Zapco ST-4B — 4-Channel Class AB Car Amplifier

Zapco ST-4B — 4-Channel Class AB Car Amplifier

The ST-4B is a four-channel Class AB amplifier from Zapco's ST-B series, rated 65 watts RMS per channel at 4 ohms, 90 watts per channel at 2 ohms, and 180 watts bridged into 4 ohms per bridged pair. Signal-to-noise is 90dB with THD under 0.2%, across a 15Hz to 30kHz bandwidth.

Like the rest of the ST-B line, it accepts speaker-level input directly from a factory system.

Speaker-Level Input for Factory Systems

A dedicated speaker-level input plug for OEM hookup lets the amplifier connect to factory speaker wiring without a separate line-output converter. RCA inputs are provided for aftermarket sources, and a pass-through RCA output can feed another amplifier from the same run.

In a vehicle keeping its factory radio, that removes a component from the signal path and one more thing to configure.

Independent Front and Rear Control

Front and rear channel pairs have their own variable gain controls and their own crossover sections. The front channels offer a crossover function selector for full-range or high-pass operation with an adjustable high-pass frequency.

Separate gain per pair matters in a four-channel install, because front and rear rarely want the same level — particularly when the rear channels are handling fill rather than a matched second stage.

Set all gains to minimum before first power-up.

System Application

Four channels at 65 watts into 4 ohms covers a full front-and-rear speaker installation, or a component front stage with the rear pair bridged for a subwoofer at 180 watts into 4 ohms.

At 2 ohms each channel produces 90 watts. Two pairs can be bridged for 180 watts each if the system calls for two bridged channels rather than four discrete ones.

Installation Considerations

Four ohms is the minimum load when bridging. Each channel sees half the load in bridged mode, so a 4-ohm bridged load presents 2 ohms per channel.

When driving the amplifier from speaker-level input, factory output levels vary considerably between vehicles — start with gains at minimum and set them against the source's usable volume range.

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

Specifications

• Configuration: 4-Channel, Class AB
• Power: 65W RMS × 4 @ 4Ω
• Power: 90W RMS × 4 @ 2Ω
• Power: 180W RMS × 2 bridged @ 4Ω
• Minimum bridged load: 4Ω
• Frequency response: 15Hz–30kHz
• THD: <0.2%
• Signal-to-noise ratio: 90dB
• Inputs: speaker-level (OEM) and RCA, with pass-through RCA output
• Crossover: front full-range or high-pass with adjustable frequency
• Independent front and rear gain controls

HF Audio Perspective

The ST-4B suits the most common upgrade path there is: factory head unit, four speakers, and a want for more than the built-in amplifier provides.

Taking speaker-level signal directly means the install is amplifier, wiring, and speakers — nothing else to specify. Bridging the rear pair for a subwoofer turns the same amplifier into a three-way system without adding a second chassis, which is often the right answer in a vehicle where space and current are both limited.