Zapco ST-500XM III — Mono Class D Subwoofer Amplifier

Zapco ST-500XM III — Mono Class D Subwoofer Amplifier

The ST-500XM III is a true mono Class D amplifier from Zapco's ST-X Mono III series, rated 200 watts RMS at 4 ohms, 300 watts at 2 ohms, and 400 watts at 1 ohm. Signal-to-noise is 100dB with THD under 0.1%, across a 10Hz to 200Hz bandwidth.

It is the entry point to the Mono III line and, like every amplifier in it, can be strapped with a second unit to double its output.

Power and Impedance

Output doubles between 4 ohms and 1 ohm, so the woofer's voice coil configuration and final wiring determine how much of the amplifier you use. A dual 4-ohm woofer wired to 2 ohms sees 300 watts. The same woofer wired to 1 ohm sees 400.

Decide the wiring alongside the woofer rather than after the amplifier is mounted.

Strapping Two Amplifiers

The ST-X Mono III amplifiers are true mono units, and two of the same model can be strapped together to drive a single voice coil at double the output. Zapco's procedure is specific:

• 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 then controls both. Every adjustment made on the master and on the bass remote transfers to the slave, which drives the negative side of the signal and has no control functions of its own.

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

System Application

A single subwoofer in a sealed or ported enclosure, driven at 4, 2, or 1 ohm depending on the driver and its wiring. The 200Hz upper limit makes this a bass amplifier only — there is no full-range mode.

The 10Hz low-frequency limit means it stays composed on ported enclosures tuned low, where the amplifier needs to remain in control below the port frequency.

Installation Considerations

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

Size power and ground wire from the amplifier's fuse rating and the distance from the battery. Use the same gauge for ground as power, keep the ground run short, and secure it to clean bare metal on the vehicle frame.

Specifications

• Configuration: Mono, Class D
• Power: 200W RMS @ 4Ω
• Power: 300W RMS @ 2Ω
• Power: 400W RMS @ 1Ω
• Minimum load per amplifier when strapped: 2Ω
• Frequency response: 10Hz–200Hz
• THD: <0.1%
• Signal-to-noise ratio: 100dB
• Bridge In / Bridge Out for strapping
• Remote control port for bass remote

HF Audio Perspective

Two hundred to 400 watts covers a single well-chosen subwoofer in a properly built enclosure — which describes the bass section of most systems, not just modest ones.

The strapping capability is the part worth planning around. It means the amplifier does not cap what the system can become: a second identical unit doubles the output later without replacing what you already own. Match its rating at your actual wired impedance to the woofer's power handling, and let the enclosure do the rest of the work.