Harmony HB 501D II — Mono Class D Subwoofer Amplifier

Harmony HB 501D II — Mono Class D Subwoofer Amplifier

The HB 501D II is a single-channel Class D subwoofer amplifier rated 300 watts RMS at 4 ohms and 500 watts at 2 ohms. Signal-to-noise is better than 90dB with THD under 0.1%, across a 10Hz to 180Hz bandwidth.

Input impedance is 20 kilohms, and the chassis measures 175mm in length — the same compact format as the HB four-channel models, which makes them straightforward to mount together.

Bass Only, By Design

The 180Hz upper limit is the defining specification. This is a dedicated subwoofer amplifier with no full-range capability, and the narrow bandwidth is what allows the design to be optimised for bass duty rather than compromised across the whole audio range.

The 10Hz lower limit matters more than it appears. In a ported enclosure the amplifier needs to stay in control below the port tuning frequency, where the port stops loading the cone and excursion rises quickly. Extension to 10Hz is what keeps that behaviour predictable.

Controls

The control panel provides a bass boost control and a frequency control for setting the low-pass point, along with the speaker outputs.

Being able to set the crossover at the amplifier means the bass section can be configured without a DSP in the signal path — relevant in a system built around a factory head unit or a simple processor.

Power and Impedance

Output rises from 300 watts at 4 ohms to 500 at 2, so the woofer's voice coil configuration and final wiring determine how much is available. A dual 4-ohm woofer wired to 2 ohms sees 500 watts; the same woofer wired to 4 ohms sees 300.

Note that the published ratings stop at 2 ohms — this is not a 1-ohm amplifier, and woofer selection should reflect that.

System Application

A single subwoofer in a sealed or ported enclosure, wired to 4 or 2 ohms. Three hundred to 500 watts covers a wide range of single-driver bass sections.

Paired with an HB 104D II or HB 154D II, the matching chassis lengths make a two-amplifier system mount tidily in the same space — a genuine consideration when the location is visible or tight.

Installation Considerations

Set the low-pass frequency to suit the enclosure and the front stage's lower limit rather than to a default position — that crossover point is where the bass either integrates with the system or sits apart from it.

Size power and ground wire from the fuse rating 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.

Specifications

• Configuration: Mono, Class D
• Power: 300W RMS @ 4Ω
• Power: 500W RMS @ 2Ω
• Frequency response: 10Hz–180Hz
• THD: <0.1%
• Signal-to-noise ratio: >90dB
• Input impedance: 20 kΩ
• Controls: bass boost, low-pass frequency
• Length: 175 mm

HF Audio Perspective

The HB 501D II is the bass channel for a compact, tidy system rather than a high-output subwoofer amplifier.

Its 2-ohm floor is the specification to plan around: woofers that only make sense at 1 ohm are the wrong match, while a driver wired to 2 ohms and rated near 500 watts is exactly right. Matched to the HB four-channels it completes a full system in two small chassis — which in a vehicle with limited mounting space is often what makes the difference between a system that gets built and one that does not.