Интегральные усилители — Line Magnetic LM-300IA

Line Magnetic builds the LM-300IA as a 300B single-ended integrated amplifier. Line Magnetic is a Chinese company founded in 2005 by two brothers that began by repairing and building replicas of historic Western Electric, Altec and Jensen equipment. The model sits in a range of integrated amplifiers built around 300B, 805, 212, KT88, KT170 and EL84 valves. The retail price is just under USD 4,000, and a separate LM-300IA Plus version is also sold.

Key specs that matter

Each channel gets 10 W of Class A output from one directly heated PSVANE 300B triode. The front end uses Tung-Sol 6SL7GT and 12AT7 (ECC81) valves, and a pair of LINLAI 274B rectifier valves handles the high-voltage supply. Frequency response is 20 Hz to 50 kHz within 1.5 dB, the signal-to-noise ratio is 87 dB, input sensitivity is 200 mV and input impedance 100 kohm. The rear panel carries three pairs of RCA line inputs, a Pre-In that turns the unit into a power amplifier, a subwoofer output and speaker terminals for 4, 8 and 16 ohm loads. The chassis measures 435 x 217 x 365 mm and weighs 34 kg.

Engineering and use context

With only one output valve per side, that valve amplifies the whole waveform, with no second valve sharing the work as in push-pull, so the 300B runs in Class A all the time and power stays low. The output transformer must carry a steady DC current without saturating, which is why Line Magnetic fits two wide-band EI output transformers of its own design, while toroidal transformers feed the power supply, each part encapsulated to limit interference. Valve rectification with the 274B gives a gradual rise of the high voltage at switch-on, and a soft start with a 30 second delay also protects the valves. Bias is set by the owner, and a built-in current meter on the top plate shows the idle current of each 300B, so the setting can be checked as valves age. Volume runs through an ALPS potentiometer, an IR remote is supplied, and the circuit is assembled by hand with point-to-point wiring rather than a printed board. The cabinet has a piano lacquer finish and a removable metal cage covers the valves.

Pairing

With its low output, the amplifier suits speakers of high sensitivity such as horns, full-range drivers or efficient two-way designs, preferably with an easy impedance curve. The separate output taps let the owner match the transformer to the nominal impedance of the speaker. The subwoofer output helps when the main speakers are small, and the Pre-In accepts an external preamplifier or the front outputs of an AV processor.

Alternatives

The Line Magnetic LM-805IA is a single-ended integrated amplifier built on 805 output valves with 48 W per channel.