Усилители мощности стерео — Doshi Audio Evolution Stereo Amplifier

The Evolution Stereo Amplifier is a hybrid power amplifier from Doshi Audio of Charlottesville, Virginia, and it is made in the USA. It belongs to the Evolution series, which also includes the Evolution Line Preamplifier. A solid-state J-FET input stage drives a valve output stage, and the price is about USD 24,995.

Key specs that matter

Output is at least 65 W per channel into 5 ohm from a Class AB circuit, with the first 50 W in Class A. Each channel uses four EL34 or 6CA7 output valves, eight in total. The output transformer is wound for 5 ohm as standard. Inputs are RCA at 47 kOhm and XLR at 20 kOhm. Voltage gain is 30 dB, frequency range is 20 Hz to 40 kHz, output impedance is 1 ohm and noise is 80 dB below 1 W. Power consumption is 130 W. The amplifier measures 457 mm wide, 508 mm deep and 203 mm high and weighs 29.5 kg.

Engineering and use context

The circuit has only two gain stages, a J-FET differential amplifier followed by the valve output stage, and it uses about 7 dB of feedback. A differential input stage rejects noise that appears equally on both sides of the signal, and keeping the number of stages low shortens the signal path. The balanced input passes through a buffer that converts the differential signal to single-ended before it reaches the amplifier circuit. A custom power transformer feeds regulated supplies, so the operating voltages of the valves stay steady when the mains fluctuates. Coupling capacitors are ClarityCap Purity Series types. The chassis is stainless steel, with isolation grommets and mass damping, and outrigger legs decouple it from the surface below. Running the first part of the power range in Class A means that at normal listening levels both output valves of each pair conduct all the time, so there is no switching between them. With the light feedback used, the amplifier's source impedance sits above typical solid-state figures, so the response at the speaker terminals follows the speaker's impedance curve to some degree. Four valves per channel share the output current, which keeps the load on each valve moderate.

Pairing

As the standard output transformer suits mid-range loads, speakers with a nominal impedance of 4 to 8 ohm are the natural match, and other transformer taps can be ordered for loads outside that range. Speakers of average or higher sensitivity in a room of about 20 m2 suit its power. The Evolution Line Preamplifier connects through XLR or RCA.

Alternatives

Doshi Audio has no other stereo power amplifier, as its Evolution Monoblock with four KT150 valves per unit is a mono design.