Усилители мощности стерео — VTL ST-85

The ST-85 is a stereo power amplifier with vacuum tubes, built by VTL Amplifiers Inc of Chino, California. The amplifier and its power supply share one compact chassis.

Key specs that matter

Four EL34 tubes form the output stage, while four 12AT7 tubes handle the input and driver stages. A rear switch selects tetrode or triode operation of the output tubes. Each channel takes its signal through one gold-plated RCA jack, so there is no balanced XLR input, and input impedance is 100 kohm. VTL quotes total harmonic distortion below 3 percent across 20 Hz to 20 kHz and 95 dB of signal to noise ratio referenced at 120 Hz. The chassis measures 40 cm wide, 30 cm deep and 18 cm high, and the amplifier weighs 20.5 kg unpacked.

Engineering and use context

Changing between tetrode and triode requires the amplifier to be switched off first, followed by a wait of at least five minutes so the capacitors can drain before powering up again. Idle current for each output tube is preset at the factory to about 27 mA, and VTL asks for a bias check after every 1000 hours of listening or once a year, with four measurement points on the top deck. That work is meant for a trained technician, because a cover has to come off and exposes high voltages. A 0.5 A fast-acting ceramic B+ fuse on the back panel protects the rest of the circuit if a tube draws too much current. The mains fuse is rated 6 A slow-blow for 100 to 120 V supplies and 3 A slow-blow for 220 to 240 V, and the factory sets the operating voltage for the country of sale. VTL expects tube replacement to become necessary after roughly 3000 hours of use. A protective wire cage covers the tubes, metal handles sit on the front panel, the unit stands on four rubber feet, and a green LED shows power-on status. Each channel has its own plus and minus speaker posts, which VTL says to tighten by hand or with a nut driver without excess force. The standard warranty runs 90 days, extendable to five years through registration, with six months on tubes.

Pairing

With single-ended RCA as the only input type, the ST-85 needs a preamplifier with an RCA main output. VTL's TL5.5 Series II line preamplifier provides such an output next to its balanced one.

Alternatives

Within VTL's stereo power amplifier range the ST-150 is the other model to consider.