Усилители мощности моно — VTL Lohengrin Reference

The Lohengrin Reference is a valve mono power amplifier from VTL, the American company behind the Siegfried Reference monoblock and the S-400 Reference stereo amplifier. VTL states that development took five years. It keeps the driver stages and power supply of the Siegfried and adds an output stage built around eight power tubes per chassis.

Key specs that matter

Rated output across 20 Hz to 20 kHz is 400 W in tetrode mode and 200 W in triode mode, both specified into a 5 ohm load, with total harmonic distortion below 2.5% at full power in either mode. The tube set per unit is one 12AT7, two 12BH7 and eight 6550 or KT-88. The output stage runs in class AB1, and the optimal load range is 4 to 8 ohm. Each monoblock has one balanced XLR input, one single-ended RCA input and a single pair of 5-way binding posts. Input impedance is 47 kohm, input sensitivity varies between 1 and 2 V depending on the damping setting, and common mode rejection on the balanced input is 60 dB. Signal-to-noise ratio is quoted as -110 dB. Power draw is 350 W at idle and 900 W at full output. A single chassis is 29 cm wide, 49 cm deep and 50 cm high at 77 kg net, and a pair ships in one crate at 227 kg.

Engineering and use context

The input and driver stages are fully balanced and differential, so a single-ended source still produces a balanced signal at the output. There is no global negative feedback, only a shorter local loop, and VTL states that the amplifier is unconditionally stable without capacitor compensation. Bandwidth reaches 100 kHz at -1 dB. A user-adjustable DF control changes output impedance and damping factor to suit the loudspeaker. The heavily interleaved multi-section output transformer is a VTL design, and the low impedance of the output stage allowed a simpler transformer with less step-down. Plate and screen supplies are precision-regulated and adjustable, a separate discrete regulated supply feeds the input and driver circuits, and the bias supply is regulated so tube operating points stay put under normal mains fluctuation. A microprocessor handles automatic bias, tetrode and triode switching, layered fault sensing against output tube failure, in-rush current limiting and a standby mute that lowers tube current to extend tube life. An RS-232 port gives two-way control and diagnostics. All signal path capacitors are Mundorf silver oil types, with film bypasses on the supply capacitors.

Pairing

The wide load window and high output suit large passive loudspeakers, and a preamplifier with balanced outputs uses the XLR input directly.

Alternatives

Among VTL monoblocks, the closest relative is the Siegfried Series II Reference, source of the driver stages and power supply used in the Lohengrin.