Напольная акустика — Line Magnetic LM-3 Constant Magnetic Barrier

The LM-3 Constant Magnetic Barrier Speaker is the permanent-magnet edition of the LM-3, a three-way horn loudspeaker from Line Magnetic Audio. The company was set up in Zhuhai, China in 2005 by the brothers Zheng Cai and Zheng Xi, and it started out repairing and copying historic Western Electric, Altec and Jensen loudspeakers and electronics before moving on to its own designs. The maker files the LM-3 among its Retro Loudspeakers, next to the LM-7396 and LM-7395 barrier speakers, the LM-812 horn speaker, the LM-755A and LM-69 console speakers and the LM-222 coaxial model. The LM-3 is sold in two forms - this alnico version and a field coil version.

Key specs that matter

The published figures are a frequency response of 38 Hz to 20 kHz, 105 dB sensitivity and a power rating of 100 W. The impedance is quoted as both 16 ohm and 8 ohm, so the tap used on a valve amplifier should be checked against the unit supplied. Each speaker measures 900 mm wide, 2080 mm high and 690 mm deep.

Engineering and use context

The midrange section copies two parts from the Western Electric cinema systems of the late 1920s - the 22A horn and the 555 compression driver. The original 555-W was developed in 1926 by the Bell Labs engineers Edward C. Wente and Albert L. Thuras for talking pictures, at a time when amplifiers delivered very little power. It used a 0.05 mm aluminium dome diaphragm with a corrugated surround, a phase plug and a threaded mount so that it could be fitted to different horns. The 22A is a shortened form of the large theatre horns with an exponential flare, and it is normally crossed over at around 350 Hz or higher. In the constant magnetic LM-3, the 555 uses an alnico magnet, the bass comes from the BA-18 alnico woofer and the top octaves from the LM-302 tweeter, with the TA-7376 network dividing the three bands. The field coil LM-3 keeps the 22A with 555 and the same crossover but switches to an HT-70CU tweeter, a TA-4181 woofer and a PR-22 supply to energise the coils. Choosing alnico removes that external supply and its wiring from the system.

Pairing

Given the high sensitivity, low-powered single-ended valve amplifiers built around 300B or 2A3 triodes can drive the LM-3 to high levels, while the generous power handling also allows larger push-pull valve or solid-state amplifiers.

Alternatives

The Line Magnetic LM-812 is a horn loudspeaker modelled on an Altec design, with a sensitivity of 102 dB.