Zapco ST-104D SQ III — 4-Channel Class D Compact Car Amplifier

Zapco ST-104D SQ III — 4-Channel Class D Compact Car Amplifier

The ST-104D SQ III is a four-channel Class D amplifier from Zapco's ST-D Compact 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 better than 90dB with THD under 0.1%, across a 15Hz to 30kHz bandwidth.

The chassis measures 213mm net and 254mm overall, by 124mm wide and 50mm tall — a compact footprint carrying four full-range channels.

Crossover and Input Flexibility

The amplifier provides low-pass and high-pass filtering with a 30Hz to 3kHz frequency range. That span is unusually wide for a compact four-channel: the low end reaches subwoofer territory while the top extends well into midrange, so the same amplifier can be set for bass, midbass, midrange, or full-range duty.

Input sensitivity accommodates both source types — 0.25 to 6V on the low setting and 0.6 to 15V on high, with input impedance of 10K on low and 50 ohms on high. The high-voltage range is what allows it to take signal from a factory amplifier's speaker-level output without a converter.

System Application

One hundred watts per channel at 4 ohms drives a component front stage and rear fill, or a two-way active front stage across all four channels using the onboard crossovers.

At 2 ohms each channel produces 150 watts, and a bridged pair delivers 300 watts into 4 ohms — enough to run a front stage on one pair and a subwoofer on the other.

Installation Considerations

Bridged loads must be a minimum of 4 ohms. Each channel sees half the load when bridged.

The amplifier operates from 9 to 16 volts with a 40A fuse rating. Size power and ground wire from that fuse value and the run distance from the battery, use the same gauge for ground as power, keep the ground short, and secure it to clean bare metal on the frame.

Set input sensitivity to match your source before setting gain — using the low-voltage setting with a high-output source, or the reverse, makes gain setting unnecessarily difficult.

Specifications

• Configuration: 4-Channel, Class D
• Power: 100W RMS × 4 @ 4Ω
• Power: 150W RMS × 4 @ 2Ω
• Power: 300W RMS × 2 bridged @ 4Ω
• Minimum bridged load: 4Ω
• Frequency response: 15Hz–30kHz
• THD: <0.1%
• Signal-to-noise ratio: >90dB
• Filter mode: low-pass / high-pass
• Filter frequency range: 30Hz–3kHz
• Input impedance: 10K (low), 50Ω (high)
• Input voltage: 0.25–6V (low), 0.6–15V (high)
• Working voltage: 9–16V · Fuse: 40A
• Dimensions: 213 / 254 × 124 × 50 mm

HF Audio Perspective

The wide filter range is what distinguishes this amplifier from most compact four-channels. A 30Hz to 3kHz crossover span means the channels can be assigned to almost any role in a system rather than being fixed to front-stage duty.

That matters in an active build where amplifier channels get reassigned as the system develops. Combined with a high-voltage input that accepts factory speaker-level signal, it fits both the simple upgrade and the more considered active install — and in a chassis small enough that mounting location rarely becomes the deciding constraint.