Halo A 21 Amplifier Build Quality

At first look the Parasound Halo A 21 and A 51 are appealing and robust amplifiers accented by brushed metal finish and a "Halo" aqua blue light around the power button. But an amplifier is not just about cosmetics especially when considering that most Audio Enthusiasts seem more interested in sound quality and features. The A 21 and A 51 amplifiers are both loaded with usable features above and beyond most other amplifiers in today's market.

The understated look of the A 21 / A 51s subtle yet empowered front panel implies a need for a space aged protective shell that contains a powerful yet precious cargo. For example the "P" on the top of the front face glows red to indicate that the unit has AC power as does the "Halo" insignia around the power button which accents the power button with its soft blue glow. Once the power is turned on the soft blue changes to a brighter blue after all circuitry has been initialized. In the center bottom of the front face are the two indicators that verify proper functionality of each channel by glowing with a different shade of blue. On the right bottom is another indicator that glows red if the unit overheats with a redundancy portrayal from the "Halo" insignia that also switches to red.

The back of these two amplifiers will meet the highest consumer specifications starting with balanced and unbalanced input connectors for each channel. The designers at Parasound also added pass through connectors for those who have the need to route the signal to another device. These connectors eliminate the need for a Y-adapter dangling off of the back, but they are only provided for the unbalanced (RCA connector).

Parasound cleverly thought to make the input connector larger than the pass through connector so that end users and custom installers wouldn't confuse the two. One of the purposes of the pass through connectors would be to route to another amplifier for bi-amping speakers. Also provided for each channel are gain controls for matching to a variety of different pre-amplifiers or controllers, thereby ensuring that Parasound doesn't limit the usefulness of their amplifiers to only their products.

The speaker connection binding posts are located at the bottom-center of the rear panel. At first I thought it was unusual that the positive connectors were configured on the inside and the negative connectors on the outside but after further investigation this orientation was intentionally designed to enable bridging of the two channels. This atypical placement comes in especially handy for those who desire the more powerful 750 Watts of RMS power into 8 ohms.

There are three dedicated switches for each channel on the back of the amplifier. One can be used to switch between balanced and unbalanced inputs, while the second can be used for switching from stereo to mono or bridged mode. The third and left most is a uniquely implemented grounding switch. The ground switch can be used to help reduce hum or buzz, common with ground noise or ground loops. This hum or buzz comes from the 60 Hz AC or its related harmonics that can intrude the signal when the system ground is not exactly at 0 volts. By changing the position of the switch it separates the signal ground from the chassis ground.

On the left top of the rear panel are two connectors, a three position toggle switch, and a small potentiometer control that can all be used for "power on" options. With the toggle switch in the up position, the audio mode is chosen to bring the potentiometer into play. This mode sets up the A 21/ A 51 to sense an audio signal at the signal inputs which then triggers on the amplifier. The potentiometer is used to set the sensitivity of when the amp turns on which can vary from 50 mV to 200 mV. This is a great feature for users with older equipment that may not have a devoted triggering. The middle position of the switch is a manual trigger which activates the power switch on the front panel. The down position of the toggle enables the 12 volt trigger input common to most new equipment. The clever Engineers at Parasound have also provided a trigger output to carry on to another device. Parasound equipped their amplifiers with a sub-mini connector to match the Halo C 1 / C 2 processors. When connecting other processors such as our reference Integra Research RDC-7 that implements a mini-connector, an adapter is required.

In order to cope with the robust heavy weight of the Halo amplifiers, Parasound provided sturdy handles on the back panel making it easier to install or reposition.