Zapco ST-1500XM III — Mono Class D Subwoofer Amplifier

Zapco ST-1500XM III — Mono Class D Subwoofer Amplifier

The ST-1500XM III is a true mono Class D amplifier from Zapco's ST-X Mono III series, rated 800 watts RMS at 4 ohms, 1,150 watts at 2 ohms, and 1,600 watts at 1 ohm. Signal-to-noise is 100dB with THD under 0.1%, across a 10Hz to 150Hz bandwidth.

Two units can be strapped together to drive a single voice coil at double the output.

Power and Impedance

The ratio between impedances is narrower here than lower in the line — 800 watts at 4 ohms rising to 1,600 at 1 ohm, a doubling across the full range rather than a steeper climb.

That has a practical consequence: the amplifier remains substantially useful at 4 ohms. A woofer that cannot be wired below 4 ohms still receives 800 watts, so driver selection is not forced by the amplifier's preferences.

Strapping Two Amplifiers

Two ST-1500XM III units can be strapped for double the output. Zapco's procedure:

• Decide which amplifier is master and which is slave
• Connect the head unit's bass or full-range RCA output to the R and L inputs of the master
• Connect the master's Bridge Out to the slave's Bridge In with a single RCA cable
• Connect nothing to the slave's regular R and L inputs
• Connect the bass remote to the master's Remote In

The master becomes the control amplifier; the slave drives the negative side of the signal and has no controls of its own.

Each amplifier must see a minimum load of 2 ohms when strapped.

System Application

One or more high-power subwoofers in a sealed or ported enclosure, in a system where the bass section is a designed element rather than an addition.

The 10Hz to 150Hz bandwidth is bass-only by design. There is no full-range capability and none is intended.

Installation Considerations

Electrical capacity is a genuine consideration at this output. Sustained operation near 1,600 watts draws current accordingly, and the vehicle's charging system, battery, and cable should be specified for it. Voltage sag under load causes an amplifier to protect itself rather than simply play quieter.

Add up the fuse values, size power and ground from that total and the run length, match ground gauge to power, keep the ground short, and secure it to clean bare metal on the frame.

A remote control port is provided for the bass remote, which serves as the single control point in a strapped pair.

Specifications

• Configuration: Mono, Class D
• Power: 800W RMS @ 4Ω
• Power: 1,150W RMS @ 2Ω
• Power: 1,600W RMS @ 1Ω
• Minimum load per amplifier when strapped: 2Ω
• Frequency response: 10Hz–150Hz
• THD: <0.1%
• Signal-to-noise ratio: 100dB
• Bridge In / Bridge Out for strapping
• Remote control port for bass remote

HF Audio Perspective

The ST-1500XM III sits where the bass section needs real output but the electrical system is still a stock or lightly upgraded one.

Its 800-watt rating at 4 ohms is the useful characteristic. Amplifiers in this power class often only make sense at 1 ohm, which pushes you toward specific woofer and wiring combinations. Here the choice stays open — pick the driver and enclosure that suit the vehicle, calculate the final impedance, and check the rating at that point. If the answer is enough, the electrical work stays proportionate too.