Прочие устройства — Line Magnetic LM-755A

The LM-755A is an 8 inch cobalt-magnet drive unit from Line Magnetic Audio, sold as a bare driver in the company's horn units range for builders who want a Western Electric style full-range speaker without an excitation supply. Line Magnetic states that the design is based on the WE 755A and that it developed its own techniques for the paper cone and the paper pulp used to make it. The original 755A was introduced in 1947, listed in Western Electric catalogues in 1948 and launched alongside the 10 inch 756A and the 12 inch 728B and 754A, and it was widely fitted in railway stations, broadcast studios, office buildings and school public-address systems.

Key specs that matter

Line Magnetic lists a frequency response of 70 Hz to 13 kHz, a sensitivity of 92 dB, a resonance frequency of 68 Hz and a power rating of 6 W. The nominal impedance is 4 ohm, the same figure given for the original alnico 755A, whereas later ferrite versions of that driver moved to 8 ohm. The historical unit measured about 213 mm across and 79 mm deep, and Western Electric called for about 57 litres of enclosure volume, with a coverage angle of 70 degrees.

Engineering and use context

A permanent magnet makes the LM-755A self-contained, so it connects to an amplifier like any passive driver and needs no mains-powered supply for an electromagnet. Line Magnetic chose cobalt for the magnet, which places it with the alnico-era original rather than the ceramic 755C introduced around 1961 or the ferrite 755E of about 1969. The cone matters here because the original was vacuum formed from a pulp that contained silk and cotton, and replicating that part is where Line Magnetic says it put its own development work. Western Electric stopped making the 755A in late 1949, when production passed to Altec Lansing, which kept building the design under its own name. Used alone as a full-range unit, a single cone needs no crossover, so no filter network sits between the amplifier and the voice coil.

Pairing

The low nominal impedance is lower than most single-ended valve amplifiers expect, so an output transformer with a 4 ohm tap is the practical choice. The low power rating limits the unit to low-powered amplifiers, and Line Magnetic's own AM-2 is a 300B single-ended monoblock.

Alternatives

The Line Magnetic LM-755 is the field-coil version of this unit, fed from an external excitation supply.