Headphones amplifiers — Stax SRM-500T

The SRM-500T is a hybrid driver unit for electrostatic earspeakers built in Japan by Stax, pairing a FET input stage with a vacuum tube output stage in a direct-coupled amplifier. Released on 22 March 2021, it replaced the SRM-006tS, and Stax called it the first full model change of its tube driver in 11 years. The Japanese list price is JPY 187,000 with tax, while buyers in the US pay USD 1,499.

Key specs that matter

Stax quotes a frequency response from DC to 90 kHz, measured with an SR-L500 MK2 connected. Gain is 60 dB, and harmonic distortion stays at 0.01% or below at 1 kHz with 100 V rms at the output. The unit delivers up to 300 V rms at 1 kHz and supplies a 580 V DC bias to the earspeaker diaphragms. Input impedance is 50 kohm on RCA and 50 kohm per leg on XLR. Power consumption is 38 W. The chassis measures 195 mm wide, 102 mm high and 376 mm deep, and the driver weighs 3.4 kg. Operating conditions are 0 to 35 degrees C at humidity below 90% without condensation.

Engineering and use context

The first stage uses a low-noise dual FET made to order for Stax, and the output stage runs a pair of 6FQ7/6CG7 dual triodes chosen for their high voltage rating. No coupling capacitors sit anywhere in the signal path, so the circuit is a full DC amplifier. A newly designed power transformer carries windings for both the high voltage and low voltage circuits, which Stax says improves efficiency and lowers consumption compared with earlier tube drivers. The chassis is a reinforced hybrid structure of non-magnetic aluminium intended to reduce interference from external physical noise. Level is set with a custom double-axis four-channel volume control, so the left and right channels can be adjusted separately.

Pairing

Inputs comprise a single RCA pair and a single balanced XLR socket, which work on a mutual exclusion basis, so only one source is active at a time. That suits a DAC or preamplifier with either output type. RCA parallel output terminals forward the source signal to a further component, which lets the driver share a line output with a power amplifier or a second headphone amplifier. The earspeaker sockets follow the Pro-bias standard used across the current Stax range, from the Lambda models such as the SR-L500 MK2 used for the frequency response measurement upward.

Alternatives

Higher in the Stax tube range sits the SRM-700T, a hybrid driver with a pair of 6SN7 output tubes, 340 V rms maximum output and a larger 240 mm wide chassis.