Усилители мощности стерео — Line Magnetic AS-135

The AS-135 is a low-power single-ended Class A valve amplifier made by Line Magnetic Audio of Foshan, China, and sold under its Analog Sound label. Line Magnetic calls it the first model in that series built as both an integrated and a pure power amplifier at low output, and as its most cost-effective way into a Western Electric style circuit. A lower-rated sibling called the AS-134 is part of the same design, and an SG Series 2.0 edition of the AS-135 has also been offered.

Key specs that matter

Rated output is 7 W per channel RMS, delivered by directly heated triodes, and the amplifier accepts either 300B or 2A3 output valves. Speaker taps are provided for 4, 8 and 16 ohm loads. Input sensitivity is 250 mV and input impedance is listed as 100 kohm / 600 ohm. Frequency response runs from 20 Hz to 30 kHz within 2 dB. Measured with AC heating on the output tube filaments, the signal-to-noise figure is 87 dB, and total harmonic distortion is 1 percent. The chassis measures 480 x 255 x 470 mm and weighs about 37 kg. The supply voltage is specified as 115 to 230 V at 50 Hz with a 5 percent tolerance, and current draw is 0.5 A at 220 V.

Engineering and use context

A valve rectifier, the 5Z4 or its equivalent 5Y3, produces the high-voltage supply instead of silicon diodes. The high-voltage rail and the secondary high-voltage rail each have their own independent power supply. Line Magnetic states that the layout follows the structure of the Western Electric WE12 amplifier line. Because both 300B and 2A3 triodes can be fitted, one chassis gives access to two classic output valves, and the maker says the circuit was designed to bring out what each type does well while working around its limits. The option of running the unit as a pure power amplifier means a separate line stage can be placed in front of it later without replacing the amplifier itself, while the integrated mode covers a system with no preamplifier at all.

Pairing

With so little power on tap, the AS-135 needs loudspeakers of high efficiency, such as horn-loaded designs or large full-range drivers, and the three output taps allow matching to the speaker's nominal load. Its input sensitivity is within reach of ordinary line-level sources, and in power amplifier mode it can sit behind an external valve preamplifier from the same brand.

Alternatives

The Line Magnetic AS-134 is the 3 W per channel RMS stereo version of this design, which the maker also offers for use as a pure power amplifier.