Integrated amplifiers — Line Magnetic Audio LM-805MK II

The LM-805MK II is a two-channel integrated valve amplifier made by Line Magnetic whose output stage runs single-ended in Class A on the 805 transmitting triode. In each channel one 805 output valve is driven by a 300B, and the whole unit is hand-made, with the internal wiring laid out point-to-point instead of on a printed board.

Key specs that matter

The rated output is 50W per channel into 8 ohms, all of it in single-ended Class A, and the amplifier is built in China. The frequency response is 20Hz to 45kHz at -3dB, total harmonic distortion is 1% at 1kHz and signal-to-noise is 87dB A-weighted. Line sensitivity is 200mV and the pre input needs 590mV, both into an input impedance of 100k ohms. Power consumption reaches 490W. The case measures 432 x 465 x 279mm and the net weight is 53.5kg.

Engineering and use context

Eight tubes are fitted in all - two 12AX7 in the input stage, two 12BH7 in the driver stage, two 300B and two 805 triodes. The power supply is built on a custom toroidal transformer described as a 900W Nippon Steel ultra-capacity core, while the signal leaves through a pair of Japanese Z11 audio-grade EI output transformers with wide bandwidth. Coupling is handled by German Mundorf capacitors and the volume control is a Japanese ALPS potentiometer. Transformers and chokes are encapsulated and mounted separately to keep interference down. A rear-illuminated meter on the front panel serves for bias adjustment, and external bias and hum-balance controls make the job possible without removing covers. High-voltage filament preheating and buffer circuits, a soft start and a 30-second time delay protect the valves at power-up, and the design is open to tube rolling. A metal tube guard cage can be removed.

Pairing

Five selectable inputs are fitted, four RCA line inputs and a single pre-in connection that allows a separate control amplifier to drive the output stage directly. Speaker cables attach to screw posts compatible with banana plugs, with output taps for 4, 8 and 16 ohm loudspeakers, and a subwoofer output is provided for adding a powered bass unit. Operation from the listening seat is by the supplied infrared handset, and the front display has a backlight that can be switched off. In India the amplifier was listed at INR 633,000 including taxes.

Alternatives

Line Magnetic also produces the LM-845IA, an integrated model with 845 output triodes rated at 22W per channel.