Understanding the Class A and A/B Amplifiers: Exploring the ONKYO TX-866
Introduction to Amplifier Classes
Amplifiers are critical components in audio systems, converting low-level electrical signals to a form that can drive speakers efficiently. Amplifier classes are categorized based on the way they operate and distribute their output power. Two of the most common classes are Class A and Class AB. Let's explore these in more detail, focusing on the ONKYO TX-866 amplifier.
Class A Amplifiers
Class A amplifiers are simple in design and operate in a way where the output device remains active and conducts even when the input signal is at zero volts. This means that the transistor operates in its full active region, leading to high efficiency and excellent sound quality. However, the sacrifice of efficiency is a significant drawback, as the power amplifier dissipates a lot of heat and consumes a lot of power when the input signal is low. Despite this, Class A amplifiers are favored for their superior sound performance.
Class AB Amplifiers
Class AB amplifiers combine the best features of both Class A and Class B amplifiers. In a Class AB amplifier, the output devices are biased in such a way that they conduct throughout the entire output cycle but not at zero volts. This configuration offers better efficiency compared to Class A, but at the expense of some crossover distortion when the input signal is at zero volts. The crossover distortion is usually minimal and not noticeable to the human ear.
Exploring the ONKYO TX-866 Amplifier
The ONKYO TX-866 is a powerful 120 Watt per channel amplifier, featuring a Class AB output stage. According to Onkyo's service manual, the TX-866 utilizes discrete BJT (Bipolar Junction Transistor) devices, with specific parts being the pull-up and pull-down transistors. The service manual specifies that the pull-up is the 2SC3856 NPN transistor, while the pull-down is the 2SA1492 transistor. There is also the option to use alternative parts, such as the 2SC3907 and 2SA1516 transistors, as shown in the schematic.
These components work together to ensure a balanced and efficient output, optimizing both power delivery and sound quality. The Class AB design in the ONKYO TX-866 strikes a balance between the superior performance of Class A and the higher efficiency of Class B, making it a popular choice for audio enthusiasts who demand both power and quality.
Key Considerations for Amplifier Selection
When selecting an amplifier, it's important to consider several factors, including the intended use, the type of speakers, and the available budget. Class A amplifiers are highly sought after for home audio systems and professional recording studios, while Class AB amplifiers are more common in car stereos, home theater systems, and general audio applications.
Conclusion
In conclusion, the ONKYO TX-866 is a notable example of a Class AB amplifier, built with discrete BJT devices and offering 120 Watts per channel. Understanding the differences between Class A and Class AB amplifiers can help consumers make informed decisions and choose the best tool for their audio needs.
The amplifier section of the ONKYO TX-866 has conventional Class A/B output stages, delivering clear and powerful sound without the inefficiencies of a purely Class A design.