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What Size Amplifier Do I Need for Subwoofer?

When adding a subwoofer to your car audio system, you’ll need to choose an amplifier that is powerful enough to drive your new speaker. But how do you know what size amplifier you need? Let’s find out.

Generally, the amplifier for a subwoofer should deliver the power of 75% to 150% of the sub’s RMS continuous power rating. For example, a 400W 4 ohm subwoofer will require an amplifier that can produce between 300W and 600W at a 4-ohm load.

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Tip: The good practice when installing a DVC (double voice coil) subwoofer is to use a stereo amplifier with one amplifier channel for each subwoofer coil.

However, if you have installed monsters like Skar VXF-12 with 1,500W RMS (link to Amazon), it is better to use two large monoblocks.

An excellent amp for such a strong sub is Skar Audio SKv2-2500.1D with its 790W RMS at 4 ohms or 1,450 W RMS at 2 ohms (links to Amazon)

This way, you will maintain proper impedance loading, and the amp will deliver the most power to your sub in a direct connection.

In this article, I’ll help you figure out the right amp for your subwoofer and show you how to install it in your car.

What Amplifier Is Best for Subwoofer?

There are two types of amplifiers that are commonly used to power subwoofers:

#1. Class A/B

#2. Class D

Class A/B amplifiers are the most common type used in car audio systems. They are typically more expensive than Class D amplifiers but offer better sound quality.

Class D amplifiers are more efficient than Class A/B amplifiers and generate less heat, and because of that, they are commonly used for the strongest subwoofers.

Another fact that makes the D class amplifiers more suitable for most subwoofers is the quality of the bass tones, which, compared to the mid or high notes, do not require as much precision when it comes to amplification.

In addition to the amplifier class, you must look at the impedance when matching with a subwoofer. Most subwoofers have an impedance of 4 ohms, but you can find models with 2 or even 1 ohm.

When matching an amplifier to a subwoofer, you must ensure that the amp’s output matches the sub’s impedance.

For example, if you have a 4 ohm subwoofer, you’ll need an amplifier that can produce power at 4 ohms, and if not, you’ll need to wire multiple subwoofers in parallel or series to the amplifier.

Generally, you should avoid wiring an amplifier to a load lower than the amp’s minimum impedance rating. Doing so can damage your amplifier.

When you use one single voice subwoofer, figuring out its impedance is straightforward, making it for more than one sub or DVC subs more difficult.

Below I listed the common combinations of total subwoofer’s impedance depending on the either serial or parallel connection to the car amplifier.

Subwoofer TypeTotal Impedance in Parallel ConnectionTotal Impedance in Series Connection
1 DVC 2 ohms1 ohm4 ohms
1 DVC 4 ohms2 ohms8 ohms
2 SVC 2 ohms1 ohm4 ohms
2 SVC 4 ohms2 ohms8 ohms
2 DVC 2 ohms2 ohms8 ohms
2 DVC 4 ohms1 ohm4 ohms
3 DVC 2 ohms1.3 ohm3 ohms
3 DVC 4 ohms2.7 ohms6 ohms
4 SVC 2 ohms2 ohms8 ohms
4 SVC 4 ohms1 ohm4 ohms
4 DVC 2 ohms1 ohm4 ohms
4 DVC 4 ohms2 ohms8 ohms
SUbwoofers’ impedance depending on their connection type

As you can see, for DVC subwoofers, you can wire it in series or parallel and get different total impedance loads depending on the total subs connected.

Based on this, you can quickly determine the best connection type and total impedance for your subwoofers and amplifier.

What Does RMS Mean for Subwoofers?

Now that you know how much power your subwoofer’s amplifier should be able to deliver let’s talk about RMS ratings.

RMS stands for “root mean square”, and it’s a measure of the continuous power that an amplifier can deliver to a speaker.

It’s important to note that the RMS rating of an amplifier is not the same as its peak power output.

Peak power output measures the maximum power an amplifier can produce for a brief moment, and it’s usually much higher than the RMS rating.

For example, an amplifier with an RMS rating of 1000 watts might be able to produce 2000 watts of peak power.

When matching an amplifier to a subwoofer, you should always look at the RMS rating rather than the peak power output. That’s because subwoofers are designed to reproduce low frequencies, which require more continuous power than high frequencies.

If you’re looking at the peak power output, you might end up with an amplifier that can’t provide enough continuous power to drive your subwoofer properly.

As I mentioned earlier, you should look for an amplifier with an RMS rating between 75% to 150% of your subwoofer’s RMS rating.

So, if your subwoofer has an RMS rating of 1000 watts, you should look for an amplifier with an RMS between 750 and 1500W.

How Many Amplifiers Do I Need for a Sub?

Now that you know how to match an amplifier to a subwoofer, you might wonder how many amplifiers you need.

The short answer is that it depends on several factors:

#1. Number of subwoofers

#2. RMS rating of your amplifiers

#3. Subwoofers’ impedance

#4. Number of the voice coils in each subwoofer

When you have all those information, you can evaluate how many and how powerful amplifiers you need in your car:

#1. If you have one subwoofer and one amplifier, you can connect them directly.

#2. If you have two subwoofers and one amplifier, you can wire the subwoofers in parallel or series.

If you wire them in parallel, you’ll need to make sure that the amplifier can handle a 2-ohm load.

#3. If you have two subwoofers and two amplifiers, you can wire each subwoofer to its own amplifier.

If those are DVC subwoofers, you can use two stereo amplifiers or one four-channel to power both, and this is the best way to get the most power out of your system.

#4. If you have four subwoofers and two amplifiers, you can wire the subwoofers in any combination of parallel or series.

Make sure that each amplifier can handle the total subwoofers’ impedance.

These combinations can be ongoing, but as a general rule of thumb, you should have one amplifier for every two subwoofers.

This will ensure that your amplifiers are not overworked and that your subwoofers are getting all the needed amperage. Especially if you use powerful subs like SKAR EVL-12 D2 with 1,250W RMS each (link to Amazon).

Can an Amplifier Overpower a Subwoofer?

An amplifier can overpower a subwoofer.

However, it’s important to note that this doesn’t necessarily mean that the amplifier is too powerful for the subwoofer. In most cases, an amplifier will produce more power than the subwoofer can handle.

This is because the RMS rating of an amplifier is not the same as its peak power output.

The RMS rating is a measure of the continuous power that an amplifier can deliver to a speaker, while the peak power output is a measure of the maximum power that an amplifier can produce for a brief moment.

Skar Audio RP-4500.1D Monoblock Class D MOSFET Amplifier with Remote Subwoofer Level Control, 4500W

This of course, does not mean you have to underpower your subwoofer. 

In fact, it’s generally better to have an amplifier that is slightly more powerful than your subwoofer. This will give you some headroom and allow the amplifier to deliver its full power without clipping.

Regardless of the amplifier you use, to make it work, first you have to make correct connections, and secondly, adjust its gain to the optimal level.

An amplifier that is too powerful for a subwoofer can damage the subwoofer, so it’s important to be careful when choosing an amplifier, and if you’re not sure which amplifier to choose, it’s always best to consult with an expert.

How To Set Amp Gain to Match Subwoofer With Lower RMS?

This is the most critical adjustment you have to make, and when overlooked, you can damage the amp or woofer.

To set the correct amplifier’s gain for the subwoofer, you need to find a voltage on the amplifier’s output at the gain completely turned down and then slowly turn it up to get to your target voltage level. Sound scary? Let’s go step by step.

Skar Audio VXF-12 D4 12" 3000 Watt Max Power Dual 4 Ohm Competition Car Subwoofer

#1. Take the subwoofer’s RMS wattage and impedance, and use the following formula to define the target Voltage at the amplifier’s output:

For example, if your subwoofer has 2500W at 4 ohms, you have :

root square from (2,500 W x 4 ohms) = root square from 10,000 = 100V

You need to set the amplifier’s gain to the 100V measured at the output (speaker’s terminal)

#2. Take the amplifier, turn the gain to zero, and connect to the stereo but disconnect the speakers.

#3. Turn the radio on and set the volume to the maximum.

#4. For this step, you have to use a multimeter, set it to AC and touch both probes to the speaker’s terminals. Red lead to the positive terminal, and black lead to the negative terminal.

#5. Look at the multimeter’s screen and slowly turn the gain up. Watch how the voltage increases and keep turning the gain until you see approx 100V on the multimeter’s screen.

#6. Now you have the gain set to the optimal level. Lower the volume on the radio and turn it off. Your amplifier is ready to be connected to the subwoofer.

The above steps will allow you to match the amplifier’s output with the subwoofer’s power requirement, but the voltage does not tell at which point the amplifier may start distorting signals.

To make a precise measurement at which point the amplifier starts to distort the signal, you’ll need an oscilloscope and a sine wave generator. 

If you don’t have access to those, you can use a multimeter, but the measurements won’t be as accurate.

Now, you can reconnect the RCA cables and enjoy your music. Remember to never turn the amp’s gain up too high, as this can damage the amp or woofer.

Can an Underpowered Amp cause a Sub to Blow Out?

An underpowered amplifier can damage a subwoofer. This typically happens when the amplifier is pushed to its limits and starts clipping the audio signal.

When an amplifier clips the audio signal, it means that the amplifier limits the amplitude of the signal. As a result, clipping distorts the sound and can ultimately lead to the subwoofer being damaged.

This is because the clipping amp will create signal spikes that may cause the voice coil to exceed the movement and damage the subwoofer’s suspension or cone.

To avoid this, make sure that the amplifier you’re using is powerful enough to drive the subwoofer, and if it does not, do not turn the volume high.

Conclusion

When choosing an amplifier for your car subwoofer, it’s important to consider the amplifier’s power output and the subwoofer’s RMS rating.

It’s also important to connect your amp and subwoofer correctly and adjust the gain to match the subwoofer’s RMS input.

Matching an amplifier with a subwoofer can be tricky, but following the steps above will help you get it right. If you’re ever in doubt, consult with an expert, a car audio installer, or shoot me a short message.