Прочие устройства — Line Magnetic PR-3

The PR-3 is a tube-rectified excitation power supply made by the Chinese company Line Magnetic Audio and placed as the high-voltage member of the PR series next to the PR-5, PR-12 and PR-22. Its full catalogue name is Tube Rectifying Excitation Power (High Voltage). It does not process audio at all. Its only job is to put a steady DC current through the field coil of a loudspeaker driver, and one unit is used per speaker, so the LM 755i field coil system runs with a pair of them.

Key specs that matter

Mains input is 220 to 240 V AC or 100 to 120 V AC, in both cases at 50 to 60 Hz. The unit is rated for excitation voltages of 60 to 230 V DC at excitation currents of 20 to 60 mA DC, far higher voltage and far lower current than the low-voltage models in the range. Rectification uses one 300B directly heated triode per unit, an unusual choice for a supply, and the case measures 240 x 190 x 160 mm at 3.9 kg per unit. A pointer meter reports the output current.

Engineering and use context

In a field coil driver the permanent magnet is replaced by a winding, and DC fed through that winding creates the magnetic field in which the voice coil moves. Supporters of the approach argue that such a motor does not lose strength over the years the way a permanent magnet can, and that it controls the cone more precisely. The PR-3 is built for field coils wound for high voltage and small current. On the LM 755i installation each supply carries a small knob at the front left corner and the current meter in the middle of the top plate, so the excitation level can be changed and checked at the same moment. The 755i driver calls for about 200 V at 50 mA, well inside the range of the unit. Line Magnetic advises keeping the supplies and field coils switched on for at least 48 hours before moving them much.

Pairing

Line Magnetic lists the drivers the PR-3 is meant to energise - the HT-70 and HD-60 horns, the 1428 and MD1428-2, the 755 and DD-2, and the 555 and 597 compression drivers. Because the unit sits outside the signal path, any amplifier suitable for the speaker can be used, and a stereo pair of speakers needs two PR-3 units. Field coil drivers that call for low voltage at several amperes are outside its rating and belong on a different PR model.

Alternatives

The Line Magnetic PR-22 is a three-way excitation supply for low-voltage, high-current field coils, delivering 3 to 24 V DC at 0.5 to 2 A through four rectifier tubes.