Class XD Amplifier Technology: Class XD, a proprietary Cambridge Audio
technology, is a unique amplifier circuit design created to offer the optimum
sound quality currently available from existing amplifier configurations, while
retaining the high power levels required for truly dynamic performance. It was
conceived by our London-based engineering team to combine the best of both
worlds from Class A and Class A/B amplifier operation. Class XD (crossover
displacement), gives the benefits of pure Class A operation at low levels and
eliminates the distortion associated with conventional Class B operation as the
fragile audio signal passes though the zero crossing point from transistor to
transistor. Distinct from Class AB, XD technology feeds a controlled current
into the output stage in such a way that the usual Class B crossover points no
longer occur either side of zero volume; the worst possible position in terms of
distortion - but displaced to a point where the transfer functions of the
transistors are better matched, at a significant output level where it is less
audible. The system operates outside of the feedback loop so is not directly
involved in the signal amplification itself. The result is a smooth and linear
transition between the two key operating modes, Class A and B. The benefits of
this approach give the sonic refinements of Class A operation and the
uncompromising efficiency and power of Class B operation. In fact, the new 851A
delivers a gigantic 120 watts per channel; enough for the most demanding speaker
120W x 2Ch Integrated Amp: The Cambridge Audio Azur 851A's
Class XD amplifier delivers an impressive 120 watts per channel into 8 ohms,
at 20-20kHz, with 0.03% THD at 80% rated power (or 200 watts per channel in 4
Audiophile-Grade Toroidal Transformers: Capable of delivering an
enormous 120 watts of high-quality power, the 851A benefits from two
audiophile-grade mains Toroidal transformers; one for the sensitive preamp
section and one for the power stages, allowing the 851A to deliver its immense
output without stress. The 851A is actually two amplifiers in one: a preamp and
a power amp. Underneath the hood it benefits from two high specification,
custom-designed toroidal transformers to deliver on its promise of a powerful
yet beautifully detailed sound. Following a painstaking assessment period,
Cambridge Audio engineers have selected uncompromising, oversized,
"audiophile-grade" components for the plethora of benefits they bring such as
lower noise, quieter PSU operation, and sublime bass response. Towards the back
of the amplifier is the separate, isolated transformer supplying the preamp
stage, and located in the center is the main transformer supplying the main
power amp stage. These critical components are key to delivering the sonic
performance and the power reserves you will experience from the 851A.
Balanced Volume topology: The 851A integrated amplifier can be
precision-controlled with Cambridge Audio's new fully balanced silicon gate
volume control, which gives very fine adjustment (1dB steps) across the volume
range and highly accurate channel-balancing.
5-Way Protection System: The 851A benefits from the latest
version of Cambridge Audio's sophisticated proprietary protection system. CAP5
(Cambridge Audio Protection 5-way) is an innovative and intelligent circuit that
constantly monitors the amplifier, without degrading signal purity, ensuring
ultimate reliability. Designed to protect the amp in operation, the system has
been developed to leave sound quality unaffected. It incorporates a clipping
sensor that reduces the volume to keep levels in a safe range, short-circuit
detection, and current and voltage overload detection plus a solid-state
temperature sensor for over-heat cut out. Combined, these technologies ensure
that Cambridge Audio amplifiers will go on to deliver years of reliable,
- DC Detection: CAP5 offers loudspeaker protection if the output of
the amplifier goes to a high constant voltage (DC). T
- Over Temperature Detection: AP5 includes
temperature detection which constantly monitors the heat generated by the
output transistors. If the monitored temperature reaches a high level
(suitably within the limits of the output devices) the amplifier will
automatically switch into a fault mode.
- Over Voltage Detection: CAP5 offers V/I protection by constantly
monitoring the output transistors to keep them working inside their Safe
Operating Area (SOA). The SOA is a set of limits given by the output
transistor manufacturer to ensure reliability. V/I also protects the
amplifier against short-circuits on the speaker terminals during use.
- Short-Circuit Detection: During power up from Standby, CAP5
performs a check on the loudspeaker terminals to see if a short across the
terminals has been accidentally introduced. If the resistance measured
across the loudspeaker terminals is too low, the unit will remain in Standby
mode until the fault has been removed and power up is re-attempted.
- Intelligent Clipping Detection: CAP5 has the ability to detect
when the amplifier starts to clip or overdrive at its output, which can
damage loudspeakers and degrade the sound. Clipping distortion is caused at
high volume levels when the output signal briefly goes outside the maximum
voltage that the amplifier can provide, causing the tops of the signal to
flatten off. When CAP5 detects clipping, the volume will be automatically
nudged down until CAP5 detects an undistorted output.
Full Metal Chassis: The 851A integrated amplifier features an
acoustically damped, low resonance full-metal chassis with a thick, brushed
Standby Circuit: This innovative new circuitry design allows
851A integrated amplifier to consume less than 0.5 watt of power when in
Standby, but itself remains totally isolated and non-detrimental to the sound
quality when the amplifier is in operation.
Main Power Switch: The 851A integrated amplifier's main
power on/off switch is located on the rear-panel.
Standby/On: Once the amp's rear-panel main power switch has been
turned On, you can switch the 851A On/Off using the front-panel Standby/On
Volume: Use the front-panel rotary volume control to increase or decrease the
level (-95dB to 0 dB) of the sound from the outputs of the amplifier. The volume control affects the level of the loudspeaker output, the preamp output,
and the headphone output. It does not affect the Rec Out connections.
Input Gain Trim: The relative levels of the 851A's inputs can be
adjusted by the gain trim feature. This allows each input to be adjusted so that
each sounds the same in terms of average loudness when you switch between them.
Pick the loudest sounding source and trim its level (0 to -12 dB) until it
matches the average perceived level of the others. Repeat this process, if other
sources also stand out as louder than the average.
A/B Operation: The Cambridge Audio 851A integrated amplifier lets you
play A speakers only, B speakers only, or A and B speakers together.
Direct: This control gives the audio signal a more direct path to the
power amplifier stage of your amplifier, bypassing the bass and treble control
circuits for the purest possible sound quality.
Bass and Treble: The 851A integrated amplifier has independent Bass
(20Hz) and Treble (20kHz) controls which can be adjusted from +/-7.5dB. The
Bass and Treble controls only effect the speaker and preamp outputs; they do not
effect the record outputs.
Balance: This control allows you to adjust the relative output levels
of the left and right speaker channels. The Balance control only effects the
speaker and preamp outputs; it does not effect the record outputs.
IR Remote Control: The 851A is supplied with an Azur
Navigator IR remote control that operates both this amplifier and Cambridge
Audio Azur-series CD players. The Azur 851A/C Navigator remote control can also
control the basic functions of Apple devices, such as Apple TV and Apple's iPod/iPhone/iPad
when docked in a Cambridge Audio or Apple dock (sold separately).
Note: The remote operates off of three "AAA" batteries
Balanced and Unbalanced Audio Inputs: The Cambridge Audio
851A integrated amplifier is fitted with both balanced and unbalanced analog audio
- Unbalanced RCA Inputs: The Cambridge Audio 851A is
fitted with ten pair of gold-plated analog audio unbalanced inputs (RCA) to connect your external audio
sources; such as your Blu-ray/DVD player, AM/FM tuner, CD/SACD player, or
- Balanced XLR Inputs: The 851A integrated amplifier features two
sets of balanced XLR inputs to connect high-resolution CD and SACD players.
XLR is a higher-quality input that can reject noise and interference from
equipment with balanced outputs.
Note: The 851A integrated amp is not equipped with digital
inputs/outputs or video inputs/outputs. There is also no Phono input.
Record Input/Output: The 851A integrated amplifier features a set of gold-plated analog audio
inputs/outputs (RCA) that can be connected to a CD
recorder, MiniDisc recorder, or tape recorder.
A-B Speaker Terminals: The Cambridge Audio 851A integrated amplifier
is equipped with two sets of gold-plated, binding-post speaker terminals
(labeled A and B). The use of A and B speaker terminals allow you to hook up
speakers in another room. When using a single pair of speakers, use speakers
with a nominal impedance of 4- to 8-ohm. When using two pairs of speaker, use only speakers with a
minimum nominal impedance of 8 ohms.
Preamp Output: The Cambridge Audio 851A is equipped with two RCA
preamp outputs to connect the integrated amp to an external power amplifier or
Headphone Output: The 851A integrated amplifier offers a front-panel
1/4" headphone output to connect a set of headphones for private listening. When
headphones are connected, the output to both A/B loudspeakers is muted. Use
headphones with an impedance of between 32 and 600 ohms.
IR Emitter: The 851A features a IR Emitter input (3.5mm) which allows
modulated IR commands from multi-room systems or IR repeater system to be
received by the amplifier.
Control Bus: The 851A features a Control Bus Input and Output (RCA),
which allows un-modulated commands from multi-room systems or other components
to be received by the amp and loop out to another unit.
RS232 Port: The integrated amp's RS232 port is used for
control of the 851A in custom install situations from an external controller. A
full protocol is available for the 851A on
Cambridge Audio's website.
AC Power Cord: The Cambridge Audio 851A operates off of standard
household AC current using the supplied detachable 6' AC power cord.
Upsampling CD Player/DAC/Pre-Amp: The
Cambridge Audio Azur 851C single
disc compact disc player/DAC/pre-amp makes an excellent addition to the
851A. The 851C has been designed to play CD-Audio, CD-Text, recordable CD
(CD-R) and re-writable CD (CD-RW) type discs.
External DAC: The
Cambridge Audio Azur DacMagic Plus is a high-quality up-sampling digital-to-analog converter. The DacMagic Plus features a range of components to enhance your music listening
experience; these include a 32-bit Analog Devices Digital Signal Processor
(DSP), dual Differential Virtual Earth balanced filter topology, and twin
Wolfson WM8740 high quality DACs in dual differential mode for excellent stereo
imaging. Other features include a choice of digital filters (linear
phase/minimum phase/steep filter) and an incoming sampling rate indicator which
shows the original source quality.
Network Player/DAC: The
Cambridge Audio Azur 851N network player enables you to play your favorite
Internet radio stations and provides digital inputs to use with the Cambridge
Audio 851A integrated amp. Using a wired or wireless Internet connection to your
network, it includes Apple AirPlay for iOS devices, Spotify Connect (premium
subscription required), Tidal control (subscription required), and the ability
to plug in an external hard drive filled with high-res audio. Its built-in
audiophile-grade DAC is a perfect complement to the ultra-clean sound from 851A