Усилители для наушников — Stax SRM-270

The SRM-270 corresponds to the SRM-270S, a compact driver unit made in Japan by the electrostatic headphone specialist Stax. Stax earspeakers do not connect to a normal headphone jack - they need a dedicated driver unit that supplies both the diaphragm bias and a high-voltage drive signal, and this box does that job. It replaced the SRM-252S as the entry-level driver in the range and is also sold as half of the SRS-X1000 system, where it is paired with the SR-X1 earspeaker and takes over from the long-running SRS-3100 set. The US price of the driver alone is USD 445.

Key specs that matter

The response extends from DC to 35,000 Hz within +0 and -3 dB, so the amplifier is direct-coupled with no low-frequency roll-off. Amplification is 58 dB, a factor of 800, and harmonic distortion is 0.01% or less at 1 kHz with an output of 100 V rms. Maximum output reaches 280 V rms at 1 kHz, and the bias supply for the earspeaker diaphragms is 580 V DC. Input impedance is 25 kohm. The case measures 132 mm wide, 38 mm high and 153 mm deep excluding protrusions, and the unit weighs 540 g. Power comes from an external AC adaptor rated at 12 V DC and 500 mA.

Engineering and use context

Compared with the SRM-252S, Stax fitted better quality RCA input jacks, a 3 mm thick aluminium front panel and a larger extruded aluminium case. The purpose of the bigger case is better heat dissipation from the circuitry inside. The first amplifying stage uses a current low-noise FET, and the output stage is an improved emitter-follower circuit. The front panel carries a single level knob and nothing else, and the unit has no source selector and no balanced input. Stax describes the compact chassis as easy to place and portable, and in the US the unit carries a limited manufacturer warranty of one year.

Pairing

A single pair of RCA inputs accepts a line-level signal from a DAC, a CD player or the fixed or variable output of a preamplifier, and a rated input of 125 mV is enough for the reference output level. Parallel output terminals pass the incoming signal on, so the driver can be inserted into an existing hi-fi chain without taking over its only line output. The earspeaker socket follows the Stax Pro-bias standard, and owners run models such as the SR-L300 and SR-L500 MK2 from it as well as the SR-X1 supplied in the system set.

Alternatives

A step up in the Stax range is the SRM-400S, a solid-state driver introduced in 2021 with FETs in its first and last stages, balanced XLR and RCA inputs and 400 V rms output.