Sony XM-754HX - Service Manual

The XM-754HX is a stereo power amplifier designed for car audio systems, offering robust power output and a range of features for enhanced sound customization and protection. It is suitable for use in various car audio setups, including those with multiple speakers or bridging configurations.

Function Description

The primary function of the XM-754HX is to amplify audio signals from a car audio source, delivering high-quality sound to connected speakers. It utilizes an OTL (output transformerless) circuit and a pulse power supply to ensure stable and regulated output power. The amplifier can operate in different modes to accommodate various speaker impedances and power requirements.

Usage Features

The XM-754HX offers several features that allow users to customize their car audio experience:

High Power Output: The amplifier provides a maximum power output of 170 watts per channel (at 4 Ω) and can be used as a bridging amplifier with a maximum output of 400 watts, making it suitable for driving demanding speaker systems.
Flexible Input Connections: It supports both RCA pin jacks for line-level inputs and a high-level input connector, allowing direct connection to speaker outputs of car audio units that lack line outputs. This versatility ensures compatibility with a wide range of car audio sources.
Variable Filters and Boost: The unit includes built-in variable Low-Pass Filter (LPF), High-Pass Filter (HPF), and a low boost circuit. These controls enable users to fine-tune the audio output, optimizing sound quality for different speaker types and listening preferences. The cut-off frequency for the LPF and HPF can be adjusted from 50-200 Hz, with a -12 dB/oct slope. The low boost control allows boosting frequencies around 40 Hz by up to 10 dB.
Input Level Adjustment: An input level adjustment control allows users to match the amplifier's input sensitivity with the output level of their car audio source, ensuring optimal signal processing and preventing distortion.
HI-CURRENT/HI-VOLTAGE Mode Switch: A switch located on the bottom of the unit allows users to select between HI-CURRENT mode (1-2 Ω speaker impedance) for powerful sound via parallel circuits, and HI-VOLTAGE mode (2-4 Ω speaker impedance) for clear sound with a dynamic range. This flexibility caters to different speaker configurations and desired sound characteristics.
Speaker Configurations: The amplifier supports various speaker setups, including 4-speaker systems, 3-speaker systems (e.g., two full-range speakers and one subwoofer), and 2-speaker systems (e.g., two full-range speakers or two subwoofers in a 2-way system).
Protection Circuit and Indicator: The unit is equipped with a protection circuit that safeguards the transistors and speakers from malfunctions. An indicator lights up in green during normal operation and changes to amber to signal conditions such as over current, offset (high voltage at speaker terminal/pin jack), or thermal overload (unsafe temperature). This visual feedback helps users monitor the amplifier's status and ensures longevity.
Pulse Power Supply: The integrated pulse power supply converts the car battery's DC 12V into high-speed pulses using a semiconductor switch. These pulses are then stepped up by a built-in pulse transformer and separated into positive and negative power supplies before being converted back to direct current. This system regulates fluctuating voltage from the car battery, providing efficient power with low impedance output.
Direct Speaker Drive Circuit: A new circuit design removes source resistance from the final MOS FET output stage, driving the speakers directly for improved performance.

Maintenance Features

For servicing and maintenance, the XM-754HX includes a specific feature:

Protector Cancellation for Servicing: During servicing, the protector circuit can be temporarily canceled by cutting JW132 on the amplifier board. This allows technicians to perform diagnostics and repairs without the protection circuit interfering. It is crucial to restore JW132 to its original state after servicing to ensure the protection circuit is active during normal operation.

The XM-754HX is designed to be a reliable and high-performance stereo power amplifier, offering a rich set of features for car audio enthusiasts to achieve their desired sound quality and system configurations.