Усилители для наушников — Fosi Audio GR70

The GR70 is a valve headphone amplifier and preamplifier from Fosi Audio, a Chinese maker of compact, affordable amplifiers. It uses four valves in an output-transformerless design and adds bass and treble controls and an RCA preamp output, so it can drive powered speakers or a power amplifier. It was launched at USD 239.99.

Key specs that matter

The valve complement is two 5654W for voltage gain and two 6Zh4 or 6AC7 types for power amplification, depending on source. Maximum output is 300 mW into 32 ohms, with rated output of 90 mW into 32 ohms. Headphone impedance is switchable, low mode for 16 to 120 ohms and high mode for 120 to 300 ohms. Frequency response is 20 Hz to 20 kHz within 1 dB, THD is 0.5% or less at the headphone output and 0.26% at the RCA output, and signal-to-noise and dynamic range are both at least 97 dB. Inputs are RCA, outputs are 6.35 mm and 3.5 mm headphone jacks and an RCA preamp output. Power comes from a 12 V/3 A adapter. The amplifier measures 129 x 46 x 192 mm and weighs 0.9 kg.

Engineering and use context

The 5654W is a military-grade pentode commonly used as a voltage amplifier, and the second pair of valves provides the output current, forming a two-stage all-valve signal path. In an output-transformerless design, the valves drive the headphones directly without a transformer, which works best with higher-impedance headphones. The impedance switch changes the output configuration to suit low-impedance models and in-ear monitors, or high-impedance full-size headphones. The bass and treble controls have centre detents so the flat setting can be found easily. A large damped potentiometer on top sets the volume. The chassis is CNC-machined aluminium with a sandblasted finish, and ventilation holes on both sides and a perforated top panel release the valves' heat. Tube protection covers come standard, and both pairs of valves are supplied with the amplifier. The RCA preamp output delivers 1.2 V. Used as a preamplifier, the GR70 places its valve stage between a DAC and a pair of powered speakers or a power amplifier. A two-year warranty is given. Some units ship in an orange finish.

Pairing

The GR70 suits in-ear monitors in low mode and full-size headphones in high mode. Its RCA output feeds powered monitors or a Fosi power amplifier on a desk.

Alternatives

Fosi Audio's K5 Pro is the brand's solid-state DAC and headphone amplifier.