Zapco ST-501D SQ MINI — Mono Class D Compact Subwoofer Amplifier

Zapco ST-501D SQ MINI — Mono Class D Compact Subwoofer Amplifier

The ST-501D SQ MINI is a true mono Class D amplifier rated 180 watts RMS at 4 ohms, 320 watts at 2 ohms, and 530 watts at 1 ohm. Signal-to-noise is 100dB with THD under 0.1%, across a 10Hz to 350Hz bandwidth.

It is the bass amplifier in Zapco's Mini series — the same compact approach applied to the subwoofer channel.

Power and Impedance

Output nearly triples between 4 ohms and 1 ohm, so the woofer's voice coil configuration and final wiring determine how much of the amplifier is available. A dual 4-ohm woofer wired to 2 ohms sees 320 watts. The same woofer wired to 1 ohm sees 530.

Worth settling at the design stage, since the wiring options follow from the woofer you choose.

Strapping Two Amplifiers

The ST-501D SQ MINI is a true mono unit and two can be strapped together to double the output. Zapco's procedure: connect the slave amplifier's speaker output positive terminal to the negative terminal of the woofer, and tie the two amplifiers' negative terminals together.

As with any bridged or strapped setup, the speaker load must be a minimum of 4 ohms.

Zapco recommends a minimum of 12-gauge speaker lead when strapping, and 10 or 8 gauge for best performance — at that output the speaker wire becomes part of the circuit's capability, not just a connection.

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 10Hz low-frequency limit means it stays composed on ported enclosures tuned into the low thirties.

Paired with an ST-64D or ST-104D SQ MINI, it completes a full system in two compact chassis — useful where a single full-size amplifier would not fit at all.

Installation Considerations

The 350Hz upper limit makes this a bass amplifier only. There is no full-range mode.

At 530 watts into 1 ohm, current draw is real despite the chassis size. 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.

Compact chassis are often mounted in tight, enclosed locations. Allow some airflow rather than sealing the amplifier in.

Specifications

• Configuration: Mono, Class D
• Power: 180W RMS @ 4Ω
• Power: 320W RMS @ 2Ω
• Power: 530W RMS @ 1Ω
• Minimum strapped load: 4Ω
• Frequency response: 10Hz–350Hz
• THD: <0.1%
• Signal-to-noise ratio: 100dB
• Strappable: two units for doubled output

HF Audio Perspective

The ST-501D SQ MINI answers a specific problem: a subwoofer stage in a vehicle with no room for a conventional bass amplifier.

One hundred eighty to 530 watts depending on wiring covers a wide range of single-woofer systems, and the strapping option means the compact format does not cap what the system can eventually become. Match its output at your actual wired impedance to the woofer's power handling, and give the enclosure the same attention you would in any other build — the box still decides most of what you hear.