STAX - SRM-400S - Drive Unit Review: In the world of electrostatic headphone amplification, the SRM-400S stands out as a game-changer. As the successor to the acclaimed SRM-353X, this semiconductor driver unit inherits the sonic virtues of STAX’s high-end SRM-700S while introducing targeted innovations that elevate detail, dynamics and musicality.

Technical Innovations and Circuit Topology

The SRM-400S employs a full FET configuration with custom-made low-noise dual FETs on its first stage and precision-matched FETs on the output stage. This meticulous component selection mirrors the design philosophy of the flagship SRM-700S, delivering:

• DC amplification without coupling capacitors for an uncoloured signal path.
• Extended frequency response from DC to 90 kHz, preserving every harmonic nuance.
• A beefed-up power transformer with dedicated low-voltage winding for improved efficiency and reduced distortion.

Unrivalled Sound Signature

From the deepest bass to the most ethereal air, the SRM-400S achieves a harmonised tonal balance:

• Bass Control: Tight, articulate low end with zero smear, thanks to the 580 V bias and 400 Vrms maximum output capability.
• Midrange Clarity: Voices and instruments emerge with lifelike presence, underpinned by ultra-low harmonic distortion of just 0.01%.
• Treble Extension: Sparkling detail without glare, courtesy of the amplifier’s DC-coupled topology and extended bandwidth.

Build, Connectivity and Ergonomics

Housed in a rigid hybrid chassis utilising non-magnetic aluminium, the SRM-400S is engineered to suppress environmental vibrations. Inputs include both RCA and XLR (mutually exclusive), and a concentric dual-knob volume control retains the iconic STAX aesthetic. An RCA parallel output further enhances system flexibility.

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Who Should Consider the SRM-400S?

This driver unit bridges the gap between entry-level electrostatic amplification (e.g. SRS-3100) and the elite SRM-700S. It’s ideal for:

• Auditioning high-resolution recordings on STAX Lambda or SR and Pro series headphones.
• Audiophiles seeking a low-noise, high-efficiency amp with minimal heat output (30 W consumption).
• Home studio enthusiasts requiring pinpoint accuracy for critical listening and mixing.

Pros & Cons

• Pros:
  • Exceptional detail retrieval and dynamic range.
  • Low noise floor with 0.01% THD at 1 kHz/100 Vrms.
  • Efficient power transformer design for improved performance.
  • All-DC circuit path ensures purity of signal.
  • Solid, non-magnetic aluminium chassis minimises vibration.
• Cons:
  • Single input per type (RCA or XLR) may require switching for multi-source setups.
  • Bias voltage and high-voltage configuration demand careful headphone matching.

Why the SRM-400S Will Transform Your Listening Experience

If you’re chasing electrostatic purity without stepping into flagship pricing, the SRM-400S is a compelling proposition. Its blend of semiconductor finesse, DC-coupled circuitry and refined power architecture delivers a soundstage that’s expansive, transparent and utterly involving.

Ready to explore the next level of electrostatic listening? Speak with our specialists at The Audio Tailor today—our experts Gabby, Nathan, Vaughan and Isaac are on hand to tailor the perfect rig for your space and budget. Discover the SRM-400S Drive Unit or call us on 1300 308 711 / email sales@theaudiotailor.com.au.