Усилители мощности стерео — Wavac Audio Lab EC-300B

The EC-300B is a stereo single-ended power amplifier from WAVAC Audio Lab of Japan, built around a pair of 300B directly heated triodes running in Class A. It was designed by Nobu Shishido, whose Inverted Interstage Transformer Coupling circuit, known as IITC, is the basis of the amplifier, and WAVAC still lists it among its single-ended directly heated triode power amplifiers. In the US it sold for USD 24,000, later raised to USD 24,490 because of higher production and shipping costs.

Key specs that matter

Output is 10 W per channel, with output transformer taps for 4, 8 and 16 ohm loads. Frequency range is 20 Hz to 100 kHz. Input sensitivity is 1.0 V rms and input impedance 100 kohm, with 80 dB of signal to noise ratio. Power consumption is 260 W, and the mains voltage can be set to 100, 110, 120, 220, 230 or 240 V at 50 or 60 Hz. The chassis measures 485 mm wide, 350 mm deep and 220 mm high, and the amplifier weighs 27 kg.

Engineering and use context

The EC-300B takes its input through a Western Electric 435A wired as a triode, a small-signal valve originally made for telecommunication equipment. WAVAC's adaptation of the IITC concept cancels the DC magnetisation in the interstage transformer, which in a normal single-ended design would push the core towards saturation and limit the low end. The circuit is direct-coupled throughout, without any coupling capacitors, and the output stage biases itself. The 300B filaments are heated from a filtered DC supply to keep hum and noise low, which matters for a triode whose filament is also its cathode. All transformers are custom-wound for WAVAC by TANGO, the Japanese transformer maker. The 10 mm thick chassis is NC machined from solid aluminium in a champagne-gold finish, with a perforated aluminium bottom plate for cooling. The valve sockets are machined from solid Teflon with gold-plated phosphor bronze contacts and sit on elastomer vibration isolators, and a glass canopy on elastomer supports covers the output valves. Gold-plated speaker terminals are mounted on the top plate.

Pairing

With single-ended triode output, the amplifier suits high-sensitivity loudspeakers such as horns and full-range designs, ideally with an easy impedance curve so the output transformer tap can be matched to the load. The high input impedance and moderate input sensitivity allow almost any valve or solid-state preamplifier to drive it to full output.

Alternatives

The WAVAC MD-300B is also a single-ended 300B stereo power amplifier, using 6Y6 drivers and 12AT7 input valves and adding a passive volume control with input switching.