Cambridge Audio Azur 851W Class XD Reference Power Amplifier

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A Monster of a Power Amplifier with Extremely Refined SoundWhen Cambridge Audio set out developing its Azur 851 Series of hi-fi separates, the brief was simple: to make a hi-fi system that sounds better than anything else out there. The... Read More

A Monster of a Power Amplifier with Extremely Refined Sound
When Cambridge Audio set out developing its Azur 851 Series of hi-fi separates, the brief was simple: to make a hi-fi system that sounds better than anything else out there. The award-winning 851 series is designed with one goal in mind – perfection.
By separating out preamp and power amplifier functionality, Cambridge Audio's engineers were able to give each component the time and dedication that it needs to deliver the ultimate performance. The 851W power amplifier is Cambridge Audio's best ever sounding amplifier. It's an incredibly flexible performer that can be used in stereo, bi-amped, or bridged mono configurations.

Key to its superior sound is a refined version of the company's proprietary Class XD amplification. This combines the best of Class A and Class AB amplification, delivering the sound quality of a Class A amp while retaining the efficiency of a Class AB amp.
The 851W is designed to work in perfect synergy with the new Cambridge Audio 851E preamplifier – from the fully balanced XLR audio connectivity, to neat touches such as power syncing with each other - this pre and power amplifier combination not only sounds incredible, but works in perfect harmony too.

Absolutely Immense Power
Helping the Azur to achieve its incredible sound are some other key features rarely seen at this price point, including the 851W's immense power. Into 8 Ohms, this amp outputs 200 watts-per-channel in stereo mode. Into 4 Ohms, this figure increases to 350 watts-per-channel. In bridged mono mode into 8 Ohms the 851W becomes a 500-watt monoblock amplifier. Into 4 Ohms, this figure increases to an incredible 800 watts.  In fact, the 851W is so powerful that a normal AC power cable can’t deliver the current required. The 851W is supplied with its own dedicated, high performance 20 amp AC power cable.

Allowing you to get the best possible performance from the 851W, it provides both balanced and unbalanced analog audio inputs for superior connectivity. Also provided are two pairs of speaker outputs per channel.

Class XD Technology
The 851W runs Cambridge Audio's proprietary Class XD amplification. The unique design of Class XD gives pure Class A operation at low levels, moving smoothly into an enhanced version of Class B at higher levels. This system should not be confused with Class AB, which inherently generates greater distortion at high levels than a pure Class B design.  XD technology feeds a controlled current into the output stage, actively displacing the crossover point in a new way so that the usual Class B crossover point no longer occurs at zero volume - the worst possible position in terms of distortion – but at a significant output level. The result is a smooth and linear transition between the two modes of operation, which differs significantly from the abrupt gain transition of a Class AB amplifier.

Ultra-Low-Distortion Terrapin Modules
The 851W uses Cambridge Audio's proprietary Terrapin impedance buffering modules, for absolutely consistent sound across all sources. Nicknamed the Terrapins (because of their 10 pins) these modules add yet another dimension to Cambridge Audio's commitment to audio excellence.  Specifically, the Terrapin technology offers lower noise and distortion with excellent channel separation. This obviously gives the most natural sound quality possible, coming from a 'black' background due to low noise and with excellent soundstaging due to the isolated left and right channels.

Large Toroidal Power Transformer
A high quality power supply is critical to the performance of a power amplifier, and the Azur doesn't disappoint. The 851W uses large toroidal transformers to deliver perfect power, with extremely low distortion. In fact, it uses two of them – one for the power amplifier itself and one for the inputs and other circuitry.

Other features include audio detect (Auto Power Up and Down) for power synchronization with the 851E preamplifier, CAP5 protection technology, and eco-friendly <0.5W Standby power consumption.

Available in Black or Silver Finish.


  • Power output – stereo/mono: 200 watts per channel into 8 Ohms; 350 watts per channel into 4 Ohms
  • Power output – bridged mono: 500 watts into 8 Ohms; 800 watts into 4 Ohms
  • THD (unweighted): < 0.001% 1 kHz; < 0.005% 20 Hz - 20 kHz
  • Frequency response: 5 Hz – 80 kHz
  • S/N ratio (ref 1W / 8 Ohm): >90dB (unweighted)
  • Sensitivity – stereo or mono: 1.5V rms unbalanced 1.5 + 1.5V rms balanced
  • Sensitivity – bridged mono: 0.775V rms unbalanced 0.775V + 0.775V rms balanced
  • Input impedances: Balanced input 38 k Ohm; Unbalanced input 68 k Ohm
  • Damping factor: > 110 at 1 kHz
  • Max. power consumption: 2400 watts
  • Standby power consumption: <0.5W
  • Dimensions: 16.9" wide, 5.8" high, 14.4" deep
  • Weight: 42.2 lbs/19.1 kgs.
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