Hertz SP4-900 4-Channel D-Class Car Amplifier

The Hertz SP4-900 is a compact four-channel car audio amplifier designed to add substantial, configurable power where installation space is limited. It can run four speakers, a front speaker pair with a bridged subwoofer channel, or two bridged speaker/subwoofer loads. Its IP55-rated enclosure and sealed connections make it especially relevant for vehicles and powersports applications exposed to humidity, vibration, or dust.

At 14.4V, the SP4-900 delivers 160 watts RMS to each of four 4-ohm channels or 250 watts RMS per channel at 2 ohms. For systems needing more output from fewer channels, it can be bridged to provide 500 watts RMS x 2 at 4 ohms. This flexibility supports upgrades ranging from amplified component speakers and coaxials to a speaker-and-subwoofer arrangement.

The amplifier uses Hertz ADC, an Advanced D-Class output stage specified for up to 85% efficiency. Its published noise, distortion, and damping specifications support a clean, controlled signal path for properly matched speakers. A fanless layout avoids an internal cooling fan, helping reduce the buildup of dirt inside the chassis.

Factory-system integration is supported through both pre-level and speaker-level inputs. Hertz USS, or Universal Speakers Simulator, is built into the high-level input section to help the amplifier work with head units that monitor connected speaker loads. It also supports remote turn-on through a conventional remote input or DC sensing from compatible BTL speaker outputs.

Independent channel-pair crossover controls allow the SP4-900 to be configured for full-range, high-pass, or low-pass operation. Both channel pairs offer 50 Hz to 5 kHz filters with 12 dB-per-octave slopes, while channels 3-4 add adjustable 45 Hz bass boost when low-pass mode is selected. The compact footprint fits under-seat, behind-seat, motorcycle, and other space-conscious installations when mounted with appropriate ventilation.

Key Features

• Four-Channel D-Class Design: Supplies configurable power for speakers, subwoofers, or mixed systems.
• 160W RMS Per Channel: Delivers rated four-channel output into 4-ohm speakers.
• 2-Ohm Stable Channels: Provides up to 250W RMS per channel at 2 ohms.
• Bridgeable Outputs: Produces 500W RMS x 2 into 4-ohm loads.
• USS High-Level Integration: Helps maintain compatibility with speaker-load-detecting factory radios.
• IP55 Aluminum Chassis: Adds protection against dust and water ingress.

Design & Installation

The SP4-900 uses an aluminum IP55-rated chassis, sealed power, line-input, and speaker-output connections, plus a conformal-coated main board intended to resist harm from internal condensation. A removable aluminum control cover protects the adjustment section, and all crossover controls are grouped on one side for easier access after installation. The fanless design eliminates a cooling fan that could draw dirt into the amplifier.

Install the amplifier in a 12V negative-ground vehicle or boat electrical system, using the included power-supply and speaker-output harnesses. Select low-level input mode for pre-out signals or high-level USS mode for speaker-output signals, then choose 2-channel or 4-channel input routing as needed. Mount it in a ventilated location with clearance around the chassis, keep it out of the engine compartment, and observe the rated minimum impedance for the selected channel configuration.