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[FAQ] Understanding the parameters and architecture of speaker amplifiers

Types of Speaker Amplifiers

This section will focus on speaker amplifiers and understanding the parameters that go into each device.

 

There are many different kinds of audio amplifiers. When most people think of audio amplifiers, they think of speaker amplifiers which drive loud speakers capable of producing milliwatts to killowatts. When thinking about driving speakers, the first think to think about should be output wattage. To give some perspective, simple earbuds and headphones could be outputting somewhere around 20 milliwatts of power, laptop speakers could be anywhere from 5-15 watts of power, car stereos can reach up to 50-300 watts of power.

 

The second key characteristic for many designers is efficiency. Efficiency here is the ratio of total output power to the input power. The architecture of the amplifier will heavily influence the efficiency. The different classes of audio amps will be summarized below:

 

  • Class A

 

  • Single Transistor conducting all the time with or without an input as long as the circuit has power
  • This design typically has the lowest distortion at the cost of efficiency (15-35%) which is well liked by audiophiles
    • However these designs typically don't meet efficiency requirements
  • Class B

 

 

 

  • This design utilizes a push-pull transistor architecture where each transistor is conducted with the negative or positive portion of the input signal (PNP takes in the negative component while NPN takes the positive component)
  • Crossover distortion is introduced for voltages in the input signal that are in between the saturation points of both transistors.
    • At the cost of this distortion, the efficiency of this device is much higher than it class A counterpart (around 75% efficient).
  • Class AB

 

  • Made to mitigate Class B cross over distortion by introducing diodes to ensure that both transistors operate in their exclusive linear regions
    • This introduces some efficiency losses om the diodes but takes away the effects of crossover distortion (typically 50-75% efficient)
  • Class D

 

  • A PWM signal is created in between the input and the output to control the push pull of the transistors
    • This ensures that transistors can be fully on or fully off but adds EMI from the PWM
    • The EMI can be reduced with proper filtering at the output
    • Comes with up to 90 Percent efficiency and is popular with smaller portable speakers
  • TI Smart Amps
    • Adds to the class D amplifier by adding the ability to monitor the speaker and speakers interface in real time to increase the overall quality and sound pressure level (effective loudness)
    • This is done by adding DSP and DAC chips to the overall

 

For more information on TI's smart amps, view the smart amp quick start guide below.

Smart amp quick start guide

 

Speaker amp parameters

 

Along with knowing architecture to know what to expect, it's also important to know how a couple other parameters work to make an informed decision on a speaker amplifier.

 

Load Impedance

Most speakers have an inherent resistance from 2-32 ohms. This will show how difficult the speaker is to power; the lower the impedance, the more efficiently a signal will be able to pass. If you already have a speaker, be sure to find it's resistance. If you don't know where your speaker came from, you may put a ohmmeter at the input of the speaker to get a nice estimate.


TI gives a minimum load impedance that your speaker should meet. If your speaker is below the minimum, the speaker amp could damage your speaker.

 

Speaker channels

 

Having more channels allows for audio playback to be routed to different speakers and create the perception of width and surround sound. Mono audio routes all audio playback to one place. Stereo breaks up the audio play back into two, usually a left and a right. The more channels you have, the more width you may have. With four channels, you could have the audio be directed to a left, right, up, and down creating a surround sound experience. Certain audio files have left and right channel information that will tell a two speaker system when to conduct, so think about what files you will be primarily playing from your system.

 

SNR

SNR stands for signal to noise ratio. It's a measure of a desired signal over background noise. The bigger the SNR, the better the quality of the signal.

 

Power Stage supply range

This range is the operating voltage in order for the speaker to conduct sound. If you are below the minimum range, then no sound will play.