As of my last update in September 2021, transformers are not commonly used in modern audio amplifiers, especially in low-power applications. Instead, they are more frequently found in tube-based or vintage audio amplifiers. The role of transformers in such amplifiers can be understood as follows:
Impedance Matching: Transformers are used to match the impedance between different stages of the amplifier. For example, in tube amplifiers, the output stage might have a high output impedance, while the speaker requires a low input impedance. A transformer can efficiently transfer the signal from the high-impedance output of the tubes to the low-impedance input of the speaker, preventing signal loss and distortion.
Voltage Step-Up or Step-Down: In some cases, a transformer can be employed to step up or step down the voltage level. This is commonly seen in power amplifiers, where the output transformer steps up the low voltage from the tubes to the higher voltage required to drive the speakers.
Isolation: Transformers provide electrical isolation between different stages of the amplifier, preventing the direct flow of DC current between them. This isolation helps avoid ground loops and reduces the risk of hum and noise in the audio signal.
Filtering: Some transformers may also have filtering properties, helping to remove certain unwanted frequency components from the audio signal.
It is essential to note that in modern solid-state audio amplifiers, transformers are typically not used in the signal path due to their size, weight, and cost. Instead, modern amplifiers generally utilize semiconductor components like transistors and integrated circuits to achieve similar or better results in a more compact and efficient manner. Transformers might still be employed in high-power audio amplifiers or professional audio equipment, but they are less common in consumer-grade audio products today.