Parasound Halo A 51 Five Channel Power Amplifier - Black

Five Channels That All Get the Same 250 Watts

A multi-channel amplifier is only as good as its weakest channel. The Halo A 51 gives all five the same 250 watts into 8 ohms, with every channel driven at once. Into 4 ohms each channel makes 400 watts.

Parasound rates both figures continuously, from 20 Hz to 20 kHz. Every channel takes a balanced XLR or an unbalanced RCA input, and the RCA side carries a ground lift switch for when a system hums. The chassis is 4U and weighs 80 pounds.

Enough Channels for the Whole Bed Layer

Five channels covers left, center, right and both surrounds on its own. Rack brackets, banana plugs, an AC power cord and a control wire all come in the box, so a rack installation needs nothing bought separately.

A 2.2 kVA Transformer and Where It All Goes

The power supply centers on a 2.2 kVA encapsulated toroidal transformer with a separate secondary winding for every channel, and 164,000µF of filter capacitance behind it. Peak current capacity is 60 amperes per channel.

Each output stage uses four pairs of 15-ampere bipolar transistors, forty across the amplifier. JFETs handle the input stage and MOSFETs the drivers. No capacitor or inductor sits between input and output, and the slew rate runs above 130 volts per microsecond.

What It Measures, and What It Draws

Total harmonic distortion stays under 0.2 percent at full power and under 0.03 percent at normal levels. Intermodulation distortion is under 0.04 percent. Signal-to-noise ratio beats 112 dB A-weighted, and damping factor beats 1100 at 20 Hz.

That performance costs electricity. The chassis is rated at 1800 watts maximum, which is also about all a standard 115 volt outlet supplies, and it still draws 25 watts sitting in standby. Plan the outlet and the ventilation around those numbers.

What Reviewers Are Saying

Ken Stein in Audioholics, October 19, 2004, five stars for performance: "The linearity of this amplifier is excellent at any power level it is operating in, and its freedom from audible noise and distortion makes it a true winner!"

Who Is This Five-Channel Amplifier Best For?

• You want every channel treated alike — the surrounds get the same 250 watts as the center, so an effect travelling around the room keeps its weight the whole way
• Your speakers dip toward 4 ohms — each channel doubles to 400 watts there, with 60 amperes of peak current behind it, so a hard load does not squeeze the life out of a soundtrack
• Your system hums when it is all wired up — a ground lift switch sits beside the RCA inputs, so you can break a ground loop here instead of hunting through the rest of the rack
• Your gear lives in a rack — 4U of space, brackets and banana plugs supplied, and a 12V trigger that wakes the amplifier with everything around it
• You will pair it with a processor or receiver — the A 51 amplifies and nothing more, with no volume control, input switching or surround decoding aboard; feed it from the preamp outputs of one of our home theater receivers or an AV processor and it takes over from there

How Does It Connect to Your System?

Run five preamp outputs from your processor into the amplifier, using either the XLR or the RCA jacks. If you hear hum through the unbalanced inputs, flip the ground lift. Then run speaker cable to the gold-plated five-way binding posts.

Power comes on three ways: the front button, a 12V trigger between 9 and 12 volts, or audio sensing that reacts to a signal between 50 and 250 millivolts. The supplied trigger cable uses 2.5 mm sub-mini plugs, so a processor with a 3.5 mm trigger jack needs an adapter.

Pairs Well With

A power amplifier needs a few things around it. These are the categories most A 51 owners shop next.

• Home theater receivers — the preamp outputs that feed all five channels
• Interconnect cables — five balanced XLR or unbalanced RCA runs from the processor
• Speaker cables — heavier wire for five sets of binding posts
• Power conditioners — a clean feed for an amplifier rated at 1800 watts

Frequently Asked Questions

Do I need a receiver or processor with this amplifier?

Yes. The A 51 supplies power and nothing else. Feed it from the preamp outputs of an AV receiver or a surround processor.

How much power does it make with all five channels driven?

250 watts per channel into 8 ohms, and 400 watts per channel into 4 ohms. Parasound quotes both continuously, from 20 Hz to 20 kHz.

What is the ground lift switch for?

It breaks a ground loop, the usual cause of hum through unbalanced connections. Flip it if you hear hum after wiring the RCA inputs.

What comes in the box?

Rack mount brackets, banana plugs for your speaker wire, an AC power cord, and a trigger cable with a 2.5 mm sub-mini plug at each end.

Will it fit an equipment rack?

It will. The chassis is 4U with carry handles, and the mounting brackets are included. Panel height is 7 inches, or 178 mm.

How much power does it use when idle?

About 25 watts in standby. The chassis is rated at 1800 watts maximum, about the ceiling of a standard 115 volt outlet. Parasound suggests using the same outlet or strip as the rest of your system.

See It in Action

Techno Dad unboxes the A 51 and walks around the chassis:

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