Прочие устройства — Line Magnetic LM-755

The LM-755 is an 8 inch field-coil drive unit from Line Magnetic Audio of Foshan, China, listed by the maker among its horn units under the name LM-755 Gamut Horn. It is modelled on the Western Electric 755A, but where that post-war driver used a permanent magnet, the LM-755 replaces the magnet with an electromagnet that must be energised from an external DC supply. Line Magnetic also installs field-coil 755 units in its own finished loudspeakers, including the LM-755A Console Speaker and the floorstanding LM 755i.

Key specs that matter

Line Magnetic rates the unit for a frequency range of 70 Hz to 13 kHz, a resonance frequency of 68 Hz, input sensitivity of 92 dB and a nominal impedance of 8 ohm. The power rating on the driver page is 6 W, while the console speaker and the LM 755i built around the same unit are specified at 10 W. The field coil wants 200 V DC at 50 mA, which works out to 10 W of continuous power turned into magnetic flux and heat in the winding for each driver.

Engineering and use context

A field coil lets the strength of the magnetic gap depend on the supply rather than on a fixed magnet, so the excitation voltage becomes an adjustable part of the design. Line Magnetic pairs the LM-755 with its PR-3, a high-voltage excitation supply with valve rectification. In the LM 755i package each speaker gets its own PR-3, and that supply uses a 300B triode together with a voltage adjustment knob and a meter so the coil current can be set and monitored. The LM 755i itself is a single-driver design with one field-coil 755 unit in each side-vented wooden floorstanding cabinet, and it was priced at USD 8,995 per pair with the supplies included. Because the driver covers most of the audio band on its own, it can run as a full-range unit with no crossover, which keeps any filter parts out of the signal path between amplifier and cone. Line Magnetic also makes larger field-coil woofers, the 13.3 inch TA-4151 and the 18 inch TA-4181-A, for builders who want to extend a 755-based system downward.

Pairing

Being efficient and an easy load, the LM-755 suits low-powered single-ended valve amplifiers, and Line Magnetic's own 300B designs such as the LM-91A monoblock or the AM-2 fall in that range. The field-coil supply must be budgeted as a separate component for each driver.

Alternatives

The Line Magnetic LM-755A is the permanent-magnet version of this unit, using a cobalt magnet and a 4 ohm voice coil so no excitation supply is needed.