Integrated amplifiers — Line Magnetic Audio AS-125N

The AS-125N is a two-channel valve integrated amplifier in the Analog Sound series of Line Magnetic, designed as a pure Class A single-ended triode circuit around the directly heated 211 output valve. The maker's own catalogue lists the model simply as the AS-125, and an export-only sibling named AS-325N uses upgraded components on the same basis. The design borrows transformer technology from Western Electric and combines it with a set of more practical user features.

Key specs that matter

Output is 18 W per channel RMS. The frequency response runs from 10 Hz to 45 kHz within 2 dB, noise sits 88 dB below the signal and total harmonic distortion is given as 1 percent. Input sensitivity is 500 mV, input impedance is listed as 100 kohm or 600 ohm, and a gain control selects 40 or 52 dB. There are four pairs of RCA inputs, and the speaker terminals offer 4, 8 and 16 ohm taps. The chassis measures 480 x 275 x 490 mm and weighs 50 kg, rising to about 70 kg once crated for shipping. The finish is grey, and the version supplied for the Australian market is set up for 240 V mains.

Engineering and use context

A pair of 211 power triodes, one for each channel, is driven by three 12AX7 and two 6SN7 small-signal valves. A Line Magnetic copy of the Western Electric Model 618 transformer is placed at the input of the built-in preamplifier stage. Assembly is manual throughout, and components are joined with individual point-to-point leads instead of a circuit board, a construction method it shares with the AS-125PP. The power supply uses two separate mains transformers - one generates the high-voltage supply for the output stage and the other handles the low-voltage side of the circuit. The 211 is also the valve Line Magnetic uses in the push-pull AS-125PP monoblocks, which share this chassis size.

Pairing

Given its modest output, this amplifier needs speakers with high sensitivity. Line Magnetic's own LM-812 horn system is rated at 102 dB and 8 ohm, and the LM-69 console speaker offers 95 dB at 8 ohm together with a 20 W power rating that sits just above the amplifier's output. The highest tap covers loudspeakers with a higher nominal impedance while the four line inputs take several line-level sources directly, with the built-in preamplifier doing the volume control.

Alternatives

The Line Magnetic LM-845IA uses a pair of single-ended 845 triodes in a stereo integrated chassis and raises output to 22 W per channel.