How To Test A Car Stereo Amplifier?




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Have you recently purchased a used car amplifier, and do you think it is not working correctly?

Figuring out how to test your car amplifier to see if it works is much easier than you think.

To test the amplifier, you have to connect all power wires and the speaker, which you know works. Next, connect the stereo or the sound generator to the amplifier’s RCA input terminals. Finally, turn the device on, and if you hear the sounds out of the speaker, the amplifier works fine.

Tip: For more advanced amplifier testing, use a multimeter.

These include testing for wattage with a clamp meter, testing components (transistors, capacitors, resistors, etc.) for a closed circuit to see if they are faulty, and testing the incoming power source’s voltage.

The best multimeter I can recommend for this type of testing is the AstroAI 6000 (link to Amazon). In addition to checking all electrical parameters, you can test the surface temperature, which is handy when you want to check any overheating issue in the car audio system.

In the article below, I will look at some of the most important factors to consider when testing your car stereo amplifier.

How do amps work?

Basic circuitry of a car amplifier

Incoming signal

The incoming signal from the main power on the amplifier will move through filtering capacitors. These will store some energy as well as filter out some noise from your alternator or something else in your system. The signal will also move into another filter called a Choke that will filter out additional noise. These filters will now contain stored energy. 

how to test car amplifier

Pulse width modulator

The amplifier’s heart is the pulse width modulator (These were omitted from the really old amplifiers that ran on different technology). This chip on the circuit board runs the power supply. The pulse width modulator’s function is to switch on and off very quickly (between 20 000Hz and 50 000Hz) and, in turn, drives the power supply transistors. 

Power supply Transistors

Most modern amplifiers are usually now fitted with Mosfet transistors, which regulate power better and create a stronger signal. Controlled by the pulse width modulator, these transistors turn on and off very quickly, taking energy stored in the choke and filtering capacitors sending the signal to the transformer. 


The amplifier’s transformer is responsible for taking a DC current and converting it into an AC current. On a side note, the transformer does not change your amplifier’s characteristics when the signal leaves the transformer at a higher voltage and pushed the signal through to the rectifiers. 


The rectifiers have two diodes inside each, which will have a positive and negative voltage that can only pass through in one direction, creating rail voltage (both positive and negative). This type of voltage is used to drive your speakers.

It is imperative to note that the rails can carry low voltage to very high voltage, and if you are testing or fixing your amplifier with it powered on, there could be enough voltage on the rails to kill you. 

Filtering Capacitors

Once the signal leaves the rails, it will move into filtering capacitors that filter out noise again and store a small charge for your amplifier’s output section (output transistors – discussed later).

RCA head unit signal

The current from your head unit will come into your amplifier and progress through an input stage. This stage will include gain, frequency, and filters (high-pass and low-pass). Then it will move into the op-amps (operational amplifiers). 

Operational amplifiers

The op-amps will receive the signal from your amplifier’s input stage (signal from your head unit RCA). From here, the input signal from your head unit will go through a set of driver transistors because the op-amps will not have enough power to drive the output transistors.

Driver transistors

These are taking the signal from the op-amps and turning the output transistors on and off. One thing to note is that the signal turning the driver transistors on and off is not a set signal like the power supply transistors (Mosfet transistors). This signal will resemble the signal that is coming into the RCA inputs from your stereo head. 

Output transistors

Output transistors have been set for both positive and negative rails. The current stops once again here, and it cannot go anywhere because no part of your amplifier at present is telling it what to do with the voltage. This is where the driver transistors come into play by telling the output transistors to switch on and off very fast, just like the pulse width modulator does to the Mosfet transistors.

Transistors will take the rail voltage stored in capacitors (output sections power supply) and drive the signal straight out to your speakers.

Additional systems at play in a car amplifier

Reverse polarity protection diode

This diode is situated at the forefront of your circuit, where the main power is coming into your amplifier.

Suppose you accidentally connect the power and the ground reversed on your amplifier. In that case, this diode will let the current pass through it (usually, it will block the current passing through it if your amplifier is working correctly). 

This will create a dead short when the current is allowed to pass through it and, in turn, hopefully blow the main fuse (in this case, all you would need to do is replace the fuse and wire your ground and power correctly, and your amplifier should work properly).

However, there are instances where this diode fails at its job for various reasons, and the result is that there is irreparable damage to your amplifier.

Feedback circuit

At the same time that the driver transistors are sending the signal to the output transistors, they will send a small signal back into the op-amps. 

This is utilized to test the input signal with the signal sent to the output transistors, and if it does not match the op-amps, they will try to rectify it. In addition to this process, the extra benefit is noise cancelation. This is so the signal that is going into the amplifier is the same as the signal going out, just not as powerful. 

Noise cancelation

There are a lot of additional parts and ways in which an amplifier can effectively cancel noise. Many of these designs and systems include substituting parts, incorporating shielding techniques, and rearranging components.

How do I know if my amp works?

This is straightforward and is easily identified. First, check that you have connected your amplifier to your car stereo system correctly. That is, you have to be sure all three connections below are stable:

  • The head unit is connected via RCA to your amplifier
  • The power and ground wires are connected to your amplifier.
  • The speakers and subwoofer are connected to your amplifier.

All you need to do then is turn on your car stereo system and play some music. If everything is working correctly, you should hear the audio you are trying to play. 

Keep in mind that some car stereo amplifiers have remote detection, so they will only turn on when your stereo head unit turns on. 

How to test my car amp with a multimeter?

First, you have to ask yourself, what exactly are you going to test?

Is the amplifier not turning on, and you are trying to test the parts inside the amplifier to see if they are faulty or blown?

In the previous section, we covered how an amplifier works. There are many components (resistors, capacitors, transistors) you can test with your multimeter to figure out which part inside your amp is not working, causing your amplifier not to function correctly.

Otherwise, we can use a multimeter to test if the amplifier works and other aspects, like wattage. If your amplifier is working, it can still not be working correctly, and therefore you can use a multimeter to test the voltage and hence the wattage of your amplifier. 

How to test what wattage my car amp is?

You can use a clamp test to give you a rough indication of the raw power (RMS) coming out of your amplifier. 

Clamp test

Tools required to test the wattage of a car amplifier
  • Digital Multimeter (one that measures AC voltage)
  • Clamp meter (this measures amperage)
  • Mathematical skills ( Volts x Amps = watts)

This tutorial assumes that your amplifier is connected, is able to power on, and plays an audio signal through speakers. If It is not, you will require additional tools and parts. Check out my article on connecting subwoofers to an old amplifier to understand how to set up an amplifier and speakers, or subwoofer.

Step-by-step tutorial on measuring wattage with a clamp test

  • You will first connect the leads of your multimeter to the speaker outputs of the amplifier.
  • Connect the red lead to the positive terminal of the amplifier
  • Connect the black leads to the negative terminal of the amplifier

Note that your speakers still have to be connected to the amplifier for this method to work because the clamp meter needs to utilize the speaker wire’s positive output. 

If necessary, you need to connect the multimeter leads to either an unused channel or with your speaker wires. 

  • Then connect your clamp meter to the positive output of the speaker wire and shift the clamp meter as close to the amplifier as possible.
  • Turn your amplifier on 

If your amplifier is not connected to your car stereo head unit, then hook it up to another device with cables such as your smartphone. 

Furthermore, you will not use music for this test as the frequencies and volume for those frequencies will change throughout a song making your readings jump all over the place.

  • Play a test tone (40Hz will be fine)
  • Write down the readings of your multimeter and your clamp meter.
  • Multiply your volts to your amps, and that will be your wattage.

Keep in mind that adjusting the gain on your amplifier will change the readings on your meters effectively, giving you a higher reading if the gain is turned up and a lower reading if the gain is turned down. 

Using a multimeter to test components in your amplifier

Suppose your amplifier is not working at all. In that case, you may need to test multiple components that make up the amplifier, as we discussed in the first section, like transistors, capacitors, the transformer, and any others that have the voltage passed through them. 

However, modern amplifiers now come equipped with many fuses that make up part of your amplifier’s circuit. These fuses will blow if the incorrect signal is passed through them. All you need to do is check that all fuses are still working correctly, and if one or more is blown, you will hopefully only need to replace them. 

Furthermore, if they are blown, you should note that something is probably wrong with your amplifier or your car stereo setup. You should evaluate all the necessary components, wiring, and layout of these systems. 

Step-by-step tutorial for checking components with a multimeter

If you do not know much about how circuits work (especially amplifiers), you should consider consulting a professional.

  • Make sure your amplifier is turned off (you never carry out tests with the power on)
  • Remove the cover of the amplifier.
  • If you need to, unclip the circuit board from the case with screws or Allen keys.
  • Switch your multimeter to the resistance setting that has an audible alarm
  • Use the elimination process and test the various components of your amplifier.
  • You will have to connect the multimeter’s leads to the section of the receiving input component and the unit receiving the output.
  • If you can not get to a component on the circuit board, turn it over to get to the soldering points.

With the multimeter set to a resistance setting with an audible alarm, you will be testing for a closed circuit at each component. If the audible alarm goes off, then you know that component of your amplifier is working correctly. 

If you should test a component and the alarm does not go off, then you can assume with reasonable certainty that that component is blown or faulty and you will need to replace it. 

Can I test my car amplifier without speakers?

If your amplifier does not work correctly and you are worried that you will blow your speakers if you attach them to your amplifier, or if you do not have any speakers lying around, then there are three possible options you could look at for testing your car amplifier.

Test the car amplifier with a multimeter

As we discussed in the above section, you would go through all the components inside your amplifier testing for closed or short circuits through the process of elimination. In addition to this, always check that no fuses are blown 

Use a light bulb to test your car amplifier.

Although not recommended, you can test your amplifier with a lightbulb. Depending on the amplifier power, you need to keep in mind that the lightbulb will burn out if the amp is more than 800watts per channel at 8 ohms. 

Buy using a 115v lightbulb; all you would do is connect the lightbulb with wire to the speaker outputs of the amplifier connecting the positive and negative terminals to the wire correctly.

Once you switch the amplifier on, you would see the lightbulb turn on. This would indicate that your amplifier is receiving and sending the signal.

Consult a professional regarding your car amplifier

In most cases, you can take your amplifier to a store, and they will test it for you at a minimal fee. This is the best option to take if you are uncertain of the other methods.

Furthermore, you will save money in the long run because if you do not know what you are doing, you will likely damage your amplifier, causing you to purchase a new one. 

How do I know if my amp is getting enough power?

If you think that your amplifier is not getting enough power, there is an easy way to check for this. 

Use a multimeter to check the voltage from your power source.

Using your multimeter and setting the meter to voltage, use the test leads and connect them to your power source (car battery). Car batteries usually measure at 12.6 volts when the car is turned off and between 13.7 and 14.7 volts when the engine is running. 

If your amplifier comes with a separate power source, you will test it in the same way by connecting the leads to the positive and negative terminals of that power source.

Use a multimeter to check the wires from your power source.

Depending on the amount of power you are sending to the amplifier, the wire’s gauge will need to be of a certain thickness. If it is not, it could cause not enough power to be sent to the amplifier. 

The approach for this is the same, except you would connect the wire to the power source (car battery) and measure the voltage from the ends of the wire that would be connected to your car amplifier. 

If you find that the voltage is incorrect for any of these methods, you would have to troubleshoot other aspects of what could be wrong. 


Being able to test your car amplifier to see that it is working is pretty straightforward if you have the correct equipment, such as a multimeter. You can check whether it is receiving enough power and whether it is sending the proper amount of power to your speakers.

In other cases where the car amplifier does not work at all, by knowing how a car amplifier works, you can use the multimeter throughout the process of elimination and test its components such as transistors, capacitors, transformers, and resistors. 

A multimeter is the best way to troubleshoot any problem that involves power and a circuit but remember that you should consult a professional if you are not comfortable with any testing method.