Интегральные усилители — Line Magnetic AS-137N

The Line Magnetic AS-137N is a push-pull valve amplifier from the Ange range, the grey-finished line produced by the Foshan branch of the company. It can be used as a complete integrated unit or run purely as a power stage. Its output stage uses four Tung-Sol KT170 beam tetrodes, and the owner can switch it between triode and ultralinear operation.

Key specs that matter

In triode mode the AS-137N delivers 60 W per channel into 8 ohms, and ultralinear operation raises this to 120 W, with both modes specified as pure class A by the maker. Input sensitivity is 500 mV, input impedance is 100 kOhm or 600 ohms, and two gain settings of 40 dB and 52 dB are available. The frequency response runs from 10 Hz to 50 kHz within 2 dB, with a 90 dB S/N figure. The chassis measures 483 x 295 x 545 mm and weighs 52 kg.

Engineering and use context

On the way in, the signal passes through an input transformer of the Western Electric 618 type, followed by two NOS XF184 valves from the 1960s that form the preamplifier stage. In the power section a 6SH7 pentode works as the first stage, and E-6SN7 double triodes drive the KT170 output valves. The KT170 needs a main high-voltage supply of around 650 V to reach its potential, so the amplifier uses a power transformer of the Western Electric 307A type together with Ange's 16A valve-regulated supply module, which is built around one XF184 and one 6L6. The KT170 itself has a plate dissipation of 42 W, which is what allows the ultralinear rating from a push-pull pair per channel. Tung-Sol gives it an indirectly heated cathode optimised for audio and a rated service life of more than 8000 hours, and the valve is still used by relatively few amplifier makers because of its price and limited production. Before this model, Line Magnetic had built beam tetrode amplifiers around the KT66 and the KT88. The triode/ultralinear selector acts directly on the output stage, so the same unit can be set up for different loudspeakers without changing valves.

Pairing

The ultralinear setting was aimed by Line Magnetic at loudspeakers of modest sensitivity, where most single-ended valve designs run out of power, while the triode setting suits more efficient boxes. The combination of an input transformer, a switchable gain figure and a pure power amplifier mode also lets the AS-137N sit behind a separate preamplifier in an existing system. Given its weight and the heat of four KT170 valves, it needs a strong shelf with open space above it.

Alternatives

The Line Magnetic AS-127 is an Ange integrated valve amplifier that uses KT88 output valves instead of the KT170.