Zapco ST-2XP SQ III — 2-Channel Class AB Car Amplifier

Zapco ST-2XP SQ III — 2-Channel Class AB Car Amplifier

The ST-2XP SQ III is a two-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. Signal-to-noise is 95dB with THD under 0.1%, across a 15Hz to 30kHz bandwidth.

The SQ III designation marks Zapco's sound-quality-focused Class AB line — the topology chosen where dynamics and signal accuracy lead rather than power density.

Inputs: RCA and Speaker Level

The ST-2XP SQ III accepts both RCA and speaker-level input, so it works with an aftermarket head unit or directly from factory speaker wiring without a line-output converter.

Pass-through RCA outputs allow multiple amplifiers to be daisy-chained from a single front-to-back RCA run — useful in a multi-amplifier build where one signal cable can feed the whole rack.

An auto-on switch lets the amplifier derive turn-on from the incoming signal when no remote lead is available, which is the usual situation with a factory source.

Crossover and Gain

The amplifier provides a crossover type selector with independent high-pass and low-pass frequency controls, plus variable gain and a bass boost control.

Set the gain to minimum before first power-up and bring it up against the source.

System Application

One hundred fifty watts per channel into 4 ohms drives a component front stage with genuine headroom. At 2 ohms it delivers 250 per channel, and bridged it produces 500 watts into 4 ohms for a subwoofer channel.

In an active system it works as a dedicated pair of channels for midbass or midrange, with the crossover section setting the band.

Installation Considerations

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

Class AB produces more heat than Class D at equivalent output. Mount it where air can move around the heatsink rather than in a sealed cavity.

Size power and ground wire from the fuse rating and the distance from the battery. Match ground gauge to power gauge, keep the ground short, and secure it to clean bare metal on the vehicle frame.

Specifications

• Configuration: 2-Channel, Class AB
• Power: 150W RMS × 2 @ 4Ω
• Power: 250W RMS × 2 @ 2Ω
• Power: 500W RMS 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
• Auto-on switch
• Crossover type selector with HP and LP frequency controls
• Bass boost control

HF Audio Perspective

Two channels at 150 watts is a front-stage amplifier, and the ST-2XP SQ III is built for a system where that front stage is the priority.

Class AB is the reason to choose it. Where a build is oriented toward sound quality rather than maximum output, the topology is a deliberate decision rather than a compromise. Accepting speaker-level input means that decision does not force a head unit change — the amplifier integrates with what is already in the dash and lets the front stage be the upgrade.