding of car audio amplifiers in a way that, hopefully, does not make your brain explode.

For the sake of transparency, I’ll make it clear that I am not an audio engineer or an electronics engineer, however I have been in the industry for over 20 years and being involved in both the Car Audio world and Information Technology, I have quite a bit of real-world and technical electronics experience at hand.

Brain explode

So let’s roll up our proverbial sleeves and dive into the wonderful world of mobile audio amplification!!

What are Class D amplifiers and why are they used for subwoofers?

What are Class D amplifiers and why are they used for subwoofers-

Before we get too far into Class D amplifiers exclusively, let’s do a little primer on amp classes and what distinguishes one amp from another by class.

For starters, there are really only two different kinds of amps; analog amps and switching amps.

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This could be broken down as analog and digital as well, however most amps use a combination of technologies and both use forms of switching, be it analog or digital.

Out of all the different classes of amplifiers, the four most commonly used types are Class A, Class B, Class AB and Class D.

  • Class A
  • Class B
  • Class AB
  • Class D.

Class A

    By definition, Class A amps are designed to be “always on”. The internal circuitry dictates that some amount of constant current is flowing through the output transistors at all times.

    This “always on” configuration has advantages and disadvantages which makes it more or less suitable depending on requirements and environments.

    This is where efficiency comes into play. As we’ll explain a little later, Class D amp are considered very efficient compared to Class A amps.

    Another big factor in amplification technology of course, is size.

    As amp technology has progressed, it has gotten significantly smaller. If you’ve ever looked at vintage amps whether they are for live sound (such as guitar amps) or mobile audio, you’ll notice just how big stuff used to be.

    While it would no doubt sound sweet to have a fully analog/tube amp rack in your ride, it’s probably not going to fit.

    That’s why most Class A car audio amplifiers are actually a combination of Class A&B, commonly referred to as Class A/B.

    Class B

    Unlike pure Class A amps, Class B amps employs a form of switching technology.

    That means that they switch their output transistors off when there is no audio signal to amplify.

    This greatly enhances efficiency which makes them more suitable for mobile audio applications, but it comes at a price and that price is a decrease in fidelity.

    Class A/B

    As you might have guessed, Class A/B is a hybrid combination of both Class A and Class B amps.

    While their transistors always have current flowing through them, they use circuitry that is capable of reducing the amount of current flow when no signal is present.

    AB CLASS AMPLIFIER

    This feature of Class B technology vastly improves efficiency over Class A amps, while keeping distortion down and retaining most of the fidelity of Class A amps.

    Due to the benefits of this design, Class A/B amps are the most commonly used design in full range car audio applications.

    Class D

    Back in the day, all mobile audio amps were Class A or Class A/B as the more efficient Class D amps had not yet made their debut.

    With the development of more efficient technology, transistors and the shrinking size of components, Class D amplifiers were ready to make their way to the mobile audio environment.

    D CLASS AMPLIFIER

    The main factor distinguishing Class A and A/B from Class D is the “D” for digital.

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    Ok, not really, but as advancements were made in transistor switching methods, Class D amps employ a switching method in which the transistors are switched on and off at a very high rate of speed, effectively creating a digitally switched or “pulsed” output signal that’s nearly consistent with the analog input signal.

    While this creates an extremely efficient amplifier, the difference in this digital signal compared to its analog signal creates a certain amount of distortion at higher frequencies.

    However since subwoofers primarily reproduce low frequencies, and with the incorporation of low pass crossover filters, this distortion is minimized and is the reason why Class D amps are an excellent solution for subwoofer amplification.

    Hopefully this has given you some insight as to why Class A/B amps are better suited for full range i.e. front speakers, and why Class D amps are more suitable for subwoofer amplification.

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    Let’s do a quick recap in the form of a Pros/Cons list for each amp class:

     

    Pros

    Cons

    Class A:

    • High Audio Fidelity
    • Low Signal to Noise ratio
    • Low Total Harmonic
    • Distortion
    • Larger Size
    • Lower Efficiency
    • More Heat

    Class B:

    • More Efficient Than Class A 
    • Smaller Size
    • Creates Less Heat
    • Lower Audio Fidelity
    • Higher Signal to Noise ratio

    Class A/B:

    • More Efficient Than Class A
    • Less Distortion Than Class B
    • Less Efficient Than Class B
    • More Signal To Noise Ratio Than Class A
    • Slightly Higher Total Harmonic Distortion Than Class A

    Class D:

    • Extremely Efficient Compared to Class A
    • Much Smaller Size (Depending on power requirements and design)
    • Notable Distortion at Higher Frequencies
    • Higher Signal to Noise Ratio
    • Higher Total Harmonic Distortion

    As you can see, there are many different amplifier designs and technologies, and while this guide should give you a general idea of the differences between amplifier classes and how they are used, it is by no means a complete explanation (I left out the really technical stuff on purpose) .

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    External factors

    Since amplification technologies continue to advance, as well as lots of other external factors such as the mobile audio environment, power needs and availability of different vehicles and audio systems, there may be exceptions to the basic rules (as always!)

    And sometimes rules are made to be broken (engineers and physics majors hate that phrase I’m sure).

    For instance: with recent advancements in Class D technology, there are more and more Class D amps on the market that are suitable for full range applications as well.