Zapco REF 500.1 — Mono Class AB Reference Amplifier

Zapco REF 500.1 — Mono Class AB Reference Amplifier

The REF 500.1 is a single-channel Class AB amplifier from Zapco's Reference series, rated 320 watts RMS at 4 ohms and 550 watts at 2 ohms. THD is under 0.02% at 1kHz into 4 ohms at 70% rated power, with A-weighted signal-to-noise better than 103dB at full power and channel separation of 63dB.

Frequency response is 10Hz to 40kHz within ±3dB — full range, not bass-limited.

A Mono Amplifier That Is Not a Subwoofer Amplifier

This is the specification that sets the REF 500.1 apart from most single-channel amplifiers. A 40kHz upper limit means it is a full-range mono, not a bass amplifier with a low-pass filter built into its bandwidth.

That opens applications a conventional mono cannot serve. In an active system it can be assigned to a single driver — a centre channel, a dedicated midbass, or one side of a stereo pair using two amplifiers — with the same reference measurements the multi-channel models carry.

Used for bass, it delivers 320 watts at 4 ohms or 550 at 2, with distortion and noise figures well beyond what a subwoofer channel strictly requires.

Class AB in a Mono Chassis

Most single-channel amplifiers at this output are Class D, chosen for efficiency in bass duty. The REF 500.1 is Class AB, which is a deliberate decision — the topology Zapco specifies where sound quality and dynamics lead.

The practical trade is heat. Class AB dissipates more than Class D at the same output, and the mounting location should account for it.

Socket-Mounted Operational Amplifiers

Like the rest of the Reference line, the REF 500.1 carries MUSES8820 operational amplifiers on sockets rather than soldered, replaceable with the higher-specification MUSES02 or others without desoldering. Orientation matters when fitting replacements, and Zapco recommends a qualified technician does the work.

System Application

Two low-level input channels accept 0.5V to 8V with greater than 20K input impedance.

Note that no bridged rating is published — this is a single-channel amplifier used as one channel. Its role is set by the system design and the crossover ahead of it rather than by bridging.

Installation Considerations

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, use the same gauge for ground as power, keep the ground short, and secure it to clean bare metal on the frame.

Give the heatsink airflow. Class AB in a mono chassis running near rated output produces sustained heat.

Specifications

• Configuration: Mono, Class AB
• Power: 320W RMS @ 4Ω
• Power: 550W RMS @ 2Ω
• THD (1kHz, 4Ω, 70% rated power): <0.02%
• Signal-to-noise (A-weighted, full power): >103dB
• Channel separation: 63dB
• Frequency response (±3dB): 10Hz–40kHz
• Low-level input channels: 2
• Input sensitivity: 0.5V–8V · Input impedance: >20K
• Working voltage: 9–16V · Fuse: 2 × 40A
• Dimensions: 416 × 210 × 62 mm
• Socket-mounted MUSES8820 op-amps, upgradeable

HF Audio Perspective

The REF 500.1 makes most sense in a system built from single-channel amplifiers — one per driver, each with its own power supply and its own signal path.

That is a deliberate architecture rather than a default one. It costs more chassis, more mounting space, and more wiring than a multi-channel amplifier doing the same job, and what it buys is complete independence between channels. For a builder pursuing that approach, a full-range Class AB mono at reference measurements is exactly the right building block.

Used simply as a subwoofer amplifier it works well, but a Class D mono would do that job with less heat and less installed cost. The 40kHz bandwidth is the reason to choose this one — if the system is not using it, the amplifier is not being used for what it is.