Усилители мощности моно — VTL MB-125

The MB-125 is a monoblock valve amplifier from the Performance series of VTL, an American maker of hand-built tube electronics. The monoblocks are supplied as a pair, and each unit is built to order with an estimated lead time of three months. VTL positions these space-efficient monoblocks for both two-channel and home theater systems.

Key specs that matter

Each monoblock produces 100 W in tetrode operation or 50 W when switched to triode, measured into a 5 ohm load across 20 Hz to 20 kHz at under 3 percent THD. The optimum load range is 2 to 8 ohm. Input sensitivity is 700 mV and input impedance is 145 kilohm. Noise sits at -95 dB below signal, referenced to 120 Hz at full rated output in either mode. At idle a unit draws 100 W from the mains, rising to 300 W at full power. The primary mains fuse is a 6 A slow-blow ceramic type on 100 or 120 V supplies and a 3 A type on 220 or 240 V, with a 0.75 A fast-acting ceramic fuse protecting the B+ rail. Each chassis is 40 cm wide, 30 cm deep and 18 cm high, with a weight of 22 kg unpacked and 25 kg once boxed.

Engineering and use context

Four EL34 pentodes form the output stage of each unit, supported by two 12AT7 small-signal valves, and every output tube has its own bias setting. Tetrode or triode operation is chosen with a switch, trading half of the available power for the triode connection. The power supply stores more than 200 joules of energy, and the output transformer is a VTL proprietary design wound for this amplifier. The front panel is an extruded aluminium section offered in black or silver, and a window of thick smoked glass lets the glow of the valves show through. With one channel per enclosure, the pair can be placed close to each loudspeaker, and single units can be added for extra channels in a home theater system.

Pairing

VTL recommends the MB-125 for planar and electrostatic loudspeakers and for highly efficient floorstanding dynamic designs, and the 2 ohm lower limit of the load range covers the impedance dips of many panel speakers. With a 145 kilohm input, the MB-125 presents a light load to a valve preamplifier, for instance the VTL TL-2.5i from the same Performance series, which has a single-ended output matching the single-ended input of these monoblocks. Home theater owners can buy additional single units to power more channels.

Alternatives

The VTL MB-185 Series III Signature is a larger monoblock from the Signature range, rated at 225 W in tetrode and 140 W in triode from eight EL34 or 6CA7 valves.