STAX SRM-400S Electrostatic Headphone Amplifier (Wired, Silver)

Hear electrostatic truth with solid‑state speed

If you love the airy transparency of electrostatics but crave greater bass grip, lower noise, and effortless headroom, the STAX SRM‑400S delivers. This all‑FET, DC‑coupled driver unit revives your favorite STAX earspeakers with 400 Vrms of clean output, a newly designed high/low‑voltage power transformer for better efficiency, and a more rigid, non‑magnetic aluminum chassis that resists vibration. It’s the solid‑state STAX sound—fast, finely resolved, and beautifully balanced—refined for today’s listening rooms and source gear.

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Why Audiophiles Choose This Product

• Bold, resolving sound
  • High‑resolution “semiconductor” presentation with tighter, more dynamic low end than its predecessor, pursuing an even sound balance from lows through highs.
• Engineering that matters
  • All‑stage FET, complete DC‑amplifier topology with no coupling capacitors in the signal path; custom low‑noise dual FETs selected and matched as in the reference SRM‑700S.
• Built for real‑world systems
  • New dual‑winding transformer adds a dedicated low‑voltage winding for higher efficiency and improved sound; 30 W power draw helps the amp run cool and quiet. The updated, more rigid aluminum chassis further reduces environmental noise and vibration.
• Seamless integration
  • XLR and RCA line inputs (selectable), plus an RCA parallel/thru output for daisy‑chaining to a speaker amp or a second driver unit. Front‑panel concentric dual volume control lets you trim left/right channels independently. PRO‑type earspeaker sockets (5‑pin) support modern STAX “Pro bias” models at 580 V.
• A smarter upgrade path
  • Clear step up from the SRM‑353X it replaces, with trickle‑down parts and manufacturing know‑how from the SRM‑700S. Ideal with advanced Lambda‑series earspeakers or as an upgrade from entry STAX systems.

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What’s Included

• STAX SRM‑400S Electrostatic Headphone Amplifier (Silver)
  • All‑FET, DC‑coupled driver; Pro‑bias output; selectable XLR/RCA inputs
• Cables
  • AC power cable x1
  • Stereo RCA interconnect x1
• Documentation
  • User manual
  • Warranty card

Accessories verified from the official owner’s manual.

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Technical Specifications

Specification	Value	Notes
Product Type	Wired desktop electrostatic headphone amplifier	Analog line‑level inputs (XLR/RCA), STAX Pro‑bias earspeaker sockets
Topology	All‑stage FET, complete DC amplifier	Eliminates coupling capacitors in signal path for transparency
Frequency Response	DC–90 kHz (with one SR‑L500 MK2)	Wideband extension preserves overtones and transient air
Gain	60 dB	Plenty of voltage gain from modern DACs and preamps
Harmonic Distortion	≤0.01% (1 kHz / 100 Vrms)	Low‑distortion headroom for clean crescendos
Maximum Output	400 Vrms (1 kHz)	Drives demanding passages without compression
Bias Voltage	580 V DC (Pro bias)	For modern 5‑pin STAX earspeakers; not for legacy Normal‑bias models
Inputs	1x XLR balanced, 1x RCA unbalanced (selectable)	Use rear selector; do not leave both connected per manual guidance
Line Thru	RCA parallel out	Feed a speaker amp or second driver unit
Input Impedance	50 kΩ (RCA), 50 kΩ ×2 (XLR)	Easy load for most sources
Power Consumption	30 W	Energy‑efficient updated transformer design
Dimensions (W×H×D)	195 × 102 × 376 mm (max, incl. protrusions)	Rack‑friendly footprint
Weight	3400 g	Substantial, vibration‑resistant chassis
Finish	Silver	Matches modern desks and racks

All specifications from the manufacturer.

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Professional Recognition

• Heritage
  • STAX has been “the founder of earspeakers” since 1938 and introduced the first STAX electrostatic headphone, the SR‑1, in 1960—pioneering the category the SRM‑400S serves today.
• Product lineage
  • The SRM‑400S is the official successor to the SRM‑353X and inherits matched low‑noise dual FET devices and manufacturing methods from the high‑end SRM‑700S.
• Industry coverage
  • Taiwanese outlet U‑Headphone characterized the SRM‑400S as a major update with improved efficiency and sound quality, suited to STAX Lambda earspeakers and as a clear upgrade path for entry systems.
• Community feedback
  • Head‑Fi users comparing energizers noted that, in practice, performance was on par with larger solid‑state electrostatic amps they’d owned, underscoring the SRM‑400S’s real‑world capability.

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Compatibility and Setup Notes

• Designed for STAX “Pro‑bias” 5‑pin earspeakers (580 V). Not intended for legacy “Normal‑bias” models.
• XLR and RCA inputs are selectable via the rear switch. For best results, follow the manual’s guidance and avoid leaving both connected simultaneously.
• Manufacturer guidance focuses the SRM‑400S for STAX earspeakers. If you plan to connect non‑STAX electrostatics, consult the headphone maker for bias and pinout conformity.

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The Bottom Line

The STAX SRM‑400S is the sweet spot in the STAX driver lineup: the speed, air, and ultra‑low distortion electrostatics are famous for, plus firmer low‑frequency control and a more even tonal balance than prior mid‑tier models. With trickle‑down FET technology from the SRM‑700S, a more efficient power supply, and thoughtful I/O that slips easily into mixed headphone/speaker systems, it’s a confident, future‑ready amplifier for anyone serious about getting the most from modern STAX earspeakers.

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Research sources: Official STAX International SRM‑400S page and owner’s manual (specifications, features, I/O, accessories); STAX USA history (heritage); U‑Headphone coverage (product update context); retailer listings for consistency checks.