Zapco ST-750D — Mono Class D Subwoofer Amplifier

Zapco ST-750D — Mono Class D Subwoofer Amplifier

The ST-750D is a single-channel Class D subwoofer amplifier from Zapco's ST-D Compact series, rated 400 watts RMS at 4 ohms and 750 watts at 2 ohms. Signal-to-noise is better than 98dB with THD under 0.1%, across a 10Hz to 250Hz bandwidth.

The chassis measures 213mm net and 254mm overall, by 124mm wide and 50mm tall — the same compact footprint as the ST-104D SQ III, making the two a natural pair.

Power and Impedance

Output nearly doubles from 4 ohms to 2 ohms, so the woofer's voice coil configuration and final wiring determine what is available. A dual 4-ohm woofer wired to 2 ohms sees 750 watts; the same woofer wired to 4 ohms sees 400.

Note that the published ratings stop at 2 ohms. This is a 2-ohm-stable design rather than a 1-ohm amplifier, which should inform woofer and wiring selection from the outset.

Low-Pass Filtering

The ST-750D provides a low-pass filter adjustable from 50Hz to 250Hz. Combined with the 10Hz to 250Hz bandwidth, this is a dedicated bass amplifier — there is no full-range mode and none is intended.

The 10Hz low-frequency extension is what keeps it in control on ported enclosures tuned low, where the amplifier's behaviour below port frequency determines whether the woofer stays within its excursion limits.

Input impedance is 10K on the low setting and 50 ohms on high, accepting 0.25 to 6V low or 0.6 to 15V high — so it takes signal from an aftermarket head unit or directly from a factory amplifier's speaker-level output.

System Application

A single subwoofer in a sealed or ported enclosure, wired to 4 or 2 ohms. Four hundred to 750 watts covers a wide range of single-driver bass sections.

Paired with an ST-104D SQ III, the two matching chassis form a complete system that mounts tidily in the same space — a practical consideration when the install location is visible or tight.

Installation Considerations

The amplifier operates from 6.5 to 17 volts — a notably wide range — with a 2 × 30A fuse rating. Size power and ground wire from that total 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 the input sensitivity to match your source before setting gain.

Specifications

• Configuration: Mono, Class D
• Power: 400W RMS @ 4Ω
• Power: 750W RMS @ 2Ω
• Frequency response: 10Hz–250Hz
• THD: <0.1%
• Signal-to-noise ratio: >98dB
• Filter mode: low-pass
• Filter frequency range: 50Hz–250Hz
• Input impedance: 10K (low), 50Ω (high)
• Input voltage: 0.25–6V (low), 0.6–15V (high)
• Working voltage: 6.5–17V · Fuse: 2 × 30A
• Dimensions: 213 / 254 × 124 × 50 mm

HF Audio Perspective

The ST-750D is the bass channel for a compact, well-integrated system rather than a maximum-output subwoofer amplifier.

Its 2-ohm minimum is the specification to plan around. Woofers that only make sense at 1 ohm are the wrong match here; a driver wired to 2 ohms and rated near 750 watts is exactly right. The wide 6.5 to 17 volt operating range is worth noting too — it tolerates the voltage swings of a stock electrical system better than many amplifiers in this class, which suits the daily-driver installations this series is built for.