Zapco REF 150.4 — 4-Channel Class AB Reference Amplifier

Zapco REF 150.4 — 4-Channel Class AB Reference Amplifier

The REF 150.4 is a four-channel Class AB amplifier from Zapco's Reference series, rated 150 watts RMS per channel at 4 ohms, 230 watts per channel at 2 ohms, and 470 watts bridged into 4 ohms per bridged pair. THD is under 0.02% measured at 1kHz into 4 ohms at 70% rated power, with A-weighted signal-to-noise better than 102dB at full power and channel separation of 63dB.

Frequency response is 10Hz to 40kHz within ±3dB.

Four Channels at Reference Measurements

Channel separation is the specification worth attention on a multi-channel amplifier. At 63dB, crosstalk between channels sits far enough down that each channel's output remains substantially independent — which is what preserves stereo separation and, in an active system, keeps one driver's signal from appearing on another's channel.

Distortion below 0.02% and noise better than 102dB apply across all four channels, not just a headline pair.

Socket-Mounted Operational Amplifiers

The REF 150.4 carries four MUSES8820 operational amplifiers on sockets rather than soldered — one per channel, at positions U2, U3, U4 and U5 on the board. They can be replaced with the higher-specification MUSES02, or with other op-amps, without desoldering.

The factory parts are surface-mount devices carried on small PCB adapters that fit the socket. Orientation is critical: the marked terminal on the replacement must align with the marked socket pin. Zapco recommends the work be done by a qualified technician.

System Application

One hundred fifty watts per channel into 4 ohms suits a component front stage plus rear fill, or a two-way active front stage across all four channels with reference-grade measurements on each.

At 2 ohms each channel delivers 230 watts. A bridged pair produces 470 watts into 4 ohms, so one pair can drive bass while the other handles the stage.

Four low-level input channels accept 0.5V to 8V with greater than 20K input impedance, suiting any line-level source including high-output processors.

Installation Considerations

Four ohms is the minimum load when bridging. Each channel sees half the load bridged.

The amplifier operates from 9 to 16 volts with a 2 × 40A fuse rating and measures 416 × 210 × 62 mm. Size power and ground wire from the total fuse value and the run distance from the battery, match ground gauge to power, keep the ground short, and secure it to clean bare metal on the frame.

Allow airflow around the heatsink — four channels of Class AB generates real heat.

Specifications

• Configuration: 4-Channel, Class AB
• Power: 150W RMS × 4 @ 4Ω
• Power: 230W RMS × 4 @ 2Ω
• Power: 470W RMS × 2 bridged @ 4Ω
• THD (1kHz, 4Ω, 70% rated power): <0.02%
• Signal-to-noise (A-weighted, full power): >102dB
• Channel separation: 63dB
• Frequency response (±3dB): 10Hz–40kHz
• Low-level input channels: 4
• Input sensitivity: 0.5V–8V · Input impedance: >20K
• Working voltage: 9–16V · Fuse: 2 × 40A
• Dimensions: 416 × 210 × 62 mm
• Four socket-mounted MUSES8820 op-amps, upgradeable

HF Audio Perspective

A four-channel at reference measurements is aimed at a two-way active front stage where each driver gets its own amplified channel and its own crossover point.

That is the configuration where channel separation and low distortion earn their keep — four channels doing four different jobs, each needing to stay out of the others' way. One op-amp per channel means the signal stage can be tuned per channel as well, which is unusual and worth knowing about even if you never use it.

Where the system is a conventional four-speaker install, the measurements are still real but less of the amplifier's capability is being used. Match the amplifier to what the system is actually asking for.