Усилители мощности стерео — VTL S-400 Series II Reference

The S-400 Series II Reference is a two-channel tube amplifier made by VTL, Vacuum Tube Logic, the American company led by Luke and Bea Manley. The original S-400 appeared in 2004, and the Series II version uses the same circuitry as VTL's Siegfried Series II Reference monoblocks, placed in a single stereo chassis. VTL reworked the entire signal path for this version.

Key specs that matter

Output is 300 W per channel into 8 ohms in tetrode mode and 150 W in triode mode, with 400 W and 200 W into 4 ohms, at below 3.5% THD from 20 Hz to 20 kHz. The valve complement is twelve 6550 or KT88 output valves, two 12AT7 and two 12BH7. Both XLR and RCA inputs are fitted, with 48 kohm impedance and 1 to 2 V sensitivity. Maximum power consumption is 1,600 W. The chassis measures 290 x 610 x 610 mm and weighs 159 kg in the manufacturer's figures.

Engineering and use context

The balanced front end feeds a differential splitter, which in turn drives a push-pull output section of reduced impedance. The output transformer is VTL's proprietary Reference design, fully balanced, with much greater interleaving and coupling for wider bandwidth. The amplifier uses zero global negative feedback and instead applies a shorter, faster, fully balanced local loop, which needs no capacitor compensation and keeps the amplifier stable into difficult loads. A three-position damping switch changes the local feedback in three steps, low, medium and high, which alters the output impedance to suit the speaker. VTL's SmartTube system, shared with the Siegfried II, provides automatic bias and fault sensing, so the output valves are biased without user adjustment and a failing valve is detected and isolated. The microprocessor switches between tetrode and triode operation and limits in-rush current at switch-on. Precision-regulated plate, screen and bias supplies hold the output valves' operating point steady against mains fluctuations. A new current source improves operation from single-ended inputs. Bi-directional RS-232 allows control from home automation systems.

Pairing

In tetrode mode, the S-400 drives large, low-sensitivity floorstanders in rooms of 40 m2 and more. VTL's own TL-7.5 Series III preamplifier pairs with its balanced inputs, and the damping factor control helps match speakers with demanding bass loads.

Alternatives

VTL's Siegfried Series II Reference is the monoblock version of the same circuit with more power per channel.