Усилители мощности стерео — VTL S-200 Signature

The S-200 Signature is a stereo tube power amplifier made by VTL, hand built in the USA to order with roughly three months for delivery. VTL puts the features of its MB-450 and MB-185 Series III Signature monoblocks into a single space-saving chassis, and the company developed its SmartTube technology to make valve ownership simpler.

Key specs that matter

Output is 200 W per channel in tetrode and 100 W per channel in triode into a 5 ohm load setting, measured from 20 Hz to 20 kHz at less than 2.5 percent THD, with an optimum load range of 4 to 8 ohm. Input sensitivity moves from 700 mV to 1.6 V depending on the damping factor switch, and input impedance is 45 kilohm. Signal to noise is quoted as 110 dB at 120 Hz. Power draw is 330 W at idle and 1000 W at full output. Mains fusing is 15 A slow-blow on 100 or 120 V and 8 A on 220 or 240 V, with a 1.0 A fast-acting 600 V plate fuse. The chassis measures 47 x 45.7 x 23 cm and weighs 47.6 kg unpacked.

Engineering and use context

Eight 6550 or KT-88 output tubes are used, joined by two 12AT7 and two 12BH7 valves. Its differential input stage is followed by a differential phase inverter feeding a push-pull output stage of reduced impedance, and the entire circuit is balanced, so the amplifier stays balanced even with a single-ended source. Global feedback is not used at all, while the local loop is short and needs no capacitor compensation, and a JFET current source supports single-ended input. The custom output transformer is fully balanced with interleaved and coupled windings. SmartTube provides automatic bias adjustment and fault sensing, so the amplifier signals when a tube needs replacing and output tubes do not have to be matched. Bias and screen supplies are adjustable and precision regulated to keep the output operating point stable under mains variation. A three-position Damping Factor switch with LOW, MED and HI settings alters the output impedance through the amount of feedback. A microprocessor runs the unit, inrush current is limited, a Standby Mute function reduces tube current, and tetrode or triode is switchable. The chassis can be rack mounted and has an extruded aluminium front in black or silver.

Pairing

VTL pairs the S-200 with its TL-6.5 Series II Signature preamplifier, whose balanced outputs match the fully balanced input stage.

Alternatives

The VTL ST-150 is a stereo valve amplifier in the Performance range, using eight 6550 output tubes with individual bias settings and a power supply storing over 200 joules.