Zapco ST-2B — 2-Channel Class AB Car Amplifier

Zapco ST-2B — 2-Channel Class AB Car Amplifier

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

The ST-B series is built around factory-system integration. Every amplifier in the line accepts speaker-level input directly, which is what separates it from the rest of the Zapco range.

Speaker-Level Input for Factory Systems

The ST-2B has a dedicated speaker-level input plug for OEM hookup alongside conventional RCA inputs. That means it connects to a factory head unit's speaker wiring without a separate line-output converter in the signal path.

In a stock-radio vehicle this removes a component, a set of connections, and a common source of noise. RCA inputs are there for aftermarket head units, and a pass-through RCA output allows a second amplifier to be daisy-chained from the same run.

Crossover and Gain

A three-position crossover selector sets low-pass, full-range, or high-pass operation, with independent frequency controls: a low-pass selector for bass duty and a high-pass filter for mids and highs.

That range means the same amplifier can run a component front stage or a subwoofer depending on how it is set. Bass boost is switched at 0, 6dB, or 12dB.

Set the variable gain to minimum before first power-up, then bring it up with the source.

System Application

Sixty-five watts per channel into 4 ohms suits a stereo pair of component or coaxial speakers — a front stage upgrade in a vehicle keeping its factory head unit. At 2 ohms each channel delivers 90 watts.

Set to low-pass, the same two channels can drive a modest subwoofer stage instead. The speaker-level input makes either role workable without changing the source unit.

Installation Considerations

Using the speaker-level input, gain setting matters more than usual — factory head units vary widely in output level, so start at minimum and set by ear against the source's own volume ceiling.

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

Specifications

• Configuration: 2-Channel, Class AB
• Power: 65W RMS × 2 @ 4Ω
• Power: 90W RMS × 2 @ 2Ω
• Frequency response: 15Hz–30kHz
• THD: <0.2%
• Signal-to-noise ratio: 90dB
• Inputs: speaker-level (OEM) and RCA, with pass-through RCA output
• Crossover: low-pass / full-range / high-pass, independent frequency controls
• Bass boost: 0, 6dB, or 12dB switched

HF Audio Perspective

The ST-2B exists for a specific situation: a factory head unit staying in the dash, and a front stage that needs real amplification behind it.

The speaker-level input is the reason to choose it. Taking signal directly from factory speaker wiring keeps the install simple and avoids adding a converter that has to be matched and set up in its own right. Sixty-five watts a channel is enough to drive a good set of components properly, and that — not a larger number — is usually what makes the difference in a factory-source system.