STAX SRM-500T – hybrid tube driver for electrostatic headphones

The STAX SRM-500T is a hybrid driver unit for electrostatic headphones, introduced as the replacement for the SRM-006tS. It combines a low-noise FET input stage with a vacuum-tube output section, bringing two established amplification approaches together in a single purpose-built component. The SRM-500T is aimed at discerning listeners who want to build a dedicated STAX headphone system around a DAC, streamer, disc player or analogue source. Within the STAX range, it offers an accessible route to a tube-output driver while retaining practical source connectivity, substantial voltage capability and a compact format suitable for a desktop, equipment rack or permanent listening area.

Custom-made low-noise dual FET devices handle the input stage, while a pair of 6FQ7/6CG7 vacuum tubes is used for the output. This hybrid DC amplification design provides 60 dB of gain, a maximum output voltage of 300 Vrms at 1 kHz and a 580-volt DC bias supply for compatible electrostatic headphones. With one SR-L500 MK2 connected, the specified frequency response extends from DC to 90 kHz. Harmonic distortion is rated at 0.01% or less when measured at 1 kHz with a 100 Vrms output. Source components can be connected through either the RCA input or the XLR input, although the two inputs are mutually exclusive. A parallel RCA output is also included for passing the line-level signal to another component.

STAX equips the SRM-500T with a newly designed power transformer intended to improve power efficiency and support better overall performance than the conventional tube-output driver circuit used in the SRM-006tS. The amplifier consumes 38 watts and measures 195 mm wide, 102 mm high and 376 mm deep, including its maximum protruding sections. Its 3.4 kg chassis should be placed on a stable surface with sufficient ventilation around the tube section. For system matching, use a source with a controlled line-level output and select either RCA or XLR according to the available connection. Keeping the ventilation openings and connectors free from dust will help maintain dependable operation. Consider the STAX SRM-500T when completing or upgrading a dedicated electrostatic headphone setup.