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

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

The ST-4X SQ III is a four-channel Class AB amplifier from Zapco's ST-X SQ III series, rated 70 watts RMS per channel at 4 ohms, 95 watts per channel at 2 ohms, and 190 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 entry point to the SQ III Class AB line — four channels sized for a component front stage and rear fill rather than for maximum output.

Independent Front and Rear Crossovers

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

That separation is what makes a four-channel amplifier genuinely useful rather than just four copies of the same channel. The front pair can run high-passed components while the rear pair runs low-passed for bass, or both can run full-range, without the two settings interfering.

Inputs and Multi-Amplifier Installs

The amplifier accepts RCA and speaker-level input, working with an aftermarket head unit or directly from factory speaker wiring. Pass-through RCA outputs let additional amplifiers be daisy-chained from a single front-to-back run.

An auto-on switch derives turn-on from the signal when no remote lead is available.

System Application

Seventy watts per channel into 4 ohms suits a component front stage plus rear fill, or a two-way active front stage using all four channels with the crossovers set per pair.

At 2 ohms each channel delivers 95 watts. Bridged, a pair produces 190 watts into 4 ohms, so the rear pair can drive a modest subwoofer while the front pair handles the stage.

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.

Class AB runs warmer than Class D at equivalent output — mount with airflow around the heatsink rather than in a sealed location.

Size power and ground wire from the fuse rating and the run distance from the battery. Use the same gauge for ground as power, keep it short, and secure it to clean bare metal on the frame.

Specifications

• Configuration: 4-Channel, Class AB
• Power: 70W RMS × 4 @ 4Ω
• Power: 95W RMS × 4 @ 2Ω
• Power: 190W 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

Seventy watts a channel is the right amount of power for a great many component sets — enough to drive them properly without exceeding what the drivers are built to take.

The value here is the crossover layout rather than the output. Independent control over front and rear means one amplifier can cover a two-way active front stage, or a conventional four-speaker install, or a front stage plus bridged bass. Decide what each pair is doing before you buy, and the amplifier will accommodate the plan.