Zapco ST-1000XM III — Mono Class D Subwoofer Amplifier

Zapco ST-1000XM III — Mono Class D Subwoofer Amplifier

The ST-1000XM III is a true mono Class D amplifier from Zapco's ST-X Mono III series, rated 500 watts RMS at 4 ohms, 700 watts at 2 ohms, and 1,000 watts at 1 ohm. Signal-to-noise is 100dB with THD under 0.1%, across a 10Hz to 150Hz bandwidth.

Like the rest of the Mono III line, two can be strapped together to double the output.

Power and Impedance

Output doubles from 4 ohms to 1 ohm, so the woofer's voice coil configuration and final wiring determine how much is available. A dual 4-ohm woofer wired to 2 ohms sees 700 watts; wired to 1 ohm it sees 1,000.

The 150Hz upper limit is tighter than the ST-500XM III's 200Hz — this is a dedicated subwoofer amplifier with no expectation of midbass duty.

Strapping Two Amplifiers

Two ST-1000XM III units can be strapped to drive a single voice coil at 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. All adjustments transfer to the slave, which 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

A single high-power subwoofer, or two woofers wired to a common load, in a sealed or ported enclosure. Five hundred watts at 4 ohms means it remains useful even where the wiring cannot reach a low impedance.

The 10Hz low-frequency limit keeps it composed on ported enclosures tuned into the low thirties, where control below the port frequency determines whether the woofer stays safe.

Installation Considerations

A remote control port is provided for Zapco's bass remote, which becomes the single control point in a strapped pair.

At a kilowatt into 1 ohm, current draw is significant. Size power and ground wire from the fuse rating and the distance from the battery, match ground gauge to power, keep the ground short, and secure it to clean bare metal on the frame. Confirm the vehicle's charging and battery capacity suit sustained operation at this level.

Specifications

• Configuration: Mono, Class D
• Power: 500W RMS @ 4Ω
• Power: 700W RMS @ 2Ω
• Power: 1,000W 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-1000XM III suits a bass section that has moved past a single moderate woofer — a high-power driver, or two on a common load.

Its 500-watt rating at 4 ohms is worth noting. Many amplifiers at this class only make sense at 1 ohm, which constrains woofer selection to match. Here, a 4-ohm load still returns half a kilowatt, so the driver can be chosen on its merits rather than on what the amplifier requires. Settle the woofers and enclosure first, calculate the final impedance, then check the rating at that impedance against what the drivers handle.