Zapco ST-6X SQ III — 6-Channel Class AB Car Amplifier

Zapco ST-6X SQ III — 6-Channel Class AB Car Amplifier

The ST-6X SQ III is a six-channel Class AB amplifier from Zapco's ST-X SQ III series, rated 100 watts RMS per channel at 4 ohms, 150 watts per channel at 2 ohms, and 300 watts bridged into 4 ohms per bridged pair. Signal-to-noise is 95dB with THD under 0.1%, across a 15Hz to 30kHz bandwidth.

Six channels of Class AB in one chassis, with up to three pairs available for bridging.

Six Channels for an Active Front Stage

The reason to choose a six-channel amplifier is usually a three-way active front stage — tweeters, midrange, and midbass each on their own channel pair, each with its own crossover point and level.

Running that from one chassis means a single power run, a single ground, and one amplifier to mount rather than two or three. In a vehicle where trunk space or electrical capacity is limited, that consolidation is often what makes a three-way build practical.

Crossover and Gain Control

The amplifier provides crossover type selectors with high-pass and low-pass frequency controls, and gain per channel pair. That per-pair control is what allows three different drivers to be set independently from one chassis.

Bass boost is provided, along with an auto-on switch for installations without a remote turn-on lead.

Inputs

RCA and speaker-level inputs are both available, so the amplifier works with an aftermarket head unit or takes signal directly from factory speaker wiring. Pass-through RCA outputs allow another amplifier — typically a subwoofer mono — to be fed from the same run.

System Application

One hundred watts per channel into 4 ohms suits a three-way active front stage, or a full front-and-rear installation with a bridged pair for bass.

At 2 ohms each channel produces 150 watts, and up to three pairs can be bridged at 300 watts each into 4 ohms — so channel count can be traded for power as the system design requires.

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.

Six channels of Class AB draws substantial current and generates real heat. 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.

Give the heatsink room to work — this is not an amplifier for a sealed cavity.

Specifications

• Configuration: 6-Channel, Class AB
• Power: 100W RMS × 6 @ 4Ω
• Power: 150W RMS × 6 @ 2Ω
• Power: 300W RMS × 3 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
• Crossover type selectors with HP and LP frequency controls
• Gain control per channel pair
• Auto-on switch, bass boost control

HF Audio Perspective

A six-channel amplifier is a consolidation decision. It trades some flexibility — you cannot relocate or upgrade one pair independently — for a much simpler installation.

For a three-way active front stage where the driver assignment is settled, that trade is usually worth making: one chassis, one power run, one set of adjustments in one place. Where the system is still evolving and channels are likely to be reassigned, separate amplifiers keep more options open. Map the drivers to the three pairs first; if the plan holds, this is the tidiest way to build it.