Power amplifiers stereo — VTL S-400 Series II Reference

The VTL S-400 Series II Reference is a stereo tube power amplifier from VTL, or Vacuum Tube Logic, the American company led by Luke and Bea Manley. The first S-400 appeared in 2004. For the Series II version VTL reworked the entire signal path and put the circuit of its Siegfried Series II Reference monoblocks into one stereo chassis.

Key specs that matter

In tetrode mode the amplifier delivers 300 W per channel into 8 ohms and 400 W into 4 ohms. Switched to triode, it gives 150 W into 8 ohms and 200 W into 4 ohms. These figures hold from 20 Hz to 20 kHz at under 3.5% THD. The output stage runs twelve 6550 or KT88 valves, joined by two 12AT7 and two 12BH7. There are XLR and RCA inputs, both with 48 kohm impedance and a sensitivity of 1 to 2 V. Maximum power draw is 1,600 W. According to the manufacturer, the chassis measures 290 x 610 x 610 mm and weighs 159 kg.

Engineering and use context

A balanced front end feeds a differential splitter, which drives a push-pull output section with reduced impedance. The fully balanced output transformer is VTL's proprietary Reference design, with much greater interleaving and coupling for a wider bandwidth. Global negative feedback is not used. Instead, a shorter and faster fully balanced local loop is used, needing no capacitor compensation and keeping the amplifier stable into difficult loads. A three-position damping switch sets this local feedback to low, medium or high, changing the output impedance to suit the speaker. The SmartTube system, shared with the Siegfried II, handles automatic bias and fault sensing, so the output valves need no manual bias adjustment and a failing valve is detected and isolated. The microprocessor also switches between tetrode and triode operation and limits in-rush current at switch-on. Precision-regulated plate, screen and bias supplies hold the output tubes at a steady operating point when the mains voltage fluctuates. A new current source improves operation from single-ended inputs, and a bi-directional RS-232 port allows control from home automation systems.

Pairing

Running in tetrode mode, the S-400 is suited to large, low-sensitivity floorstanders in rooms of 40 m2 and more. The balanced inputs match VTL's own TL-7.5 Series III preamplifier, and the damping control helps with speakers that present a demanding bass load.

Alternatives

VTL's S-200 Signature offers VTL valve amplification in another stereo chassis.