Today, understand the car audio amplifier test

Yes, car audio testing makes its debut today.

Car audio has long been a selling point for car manufacturers and promoted to consumers. In our ears, there are also well-known audio brands that are equipped in luxury cars. In the field of car modification, audio upgrading and transformation also occupy a large proportion.

Some people say that car audio is like cooking. Is it delicious? Everyone has different tastes and there is no standard. Actually no, there are some world-recognized rating standards for audio.

You know, car audio, a standard configuration that seems to be taken for granted today, has also experienced nearly a hundred years of development.

At the beginning of the 20th century, Lee de Forest, the "father of radio", had already begun thinking about how to use radio broadcasting, which was still a new technology at the time, on vehicles. However, the battery of the car at that time The voltage was only 6 volts, which was not sufficient to support the use of vacuum tube receivers, which required 50 to 250 volts to operate, so progress was very slow.

In 1936, Delco, a subsidiary of General Motors, produced the first car radio that was integrated into the dashboard and was used in models of multiple GM sub-brands such as the "Chevrolet Master Deluxe". Radios began to gradually become a comfortable equipment in cars. Today , from our "audio perspective", the audio testing involved in the car is still a complex system. Therefore, Qicai will break down the car audio test into several systems this time. Here we will focus on car audio amplifiers , and the types of testing typically performed on these devices.

Testing gossip

Due to the desire to keep power consumption to a minimum while maintaining high power output and acceptable (low) distortion levels, most car audio amplifiers are currently Class AB or D.

A2B can be used to connect automotive audio ECUs, microphone arrays, power amplifiers, speakers, tuners, active noise reduction, etc. Ford first applied it in 2016, and then BBA, Volvo, BYD, Great Wall, Changan, SAIC, and Geely began to gradually apply it.

A2B power amplifiers are increasingly used in vehicle models, and the AP audio analyzer paired with Mentor's A2B protocol analyzer is a mature solution for testing A2B power amplifiers——

Looking below, we continue to bring you typical tests of audio amplifiers , as well as specific areas of concern related to automotive systems.

Typical audio amplifier tests include gain and level, common-mode rejection ratio, power supply rejection, frequency response, output power and harmonic distortion, intermodulation distortion, noise floor, separation, DC offset, and click and click.

The instruments required to complete these tests include an audio analyzer, a DC power supply capable of outputting 9 to 16V, a power meter, and a multimeter. Other required test items include the rated impedance of the non-inductive load resistor and the output power of the amplifier under test, plus connecting the DUT and test equipment to a common ground.

From a "black box" perspective, this test is consistent between different types of audio amplifiers. However, special conditioning of the Class D amplifier output is required to account for the out-of-band noise that is characteristic of these amplifiers.

Good grounding is critical to optimizing audio amplifier performance and is equally important to achieving best test results. Small ground potential differences between devices in a test system (such as switches, devices under test, and test instruments) can couple into the signal path and cause undesirable interference or noise due to the inherent stray capacitance between the signal wires and the chassis .

Busbar grounding sometimes seems like a convenient method, but often produces the worst results. Resistors in each leg of the link place the equipment at a different ground potential and are not as effective as a star ground.

So we recommend connecting each device's ground directly to the test instrument's ground via a low-impedance wire.

Test direct hit

gain and level

Gain and level tests can be conveniently performed by applying a step input level sweep while measuring output level and gain on both channels. As shown in the graph below, this amplifier has a gain of about 35 and has a linear response, with the input amplitude (horizontal scale on the graph) going from below 2mVrms to about 600mVrms and then starting to clip at the output. The left axis shows the measured output level in Vrms. Notice that the left and right channels of this DUT match very well.

noise floor

One of the enemies of a good sound system is noise. Any unintended signal in the channel is often called noise. This could be noise generated randomly as electricity flows through the circuit, or it could be due to crosstalk, lack of adequate power isolation and filtering, poor grounding, or electromagnetic interference from motors (such as windshield wipers, seats, sunroofs) or ignition systems And the deterministic (non-random) signals that appear in the channel. If this noise occurs before the gain stage, the noise will be amplified.

Test example

As you can see from the data in the spectrogram above, there is no major deterministic source of noise (because it would show up as a specific line or spur in the spectrum); rather, the noise is random in nature. If deterministic spurs appear in the spectrum when the input is terminated, the frequency of these spurs can provide hints about the nature and origin of the signal, which is the first step in system or design troubleshooting.

SNR and common mode rejection ratio

The method used for signal-to-noise ratio measurement is to inject a tone into the amplifier input (at the specified input amplitude of the DUT) and measure the tone amplitude at the output, then remove the input signal and measure the noise floor amplitude at that output, thus Provide the required data value for SNR. The graph below shows the measured SNR on a dual channel amplifier. This shows that, at the output, the difference between the amplitude of the applied signal and the noise floor exceeds 70dB.