Zapco ST-5B — 5-Channel Class AB Car Amplifier

Zapco ST-5B — 5-Channel Class AB Car Amplifier

The ST-5B is a five-channel Class AB amplifier from Zapco's ST-B series — four full-range channels plus a dedicated subwoofer channel in one chassis. It is rated 70 watts RMS × 4 plus 250 watts on the subwoofer channel at 4 ohms, and 100 watts × 4 plus 400 watts at 2 ohms. THD is under 0.2%.

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

A Complete System From One Amplifier

Five channels means front stage, rear fill, and subwoofer from a single chassis — one power run, one ground, one place to mount. In a vehicle with limited space or a modest electrical system, that is often the deciding factor.

The subwoofer channel is not an afterthought: 250 watts at 4 ohms and 400 at 2 ohms is enough for a single well-chosen woofer in a properly built enclosure.

Sub Mode Selector

The feature that distinguishes the ST-5B is its Sub Mode selector, which determines where the subwoofer channel gets its signal:

• 4CH mode — the subwoofer takes a mix of channels 1 and 2
• 5CH mode — the subwoofer takes its signal from dedicated inputs 5A and 5B

That matters when integrating with a factory system. If the source has no dedicated subwoofer output — which most factory head units do not — 4CH mode derives the bass signal from the front channels. If the source does provide a subwoofer output, 5CH mode keeps it independent so its level and crossover can be set separately.

Controls

Channels 1/2 and 3/4 each have their own crossover function selector for full-range or high-pass operation with adjustable high-pass frequency, and their own variable gain. Channel 5 has a low-pass frequency control, its own variable gain, and a subsonic filter.

The amplifier also includes an auto turn-on switch and a remote control port, plus 0–12dB variable bass boost.

System Application

The ST-5B suits a complete system built around a factory head unit: components or coaxials front, speakers rear, and a subwoofer, all driven from one amplifier taking speaker-level input.

The auto turn-on switch is relevant here — factory systems often provide no remote turn-on lead, and this removes the need to find or fabricate one.

Installation Considerations

Choose the Sub Mode setting based on what your source actually provides. Selecting 5CH mode without feeding inputs 5A and 5B leaves the subwoofer channel with no signal.

Set the subsonic filter to suit the enclosure — it protects the woofer from frequencies below what the box is built to reproduce.

Five channels of Class AB power draws real current. Size power and ground wire from the fuse rating and the run length from the battery, match ground gauge to power, keep the ground short, and secure it to clean bare metal on the frame.

Specifications

• Configuration: 5-Channel, Class AB
• Power: 70W RMS × 4 + 250W × 1 @ 4Ω
• Power: 100W RMS × 4 + 400W × 1 @ 2Ω
• Power: 180W RMS × 2 bridged @ 4Ω + 400W @ 2Ω
• THD: <0.2%
• Inputs: speaker-level (OEM) and RCA
• Sub Mode: 4CH (mixed from Ch 1/2) or 5CH (dedicated 5A/5B inputs)
• Ch 1/2 and 3/4: full-range or high-pass, adjustable frequency
• Ch 5: low-pass frequency control, subsonic filter
• Bass boost: 0–12dB variable
• Auto turn-on switch, remote control port

HF Audio Perspective

Five-channel amplifiers are a compromise by design — one chassis doing the work of two. Whether that compromise is the right one depends on the vehicle rather than the specification.

Where space is tight, where the electrical system is stock, or where a single tidy install matters more than maximum flexibility, the ST-5B does the whole job from one power run. Where the subwoofer stage is likely to grow later, separate amplifiers leave more room to change your mind. The Sub Mode selector is the detail to plan around: it determines how the bass channel integrates with whatever source you are keeping.