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Parasound Halo Series A-51 5 Channel Power Amplifier

Regular price $5,999.00 USD
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Halo Series A-51 5 Channel Power Amplifier

Power up your surround sound system

Parasound's Halo A 51 amplifier offers outstanding power for the front left/right/center speakers and a pair of surrounds in your home theater system. The amp delivers 250 watts per channel with all five channels driven, and it's stable at very low impedances, so it's compatible with a very wide range of speakers. With its sheer power — and thoughtful circuit design courtesy of Parasound's audio guru John Curl — this amplifier delivers impressive volume, clarity, and dynamics to your surround sound system.

Class A/AB operation for smoother sound

The A 51 uses a Class A/AB power circuit. You don't see an amplifier described that way every day, so let's break down the benefits.

Engineers and audiophiles generally agree that Class A designs offer the purest sound, but they tend to run hot and take up a lot of space. As a result, home audio amplifiers often use a compromise known as Class AB, which hands off part of the audio signal to a different set of output transistors at high power levels. AB can sacrifice a bit of signal purity, but it lets the amp run cooler and more efficiently.

So this Class A/AB amp operates in pure Class A mode up to a higher power level than most other amplifiers in its price category, before the second set of transistors kick in. The result is a sound that's smoother, less fatiguing, and more natural.

The legacy of John Curl

The A 51 features circuitry designed by John Curl, a legend in audio engineering. He started his career in the 1970s, designing tape and video recorders at Ampex, and he had a hand in building the Grateful Dead's famous Wall of Sound. Curl has worked with Parasound since 1989, designing all of their high-power amplifiers and more. The power supply, transistors, capacitors and other parts in the A 51 were chosen to offer high performance at a reasonable price.

Product highlights:

  • 250 watts per channel into 8 ohms (20-20,000 Hz) at 0.2% THD with 5 channels driven
  • 400 watts per channel into 4 ohms (20-20,000 Hz) with 5 channels driven
  • designed by legendary audio engineer John Curl
  • THX Ultra2 certified for reference home theater sound
  • high-bias Class A/AB amplifier design
  • large toroidal transformer
  • frequency response: 5-100,000 Hz (-3dB)
  • signal-to-noise ratio: 112 dB
  • handles very low impedances for use with virtually any speakers
  • five balanced XLR and five unbalanced RCA inputs
  • heavy-duty gold-plated 5-way speaker terminals
  • switchable turn-on options: manual, signal sensing, or 12-volt trigger
  • detachable power cord
  • rack mounting ears included

Dimensions and warranty:

  • 17-1/2"W x 7-5/8"H x 20"D (4U rack spaces)
  • weight: 80 lbs.
  • warranty: 5 years
  • MFR # A 51

What's in the box:

  • 5-Channel power amplifier
  • 6' AC power cord
  • 6' Mono-minijack cable
  • Rack-mount hardware
  • Owner's Guide
  • Parasound Halo A 51 owner’s manual 

Features & specs


Number of Channels


Power Output per Channel (8 ohms)


Power Output per Channel (6 ohms)


Power Output per Channel (4 ohms)


Frequency Bandwidth




Minimum Impedance


Bridged Power per Channel

0 watts

Speaker Outputs


THX Certification


Width (inches)


Height (inches)


Depth (inches)



80 pounds

Parts Warranty

5 Years

Labor Warranty

5 Years

Inputs & Outputs

Number of Inputs


Input Type


Removable Power Cord

IEC 3-prong

Level Controls


Speaker Connectors

Binding Post




5-Channel Power Amplifier: The Parasound HALO Series A 51 is an ultra-high performance Class A/AB, 5-channel power amplifier. The A 51 is built to extremely strict quality and performance standards for which Parasound is renowned. Every part within the A 51 has been carefully chosen for its accuracy, sound quality and long life. The Halo Series A 51 power amplifier is profoundly powerful and supremely musical. Its 5-channel design makes it a perfect choice to power the front, center and surround channels of your home theater.

  • 8-Ohm: 250 watts x 5 @ 20-20kHz with 0.03% THD (all channels driven)
  • 4-Ohm: 400 watts x 5 @ 20-20kHz with 0.03% THD (all channels driven)

Class A/AB Amplifier Design: Pure class A operation provides the purest sound. However, an amplifier operating entirely in class A would be enormous, highly inefficient, and generate far too much heat. Class A/B combines the main advantage of Class A with the efficiency of Class B operation. It is a compromise that reduces the heat generated in pure class A operation and the higher-order odd harmonic distortions created in class B. In class AB, the driver and output stages are always partially turned on, which provides a nominal amount of pure class A operation. At higher power levels, when the musical waveform swings from positive to negative and vice versa, each bank of transistors is allowed to rest momentarily. This resting makes it possible to deliver high amounts of power without overheating. It also makes possible the use of passive cooling and avoid fans, whose noise can be heard over the music. The result is less fatiguing, more natural sound.

Toroid Power Transformer: The heart of the power supply is a 2.2kVA toroid power transformer, chosen for its efficiency, low hum field, and high power rating. Encapsulating this massive power transformer in an epoxy-filled steel canister assures ultra-quiet performance. The A 51 power transformer employs multiple independent secondary windings so that each amplifier channel has its own power supply, assuring more than ample DC voltage at all times and under all conditions. It also reduces inter-channel crosstalk that can blur the sound and impair the correct sense of where instruments, dialogue and effects are positioned. Each channel use high-speed rectifier diodes and four 8,200UF electrolytic filter capacitors, chosen for their low Equivalent Series Resistance (ESR), dielectric absorption and resistance to high temperature. In addition, these filter capacitors are bypassed with smaller polypropylene capacitors to reduce AC ripple in the DC supply and to further eliminate noise and interference that is generated in AC power lines from computers and other appliances in the home.

Relay-Bypassed Soft Start Circuit: When the A 51 is first turned on, there is a significant amount of in-rush current required to charge the enormous power supply capacitors. Parasound employ a NTC (negative temperature coefficient) thermistor order to slow down and suppress this in-rush current and to prevent nuisance tripping of a household circuit breaker. A thermistor cuts the in-rush current by approximately 50%. In a few seconds as it heats up to its operating temperature it it’s essentially a jumper wire with zero ohms resistance. However, the A 51 goes one step further for this circuit. After the NTC resistor has done its job of suppressing in-rush current a relay with gold contacts automatically activates to jump across it to completely bypass it. This extra step insures that the thermistor does not restrict any current whatsoever when the A 51 is in full operation.

Hybrid Audio Circuit Path Topology: Parasound's circuit topology is a hybrid of carefully chosen discrete transistors that result in superior performance at each stage. Parasound uses JFETs (Junction Field Effect Transistors) for the input voltage amplifier stage; MOSFETs (Metal Oxide Field Effect Transistors) for the drivers of a second voltage amplifier stage, and bipolar transistors for the current amplifier output stage. Discrete transistors are more sonically accurate than integrated circuits commonly used by other brands.

Complementary Stage Configuration: The amplifier’s sonic characteristics are established by its input and driver stages, while the sole job of its output stage is to deliver the enormous current and voltage from its power supply to the speakers. Each stage of amplification (Input, Driver, and Output) has transistors fed by the positive DC power supply and complementary transistors fed by the negative DC power supply. Thus, half of the devices amplify the positive half of the musical waveform while the other half of the devices amplify the negative half. This complementary topology is inherently linear, which reduces distortion and improves sonic accuracy.

  • Input Stage: The A 51’s input stage uses matched pairs of discrete JFETs arranged in a differential configuration. JFETs are ideal for the input stage because their inherently high impedance is unaffected by the impedance of source components. (Ordinary bi-polar transistors are low impedance devices) Differential configuration provides superior noise reduction. These precision input JFETs are also cascaded to produce the current necessary to drive the MOSFET drivers in the following stage.
  • Driver Stage: The driver stage provides critical amplification for which we employ a complementary matched pair of MOSFETs selected for their tube-like sonic qualities. MOSFETs tend to generate less odd-order higher harmonic distortion than bipolar transistors. This is important because odd-order distortion sounds unnatural and fatiguing to the human ear, whereas even-order distortion is less offensive because it is consonant, rather than dissonant. Our MOSFET driver stage prevents the harshness and brittle sound so often found in other solid state amplifiers.
  • Output Stage: Bipolar output transistors are better than MOSFETS in the output stage because of their higher safe operating area (SOA) and inherent ruggedness. Each channel’s output stage employs four pairs of high current (15-amp) bipolar transistors to insure long-term reliability, even with continuous high power operation and challenging speaker loads. Lightning-fast (60 MHz) transistors respond instantly to complex demands in the musical signal, virtually eliminating distortions that occur with “slower” transistors. Slew rate limiting and Transient Intermodulation Distortion (TIM) are simply not an issue in the A 51.

High-Temp Indicator: The front-panel High-Temp indicator will glow red if any channel overheats. The On-Off button will also glow red if the A 51 overheats. If such a condition occurs you must provide for better ventilation around the amplifier or check speakers for faulty operation that is causing the A 51 to overheat.

Rack Mountable: The A 51 features a 4 rack space chassis with rear carry handles. Rack mounting hardware is included.


Balanced & Unbalanced Analog Inputs: The Parasound HALO Series A 51 is outfitted with both balanced XLR and unbalanced RCA analog inputs for each of the power amplifier's five channels. In most systems, balanced XLR connections will give you the best sound. Using balanced XLR input connectors results in a 6 dB higher volume level compared with using the RCA input jacks.

Note: The Balanced/Unbalanced switch is not an input selector. Its function is to optimize the signal to noise ratio for each type of input. You should not connect both the Balanced and Unbalanced jacks at the same time with the expectation of switching between two different devices such as a preamp and surround processor.

Loop Output Jacks: These stereo RCA jacks enable the incoming audio signal to pass along or "daisy-chain" from the Left and Right Input jacks to an additional amplifier or powered subwoofer. The Loop Out jacks are not affected by the volume control settings. The Loop output jacks work with either the balanced XLR or unbalanced RCA inputs.

24K Gold-Plated Binding Posts: The Halo Series A 51's speaker terminals are new, developed in collaboration with CHK. Their ample size and wide spacing accommodates even the largest gauge speaker wire, spade connector, and banana plugs (single only). High quality 24 K gold-plating on the speaker terminals, input connectors and internal audio signal connections assure sound quality that won't be compromised by corrosion.

Detachable AC Power Cord: The AC cord supplied with the A 51 is a high quality IEC type cord. Please connect it directly to an AC wall outlet or power conditioner that is always on. If possible, plug your A 51 into the same AC outlet that your preamplifier is plugged into. If different AC outlets are used for the A 51 and other components, (including a TV or video projector) the ground potential may be higher or lower between the outlets, resulting in audible hum.


Gain Control: The Parasound HALO Series A 51 offers a Gain control for each channel. The Gain control can be set to -6dB, Normal/THX, or +6dB. The Gain control knob should be left at the Normal/THX setting for most applications. When the Gain control knob is set to Normal the gain for that channel is 29. This is the THX Ultra2 Reference Level where 1V input = 28.28V output. (28.28V driving an 8O speaker equals 100 watts.) If you are unsure where to set the Gain control knobs, it is best to start with them in the Normal position and only change them if needed (see Owner's Guide for further details).

Turn-On Options: The Turn On Options switch on the back-panel determines how the amplifier turns on and off. Setting the Turn On Options switch to Audio or 12 V disables the A 51's front-panel Power button. There are 3 positions for this switch - Manual, Audio, or 12V Trigger.

  • Manual: When the Turn On Options switch is set to Manual, the A 51 is turned on and off manually only by pressing the Power button on its front-panel every time you want to listen to music.
  • Audio: When the Turn On Options switch is set to the Audio position, the A 51 will be turned on automatically when an audio signal is present at the L or R Input jacks. After the audio signal ceases the amplifier will remain on for about 10 minutes before shutting itself off. This prevents unintended turn-off during pauses in your music.
  • 12V Trigger: When the Turn On Options switch is set to its 12V position, the A 51 is turned on and off only with an external +9 V to +12V DC voltage, using its 12V Trigger input (3.5mm). When the external voltage ceases the amp will turn off within a few seconds. The 12V turn on trigger circuit in the A 51 requires a mere 5mA from the 12V trigger source.

12V Loop Out Jack: The 12V Trigger Out jack (3.5mm) lets you loop or "daisy-chain" the incoming trigger voltage to an additional A 51 or other component(s). The total load on your triggering device's 12V output is the sum of the trigger current drawn by each of the components you plan to loop together. Check the maximum capacity of your preamplifier's trigger output so you do not overload it by connecting too many power amplifiers. Typical triggering devices are rated to handle 50mA to 100mA.

Total Protection Circuitry: The Parasound A 51 features Total Protection Circuitry with the incorporation of DC servo circuits, integrated relays, current-sensing transistors, and multiple fuses to protect the power amplifier from catastrophic damage.

  • DC Servos: Parasound power amplifiers incorporate ingenious and fast-acting DC servo circuits, completely eliminating the need for coupling and blocking capacitors. The A 51 is direct (DC) coupled from its input jacks to its speaker terminals. This advanced circuitry never needs adjustment or maintenance. It operates outside the audio signal path to keep the DC offset at the output of the A 51 at a constant 0.00 Vdc. The results are startling clarity, freedom from listening fatigue, and formidable bass response.
  • Integrated Relays: Each channel of the A 51 has a high-quality protection relay with gold-plated contacts for long-term reliability. These relays function to protect either the amplifier, the speakers, or both. When the A 51 is first powered on, these relays remain open for approximately three seconds as the positive and negative power supplies stabilize and reach equilibrium with no DC offset at the speaker terminals. This prevents annoying popping or other transient noises. Relay protection also prevents damage to your speakers in case of a catastrophic amplifier failure. Any amplifier that doesn't use relay protection for its speaker outputs compromises the safety of the amplifier and your speakers.
  • Current Overload: Specialized current-sensing transistors are connected to the output stages of the A 51 to constantly monitor the current flow through the output transistors. If the current drawn by this stage exceeds a predetermined safe level due to a load impedance below 1 ohm or a short circuit at the speaker terminals, the output relay will open immediately to prevent the output transistors or other parts from failing.
  • Multiple Fuses: In addition to the main fuse on the rear panel each channel of the amplifier has four internal fuses for the positive and negative DC voltage rails of input stage and two for the output stage These fuses provide backup protection in case the over-current protection does not work in time, or if an internal part fails. In the event of a part failure, these fuses halt operation to minimize damage to additional parts.