Mega Rare Legendary Stax SR-Lambda Professional electrostatic earspeakers with the Stax SRD-X Amplifier the original Mk1 pristine condition

The STAX SR-Lambda Professional (commonly called the Lambda Pro), released in 1982, is a legendary vintage open-back electrostatic "earspeaker". It holds a significant place in audiophile history as the first STAX model to introduce the 580V "Pro" bias voltage, which became the industry standard for all modern STAX headphones and amplifiers.

The Practical Holy Grail: STAX SR-Lambda Pro (1982)

The Lambda Pro is the "Grail" you can actually own and enjoy daily. It stripped away all acoustic resonances and time-smearing, making it a pure, open-window tool for music reproduction. For listeners who value raw speed, zero distortion, and clinical detail retrieval above all else, it remains the ultimate benchmark. [1, 2]

. STAX SRD-X (Original - 1979): This base model was built before the Pro standard existed. It features a single 6-pin "Normal Bias" socket operating at 230V. It will not natively power your Lambda Pro. [1, 2, 3]

2. STAX SRD-X Professional (1986): This is the version you need. It features two sockets on the front panel: one 6-pin Normal Bias (260V) socket and one 5-pin Pro Bias (580V) socket. It is fully compatible with the Lambda Pro. [1, 2, 3]

Features & Quirks of the SRD-X Professional

Dual Power Sources: It can be powered by standard AC wall power via an adapter or via 8 AA batteries tucked into a internal tray, allowing you to carry an electrostatic setup on the go. [1]

The Deadly Polarity Trap: If you are buying a used unit that lacks the original wall adapter, be incredibly careful. The DC input jack on the SRD-X uses a center-negative (minus center pole) configuration. Plugging a modern, standard center-positive power supply into it can instantly fry the internal circuitry. [1]

Sound Performance: While it drives the Lambda Pro remarkably well for its size, audiophiles on forums like Head-Fi note that because it relies on step-up transformer technology rather than pure direct-drive high-voltage amplification (like the SRM-1/MK2), it lacks a bit of the absolute bass control and expansive staging found on desktop STAX amplifiers. [1]




Known for its incredibly fast transient response and ethereal transparency, it delivers a highly detailed sound that many enthusiasts consider superior to traditional dynamic or planar magnetic headphones. [1, 2, 3]

Key Specifications

Because it is an electrostatic headphone, its specifications differ drastically from conventional gear: [1]

Transducer Principle: Electrostatic, push-pull

Bias Voltage: 580 VDC (Pro Bias)

Frequency Response: 8 Hz – 35 kHz

Impedance: Measured in kilo-Ohms (requires a dedicated electrostatic amplifier)

Sensitivity: 108 dB @ 1 mW input

Weight: 15.1 oz (approx. 428g) [1, 2]

Sound Profile & Performance

Speed and Transparency: According to reviews on platforms like Headphonesty, the Lambda Pro is exceptionally fast, allowing it to retrieve micro-details



The STAX SR-Lambda Pro earned its "legendary" status by completely redefining what audiophiles expected from headphone resolution and clarity. When it debuted in 1982, it shattered performance limits and set a technical benchmark that manufacturers are still trying to match today.

Here is exactly why the audio community considers the Lambda Pro a true legend:

1. It Created the modern STAX Standard

Before this headphone, electrostatic models used a lower 230V "Normal Bias." The Lambda Pro introduced the 580V "Pro Bias" system, which utilized a wider gap between the stators and the diaphragm. This allowed for greater linear movement, higher volume capabilities, and drastically lower distortion. Virtually every electrostatic headphone made by STAX today still relies on the 580V standard born with the Lambda Pro.

2. Speed that Rivals $5,000+ Modern Headphones

The driver's diaphragm is only a few microns thick—lighter than the air it moves. Because it has almost zero mass, it responds instantly to electrical signals. Owners frequently note that even when compared to modern high-end flagships costing thousands of dollars, the Lambda Pro still matches or beats them in pure speed, transient response, and the ability to separate complex instruments in a track.

3. The Pro Audio Breakthrough

While earlier STAX models were loved by hobbyists, the Lambda Pro was widely adopted by professional recording and mastering studios in the 1980s. Engineers used it as a ruthless tool for evaluating mixes because it refused to hide flaws, hiss, or distortion in a recording.