Zapco ST-204D SQ III — 4-Channel Class D Car Amplifier

Zapco ST-204D SQ III — 4-Channel Class D Car Amplifier

The ST-204D SQ III is a full-range four-channel Class D amplifier rated 200 watts RMS per channel at 4 ohms, 290 watts per channel at 2 ohms, and 580 watts bridged into 4 ohms per bridged pair. Signal-to-noise is 90dB with THD under 0.1%, across a 15Hz to 30kHz bandwidth.

Zapco describes it as the first of the Studio-D series to carry the full Series III upgrades — a Class D design intended to run full range rather than bass only.

Full-Range Class D

The 15Hz to 30kHz bandwidth is the important specification. Many Class D amplifiers are bass-focused by design; this one is built to cover the whole audio range, which is what allows it to drive a front stage rather than only a subwoofer.

The practical consequence is efficiency. Class D generates less heat than Class AB at comparable output, so 200 watts per channel is available from a chassis that does not demand the mounting space and airflow a Class AB amplifier of the same rating would need.

High Power 2-Channel Mode

The four channels can be run in a dual mono configuration to work as a high-power two-channel amplifier — similar in principle to bridging a stereo amplifier to a mono woofer, but applied across both pairs.

Bridged loads must be a minimum of 4 ohms. If active crossovers are in use, both pairs must be set to the same frequency — mismatched settings on the two halves of a bridged configuration will not behave correctly.

5-Channel Mode

The amplifier also supports a five-speaker configuration — a three-way system with front stage, rear stage, and woofer — using passive crossovers. Capacitors on the front highs and a coil on the woofer positive divide the range, with the amplifier channels running full range.

This is a way to get more speakers from four channels where a passive network is acceptable and channel count is the constraint.

Controls

The rear channels have a crossover function switch offering high-pass, full-range, or low-pass operation, so the rear pair can handle highs, run full range, or drive bass.

System Application

Two hundred watts per channel into 4 ohms drives a component front stage with substantial headroom, or a full four-speaker installation. At 2 ohms each channel delivers 290 watts.

Bridged, a pair produces 580 watts into 4 ohms — enough for a subwoofer channel alongside a front stage from a single chassis.

Installation Considerations

Minimum load is 2 ohms per channel, and 4 ohms bridged.

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 a clean solid surface — preferably the vehicle frame.

Specifications

• Configuration: 4-Channel, Class D, full range
• Power: 200W RMS × 4 @ 4Ω
• Power: 290W RMS × 4 @ 2Ω
• Power: 580W RMS × 2 bridged @ 4Ω
• Minimum load: 2Ω per channel, 4Ω bridged
• Frequency response: 15Hz–30kHz
• THD: <0.1%
• Signal-to-noise ratio: 90dB
• Rear crossover: high-pass, full-range, or low-pass
• High Power 2-Channel dual mono mode
• 5-Channel mode with passive crossovers

HF Audio Perspective

The ST-204D SQ III is the Class D answer to a job usually given to Class AB — a full-range four-channel with enough output to run a real front stage.

Where it earns its place is installations with thermal or space constraints that a Class AB amplifier of the same rating would struggle with. The dual mono and five-channel modes add flexibility on top of that, though both come with conditions — 4-ohm minimum bridged, matched crossover settings, passive networks for the five-speaker layout. Work out the configuration you need first; the amplifier supports several, but each has its own requirements.