Zapco ST-4XP SQ III — 4-Channel Class AB Car Amplifier

Zapco ST-4XP SQ III — 4-Channel Class AB Car Amplifier

The ST-4XP SQ III is a four-channel Class AB amplifier from Zapco's ST-X SQ III series, rated 150 watts RMS per channel at 4 ohms, 250 watts per channel at 2 ohms, and 500 watts bridged into 4 ohms per bridged pair. Signal-to-noise is 95dB with THD under 0.1%, across a 15Hz to 30kHz bandwidth.

It is the high-power four-channel in the SQ III line, sharing the control architecture of the ST-4X SQ III with roughly double the output at every impedance.

Independent Front and Rear Crossovers

Front and rear channel pairs each carry their own crossover type selector and high-pass frequency control, with the rear pair adding low-pass. Each pair has independent gain.

At this power level that separation matters more, not less. Running 150 watts into high-passed components on the front while the rear pair drives a low-passed load requires the two sections to be set independently, and the layout allows it directly.

Inputs and Multi-Amplifier Installs

RCA and speaker-level inputs are both provided, so the amplifier works with an aftermarket head unit or takes signal directly from factory speaker wiring. Pass-through RCA outputs allow additional amplifiers to be daisy-chained from one front-to-back run.

An auto-on switch handles turn-on where no remote lead exists.

System Application

One hundred fifty watts per channel into 4 ohms drives a serious component front stage, or a two-way active front stage across all four channels with real headroom on each driver.

At 2 ohms each channel delivers 250 watts. A bridged pair produces 500 watts into 4 ohms — enough that the rear pair can run a subwoofer while the front pair handles the stage, from one chassis.

Installation Considerations

Four ohms is the minimum load when bridging. Each channel sees half the load bridged, so a 4-ohm bridged load presents 2 ohms per channel.

Four channels of Class AB at this output produces meaningful heat. Mount with airflow around the heatsink, not in a sealed cavity, and size the electrical accordingly.

Add up the fuse values and size power and ground wire from that total and the run length from the battery. Match ground gauge to power, keep the ground short, and secure it to clean bare metal on the frame.

Specifications

• Configuration: 4-Channel, Class AB
• Power: 150W RMS × 4 @ 4Ω
• Power: 250W RMS × 4 @ 2Ω
• Power: 500W RMS × 2 bridged @ 4Ω
• Minimum bridged load: 4Ω
• Frequency response: 15Hz–30kHz
• THD: <0.1%
• Signal-to-noise ratio: 95dB
• Inputs: RCA and speaker level, with pass-through RCA outputs
• Independent front and rear crossover type and frequency controls
• Independent front and rear gain controls
• Auto-on switch, bass boost control

HF Audio Perspective

The ST-4XP SQ III is the four-channel to choose when the drivers can genuinely use 150 watts a channel — not simply because more power is available.

Against the ST-4X SQ III, the difference is output alone; the crossover capability and integration features are identical. So the decision comes down to what your speakers are rated to handle and whether a bridged pair is being asked to run bass. If 70 watts a channel already covers the drivers, the extra output will sit unused. If it does not, this is the same amplifier with the headroom the system actually needs.