SLAA898 September   2022 TAS3251 , TPA3255

 

  1.   Abstract
  2.   Trademarks
  3. 1Introduction
    1. 1.1 Power Amplifiers
    2. 1.2 Discrete Power Amplifier Implementation
    3. 1.3 Class-D Amplifier Implementation
    4. 1.4 Advantage of a Class-D Implementation
  4. 2Background
    1. 2.1 Why Use Constant Voltage Audio Systems
    2. 2.2 Basic Principle of Constant Voltage Systems
    3. 2.3 Power Loss in Transformer
    4. 2.4 Auto-Transformer
  5. 3System Test (Based on TPA3255)
    1. 3.1 Transformer Characteristics
      1. 3.1.1 Turns Ratio and Resistance Match
      2. 3.1.2 DCR of the Transformer
    2. 3.2 System Build-Up
    3. 3.3 System Test
  6. 4Efficiency Analysis and Optimization
    1. 4.1 Efficiency of Three Parts
      1. 4.1.1 Efficiency for TPA3255
      2. 4.1.2 Efficiency for Step-Up Transformer
      3. 4.1.3 Efficiency for Step-Down Transformer 330-040
    2. 4.2 Improvements on System Efficiency
      1. 4.2.1 Improve Resistance Matching
      2. 4.2.2 Apply a Transformer With Less Power Loss
  7. 5Considerations on Building a Constant Voltage System
    1. 5.1 Transformer Saturation
    2. 5.2 Low DCR
    3. 5.3 Resistance Matching

2.1 Why Use Constant Voltage Audio Systems

For an audio system, sometimes a large amount of power is distributed to many small speakers. Audio systems, like those used for flight announcements at an airport or background music at a mall, often rely on a large number of speakers and only a few amplifiers to distribute audio. The constant voltage system is introduced based on these applications.

Suppose a ceiling speaker system with five or more speakers, each with a distance from the amplifier of 50 feet or more, needs to be installed. How would this system be designed? Running wire from the amplifier to each speaker would require a large amount of wire. If using a single wire to connect each speaker from the wire to ground in parallel, the resultant impedance of the paralleled speakers would be very low, and result in high current and large losses in the wire.

For example, five 4-Ω speakers results in 0.8-Ω equivalent resistance.

4 Ω || 4 Ω || 4 Ω || 4 Ω || 4 Ω= 0.8 Ω

This is too low for most amplifiers, and requires large amounts of current. The solution is called a “constant voltage”, or “distributed speaker”, system. Constant voltage systems were designed to reduce power losses across the wire in the same way electricity is distributed along power lines. In the system, a step-up transformer is used to increase the output voltage of the audio amplifier to 70 VRMS or 100 VRMS. The voltage is then converted back to a lower voltage using a small step-down transformer attached to each speaker. The high-voltage reduces the current on the wire, and results in less power losses on the wire.

In addition, constant voltage systems have the advantage of allowing different speaker impedance values to be attached to the same line. The system can be designed so that the primary and secondary transformers match the output impedance of the amplifier with the equivalent impedance of all the speakers. This is made possible by the two-stage transformer design.