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Input circuit: Snugly parked behind a mu-metal shielding plate and fixed directly above the mother board behind the XLR connectors that are directly soldered to it, the input circuit consists of a set of high-precision and low-noise operational amplifiers run symmetrically. The input circuit allows the input voltage sensitivity to be adjusted to 1 or 2 volts and to determine the mode in which the unit will work: stereo, parallel-bridged or dual mono. It also includes a detection mechanism in charge of switching the unit on as soon as a signal reaches the input terminals and for engaging the standby mode after 15 minutes without signal. This mechanism comes into play as soon as the automatic mode is activated through the main selector on the front panel of the unit. In standby mode, consumption is reduced to less than 2 watts.

Amplification circuit: The amplification section takes up the central third of the motherboard. It is based on a thrifty architectural design. Impeccable stability while running is the chief result. The topology is unique. The driver stage is organised in what is known as a double-current transfer configuration, giving the power stage a considerable tolerance to residual power supply ripple. It relies on a pair of AB-biased Mosfet type transistors assembled on each channel in a push-pull common-source arrangement (output on the drains). Chosen for their excellent audio characteristics, these parts are very carefully hand-selected. The result is so precise that the circuit requires very little negative feedback. It allows the MSA to perform smoothly under all circumstances without instability even when running in bridged mode with very low impedance loads. Above all, it is a true Nagra electronics ambassador by virtue of its transparency and integrity.

Protection circuit: The Nagra MSA is equipped with all necessary safeguards to protect against potential problems. An appropriate array of sensors and surveillance circuits will detect any overheating or output stage overload. As soon as an anomaly is detected, the control circuit triggers a drop-out relay sequence that deactivates all inputs and inhibits the power circuits. The MSA also hosts a soft-start switch-on circuit that makes sure the relays only start up a few seconds after the unit is powered up, preserving the electronics and preventing turn-on transients through the loudspeakers.

Control circuit: Fixed immediately behind the unit’s front panel, the control circuit uses a microprocessor to handle the MSA’s main functions of start, stop, standby, automatic and silence modes. It also controls the protection circuit and delivers the clock signal to the PFC circuits.

Output circuit: The support plate fixed to the back of the unit’s rear panel is equipped with two pairs of speaker terminals and banana-type sockets into which the user may insert, when required, the parallel-bridging jumper.

A description of the mechanical parts - main body: The casing of the Nagra MSA amplifier is entirely made of finely brushed anodized aluminium in true respect of the distinctive look and aesthetic standards of the brand. Its dimensions are of course adjusted to blend with other members of the Nagra audio family such as the PL-L, the PL-P and the VPS preamplifiers or the Nagra CD compact disc players. The width and the depth are identical (275 x 230mm WxD without connectors) whereas the 115mm height is slightly greater due to the hood’s heat sink construction covering the top of the unit.

The heat sink itself is a complex slab of aluminium milled from a massive aluminium block weighing 10kg before machining and 3kg once finished. Its construction plays a key role in stabilising the amplification stages. Thanks to the strong moment of inertia of its central part, the heat sink acts as an energy storage space so that the transistors fixed to it can release their peaking capacity without a sudden rise in temperature. The 10mm front panel is also machined from a solid block whereas the sides and the rear panel are made from folded sheets.

Front panel: The front panel of the Nagra MSA is equipped with the brand’s famous rotating selector to turn the unit on and off, activate manual or automatic modes and mute the power stages. The traditional Nagra modulometer which expresses the power output levels of the amplifier is escorted by a toggle switch to illuminate the display. Finally a small red LED acts as sentinel should the power stages reach saturation.

Back panel: On the back panel an independent element integrating an anti-interference RFI filter carries the mains power switch, the fuse holder and the IEC connection slot for the external power supply cable. Available connections include two XLR symmetrical input connectors, two pairs of gold  screw-down terminals  to connect the speaker cables, two sockets to accommodate the parallel-bridging jumper for the power stages and a gold-plated threaded terminal to enable the grounding of the unit’s chassis. The back panel also gives access to selectors to adjust the input sensitivity levels (each channel can be adjusted separately) and to define the operating mode - stereo, parallel-bridged or dual-mono.

Technical specifications: Class AB, 2 x 60 watts into 8Ω, 1 V or 2 V input, 1 x 120 watts RMS parallel-bridged 4Ω ;10 Hz to 75kHz +0/-3dB bandwidth; channel separation >85dB; S/N ratio typically 109dB (ASA A-weighted); TDH+N <0.08 % @ full power; input impedance >100 kOhms; automatic start with an input level >10 mV; protection deactivates the amplifier when overheating above 60° C (140° F); DC protection for loudspeakers above ±2.5 V DC; monitoring  level indication via blue LED on front panel (can be disengaged); clipping indication by red LED on front panel (for I>12 A or U>42V); operating range 90-132 V or 180-264 V, 50-60 Hz; power consumption 350W; dimensions 275 x 230 x 115mm WxDxH; weight 10kg.