The power amplifier has the critical task of transforming the low voltage signals provided by source equipment into high-current signals capable of driving the loudspeakers. An amplifier has to satisfy the conflicting demands of conveying fine detail and authentic tonal colours while at the same time providing absolute power and control even into difficult loads. This is why designing an amplifier is always a challenge.
To design an amplifier of the highest quality, three critical areas must be addressed: the circuit design, the circuit topology and the selection of components.
The circuit design of the 855 is based on a proven concept that has been systematically developed and refined. At the same time, a host of new ideas has been incorporated in the design. The circuit is superbly logical and capable of achieving the ultimate in sound quality.
We have explored a number of radically new avenues with the 855. The amplifier’s unique transparency benefits greatly from the use of an innovative topology that substantially reduces stray electromagnetic fields.
Components for the 855 have been selected with great care throughout. Only the best available components are considered and all must undergo an exhaustive analysis of their sonic influence prior to final selection.
True dual mono construction
Under the hood, the 855 Dual Mono Power Amplifier is actually two separate power amplifiers. This arrangement suppresses mutual interference between the channels via their power supplies, resulting in stable and superior sonic imaging. The amplifier electronics are located on a single 4-layer printed circuit board. This technology enables optimum component placement and the shortest possible signal paths. Hard wiring is reduced to a minimum.
New techniques have also been applied to the internal construction of the 855. For example, the large heatsinks are fitted horizontally under the top cover. Airflow is supplemented when necessary by an ultra-quiet fan whose speed is controlled by a microcontroller, ensuring that the amplifier operates at its optimum temperature irrespective of ambient conditions.
Fully balanced design
The 855’s voltage-gain stage is implemented as an ultra-linear, fully balanced cascode stage with current feedback loops. We first introduced this circuit concept in the Lindemann AMP 2 in 1994 and it has been an important part of all of our amplifiers ever since. The extremely high bandwidth available with this technology contributes substantially to the impressive transparency and high-frequency purity of the 855. Only moderate (20 dB) overall feedback has been applied, since our experience has shown that this is vital to the realistic portrayal of the liveliness and dynamics of music.
Innovative high-power output stage
A particular highlight of the 855’s design is its innovative output stage topology that sets new standards for current delivery and electromagnetic interference control. Electromagnetic interference induced by the signal current is one of the most important causes of colouration in transistor amplifiers, yet its effects are frequently overlooked. The output stages contain no less than 32 of one of today’s finest power transistors, carefully hand-selected for every unit. These transistors combine high bandwidth with excellent linearity at high power. A novel arrangement of three parallel push-pull stages ensures excellent stability and a transient capability previously unequalled in an amplifier of this power rating.
Optimized power supply
Because the power supply is such a critical part of any power amplifier, the 855 contains a generously dimensioned power supply section. Power transformers are wound on a special core material. We fit ultra-fast diodes and long-life, low-impedance capacitors. The sensitive input stages have independent power supplies.
Comprehensive protection circuits
Intelligent protection circuitry constantly monitors all aspects of the 855’s operation. Should a DC, short-circuit or overtemperature fault occur, power to the electronics is immediately shut down in order to prevent damage to the amplifier or the speakers. A built-in inrush current limiter operates at turn-on to protect the amplifier’s components and prevent the mains circuit-breaker from tripping.
The protection and microprocessor control circuits are fully isolated opto-electronically from the audio section.
The 855 is equipped with a SYSCOM communication link to allow remote amplifier turn-on via the preamplifier or CD player.