The Infinity HPS250 is a high-current powered subwoofer designed to enhance the low-frequency audio experience in a home entertainment system. It integrates an amplifier and a speaker driver into a single unit, providing powerful and controlled bass output.

Function Description

The primary function of the HPS250 is to reproduce low-frequency sounds (bass) that are often difficult for main or satellite speakers to handle effectively. It acts as a dedicated bass speaker, taking a low-level or high-level audio signal, amplifying it, and driving a large woofer to produce deep, impactful bass. The subwoofer is designed to seamlessly integrate with existing audio systems, complementing the sound produced by other speakers rather than overpowering them.

The HPS250 features a sophisticated internal architecture that includes an input buffer, gain stage, and overdrive management, ensuring clean and powerful audio reproduction. It incorporates both high-pass and low-pass filters to precisely control the frequency range it reproduces, allowing for a smooth transition between the subwoofer and the main speakers. A phase switch is also included to adjust the relative polarity of the subwoofer, which is crucial for achieving optimal sound integration and avoiding phase cancellation with other speakers in the room. The bridged output configuration of the amplifier ensures efficient power delivery to the woofer.

Usage Features

The HPS250 offers several user-adjustable controls and input/output options to facilitate its integration and optimize its performance within various audio setups.

Gain Control: This knob allows users to adjust the overall volume level of the subwoofer, enabling them to balance the bass output with the rest of their audio system.
Phase Switch: A two-position switch (0 or 180 degrees) that adjusts the phase of the subwoofer's output. This is a critical control for ensuring that the subwoofer's sound waves align correctly with those from the main speakers, preventing thin or boomy bass.
Crossover Frequency Knob: This continuously variable control (50-150 Hz) determines the highest frequency the subwoofer will reproduce. By adjusting this, users can fine-tune the blend between the subwoofer and their main speakers, creating a seamless and coherent soundstage.
LED Indicator: A green LED on the front panel indicates when the HPS250 is powered on and receiving an audio signal, providing visual feedback on its operational status.
Main Power Switch: A master power button for turning the unit on or off.
Low Level Inputs (RCA): These left and right inputs are typically used when connecting the subwoofer to a receiver or processor that has dedicated line-level "pre-amp out" or "subwoofer out" jacks. If a single mono or subwoofer source is used, a Y-cable is recommended for full output.
High Level Inputs: These inputs are designed for receivers or amplifiers that do not have line-level outputs. Users can connect speaker wires from their amplifier's main or satellite speaker outputs directly to these terminals.
Full Range Speaker Outputs: When using the high-level inputs, these output terminals allow the full-range signal from the music source to be passed through to main or satellite speakers.
AC Fuse: The unit includes a replaceable fuse (2A 250v MDL Slo-Blo for 120v models, 1A 250v MDL Slo-Blo for 230v models) to protect against electrical overloads. It is important to replace it only with a fuse of the same type and rating.
AC Plug: The standard input for the AC power cord.

The design emphasizes user convenience, allowing for flexible connectivity options to accommodate different home audio systems. The controls are intuitively placed for easy access and adjustment, enabling users to tailor the subwoofer's performance to their specific listening environment and preferences.

Maintenance Features

The HPS250 is designed with serviceability in mind, providing clear procedures for disassembly and testing, which are essential for troubleshooting and repair.

Amplifier Removal: The amplifier assembly can be accessed by laying the subwoofer on its side, removing two Allen head screws, and sliding the amplifier straight down and away from the cabinet. This exposes the internal components for servicing without requiring complete disconnection of all wires. Individual boards within the amplifier can be further removed by unscrewing Philips screws and disconnecting relevant connectors. Wires threaded through the cabinet are typically glued in place and should remain intact unless absolutely necessary.
Woofer Removal: The woofer can be removed by first taking off the rubber foot plugs and foot screws, then removing the bottom panel screws. The bottom panel and woofer assembly can then be lifted out. The woofer terminals can be unplugged, and the woofer itself can be detached by removing its mounting screws.
Bias Test: A specific procedure is outlined for checking and adjusting the amplifier's bias current. This involves exposing the amplifier assembly, disconnecting a specific connector (CD+) on the main board, inserting a DC ammeter, and adjusting potentiometers (R13 and R28) to achieve specified current readings (5 mA and 10 mA). This test helps ensure the amplifier's output devices are operating correctly and efficiently.
Audio Test: A procedure for testing the audio output of the amplifier is provided. This involves replacing the woofer with a resistive load (20 ohm, 100 watt), connecting a voltmeter and oscilloscope to the output terminals, and observing AC noise levels (should be less than 8 mV). Further testing involves connecting a signal generator to the low-level input and setting it to 60Hz, 800mV, while adjusting the gain control.
General Function Test: This test involves connecting the subwoofer to a signal generator (set to 100mV, 50 Hz) via its low-level inputs and verifying bass response from the port tube opening when the gain control is turned clockwise. It also includes testing with speaker-level inputs (2.0V, 50 Hz) from an integrated amplifier.
Sweep Function Test: This procedure uses a sweep generator (20Hz to 300Hz) to check for any rattles, clicks, buzzes, or other unusual noises from the cabinet and drivers, indicating potential issues.
Driver Function Test: After removing the driver, its DC resistance should be checked (21 ohms +/-10%). The driver is then connected to an integrated amplifier and swept from 20Hz to 1kHz at 30.0V to listen for any rubbing, buzzing, or other unusual noises, which could indicate a damaged voice coil or motor assembly.
Safety Precautions: The manual emphasizes critical safety warnings for servicing. It advises against touching the metal frame of the woofer when driving the amplifier at high power levels, as hazardous voltage could be present in case of output device failure. It also strongly recommends using a 1000W isolation transformer and an isolator plug (cheater plug) when connecting 120/230v-based test equipment to the amplifier to ensure technician safety.

These detailed maintenance and testing procedures allow for effective diagnosis and repair, ensuring the longevity and optimal performance of the HPS250. The modular design, with separate boards for power supply, main processing, AC power, shield/LED, and high-level input, simplifies component replacement and troubleshooting.

Infinity HPS250 - Main Board Schematics; Main Board Schematic Part 1

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