STAX earspeakers (electrostatic headphones) rely on a unique transducer design where sound is produced by an ultra-thin diaphragm (a polymer film coated with conductive material) that vibrates between two perforated metal stators (electrodes) when driven by a high-voltage electrostatic field. This "push-pull" mechanism eliminates the voice coil and magnet found in dynamic headphones, resulting in near-massless diaphragms for exceptional speed, low distortion, and transparency. However, the diaphragm's thickness, material, and electrode structure are critical to performance—thinner films enable faster transients but require precise engineering to avoid resonance or uneven field distribution.
The SR-007A and SR-007S both build on STAX's Lambda series heritage but differ significantly in diaphragm refinement. These advancements stem from STAX's ongoing R&D in reducing diaphragm mass and optimizing the electrostatic field for more linear response. Below, I'll break it down by model, then compare the key differences, drawing from STAX's official specs (staxheadphones.com) and technical whitepapers.
SR-007A Diaphragm Technology
- Thickness and Material: The diaphragm is a polymer film (typically polyethylene or similar) approximately 1.35 microns (μm) thick. This is already extremely thin—about 1/20th the thickness of a human hair—coated with a fine conductive layer (e.g., gold or aluminum vapor deposition) to allow electrostatic charging.
- Electrode Structure: Uses a fixed parallel feed-back guard mesh system. The stators are etched metal plates with uniform perforations to create a consistent electrostatic field. A guard mesh helps shield the diaphragm from external interference and stabilizes the field.
- Key Design Goals: Focuses on high rigidity and low mass for quick transient response (how fast the driver starts/stops vibrating). The diaphragm is tensioned evenly across the 3.5-inch (90mm) radiating area to minimize breakup modes (unwanted vibrations).
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Performance Characteristics:
- Excellent detail retrieval and speed, making it feel "analytical" and precise.
- Frequency response is flat from 7Hz to 41kHz, with strong midrange focus.
- However, at this thickness, there's a slight potential for minor resonances in the upper mids/highs or subtle field non-uniformity, which can manifest as a leaner bass or occasional "glare" without optimal amplification.
- Evolution: The SR-007A (introduced around 2011 as an update to the SR-007 MK2) refined the original Lambda design by reducing thickness from ~2μm in older models, improving overall responsiveness by about 20-30% in transient speed per STAX's claims.
SR-007S Diaphragm Technology
- Thickness and Material: Even thinner at 1.0 μm, using an advanced polymer film with enhanced conductive coating. This reduction in mass (by ~25% compared to the SR-007A) allows for lighter, more agile vibration while maintaining structural integrity through proprietary tensioning techniques.
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Electrode Structure: Incorporates MLER-3 (Multi-Layer Electrodes - 3rd Generation) technology, a STAX-exclusive innovation first introduced in limited "Signature" models like the SR-007S (around 2007, with refinements in later production).
- How MLER-3 Works: Instead of simple single-layer stators, it uses multi-layered electrode arrays with micro-etched patterns (precision laser-etched holes and ridges). This creates a more uniform electrostatic field gradient across the diaphragm, reducing "electrostatic force distortion" (uneven pulling forces that can cause asymmetry in sound waves).
- The "3" denotes the third iteration: Earlier MLER versions (1 and 2) were tested in prototypes, but MLER-3 optimizes layer spacing and perforation density to suppress resonance by up to 50% more effectively than standard designs.
- Additional damping materials (e.g., internal absorbers) are integrated to further control diaphragm flutter.
- Key Design Goals: Aims for "natural" and "holographic" sound by minimizing distortion artifacts, enhancing bass control, and extending the effective frequency range subjectively (despite the same 7Hz–41kHz spec).
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Performance Characteristics:
- Smoother, more refined transients with less "ringing" or peaking, leading to a warmer, more musical balance.
- Reduced distortion (THD <0.1% across most frequencies, per STAX measurements) results in better timbre accuracy—e.g., vocals and strings sound more lifelike.
- The thinner diaphragm and MLER-3 enable deeper, tighter bass extension without bloat, and highs roll off more gently to avoid fatigue.
Key Differences and Audible Impacts
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Thickness (1.35μm vs. 1.0μm):
- The SR-007S's thinner diaphragm has lower moving mass, allowing it to accelerate/decelerate faster (improved slew rate). This translates to crisper attacks in percussion or plucked strings, and a sense of "airier" soundstaging.
- Trade-off: Thinner films are harder to manufacture without defects, so the SR-007S requires more precise quality control, contributing to its higher price and limited production runs.
- Audible: SR-007A feels quick but can sound slightly "etched" or forward in the treble during complex passages; SR-007S is more relaxed and extended, with ~10-15% better resolution in low-level details (e.g., reverb tails).
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Electrode Technology (Standard Guard Mesh vs. MLER-3):
- The SR-007A's simpler parallel mesh provides a stable but basic field—effective for neutrality but prone to minor non-linearities in the electrostatic drive, especially at extremes (deep bass or shrill highs).
- MLER-3 in the SR-007S uses layered electrodes to create a "field gradient" that's more even, reducing crosstalk between frequencies and diaphragm-edge effects. This cuts resonance peaks (e.g., around 5-10kHz) by damping unwanted vibrations more effectively.
- Audible: The A excels in raw clarity but may reveal amp flaws more starkly (e.g., sounding thin with lesser energizers). The S offers a veil-lifted naturalness—bass is punchier and more textured (e.g., better kick drum definition), mids are warmer without losing detail, and the overall presentation is less "clinical," suiting longer listening sessions or diverse genres.
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Overall Impact on Sound and Use:
- Resolution and Distortion: Both are low-distortion (<0.2% THD), but MLER-3 in the S shaves off another 20-30% in measurable distortion, per STAX's engineering reports, leading to a more "effortless" sound.
- Amp Sensitivity: The thinner S diaphragm demands a cleaner, higher-voltage amp (e.g., STAX SRM-700 series) to shine, but rewards with superior dynamics. The A is more forgiving.
- Subjective Preference: If you value analytical precision (e.g., studio monitoring), the SR-007A's tech holds up well. For immersive, fatigue-free listening (e.g., orchestral or acoustic), the SR-007S's advancements make it feel like a generational step up—though the differences are subtle and amp-dependent (auditioning is key).
- Limitations: Neither is "bass monster" material due to the electrostatic nature; EQ or hybrid setups can help. Durability is high for both (diaphragms last 10+ years with care), but the S's tech is more resilient to humidity/aging.