OLLO Audio S4X Reference Open-Back Studio Headphones

The OLLO Audio S4X are professional open-back reference headphones developed specifically for mixing, mastering and critical studio monitoring. Their acoustic design focuses on a neutral, controlled presentation intended to help engineers make reliable production decisions without excessive tonal coloration.

The S4X use large 50 mm dynamic drivers that are individually measured and selected in matched pairs. OLLO evaluates drivers in laboratory batches and analyzes their frequency-domain performance before pairing them, helping maintain consistent left-to-right response and accurate stereo imaging.

Version 1.3 incorporates OLLO’s USC calibration, developed to further refine the frequency response for professional mixing and mastering. The objective is a balanced monitoring reference that allows engineers to assess tonal relationships, dynamics and spatial information with greater consistency.

The acoustic architecture separates the front and rear sections of the driver through a carefully controlled chamber design. This approach helps achieve a smoother frequency response while improving definition between low and mid frequencies — an important factor when balancing bass instruments, vocals and other central elements of a mix.

With a frequency response of 20 Hz to 20 kHz, the S4X concentrate on controlled reproduction throughout the conventional audible spectrum. Their open-back construction minimizes internal acoustic reflections and contributes to a spacious presentation with natural stereo separation and depth.

The headphones have an impedance of 32 ohms and a characteristic sound-pressure level of 108 dB. This relatively low impedance provides broad compatibility with professional audio interfaces, headphone amplifiers and suitably capable portable sources, making the S4X practical across different production environments.

Signal transmission is handled by a detachable dual-entry cable with an OFC conductor. Its silicone outer layer is designed to reduce mechanical cable noise and microphonics, while the replaceable architecture simplifies maintenance and long-term use.

Comfort and acoustic behaviour are supported by hybrid velour and synthetic-leather ear pads. The combination of materials helps control airflow around the driver while providing a soft contact surface for extended mixing and mastering sessions.

A self-adjusting stainless-steel headband provides structural durability, while components are designed with serviceability in mind. Parts can be replaced using conventional tools, extending the practical working life of the headphones.

The construction combines stainless steel, aluminium and CNC-machined American walnut, avoiding an extensively plastic chassis. The wooden earcups are precision-machined to provide structural consistency while giving the S4X their distinctive professional appearance.

Combining matched 50 mm drivers, USC calibration, open-back acoustics and a serviceable construction, the OLLO Audio S4X are designed as a dedicated reference monitoring tool for engineers and producers working with mixing, mastering and detailed audio evaluation.

Specifications

• Brand: OLLO Audio
• Model: S4X
• Type: professional reference studio headphones
• Acoustic design: open-back
• Form factor: over-ear
• Driver type: dynamic
• Driver diameter: 50 mm
• Driver matching: laboratory-selected matched pairs
• Calibration: USC, version 1.3
• Frequency response: 20 Hz – 20 kHz
• Impedance: 32 Ω
• Characteristic sound pressure level: 108 dB
• Cable system: detachable, dual-entry
• Cable conductor: OFC
• Connection: 3.5 mm stereo mini-jack
• Ear pads: hybrid velour and synthetic leather
• Headband: self-adjusting stainless steel
• Main materials: American walnut, stainless steel and aluminium
• Wooden components: CNC-machined
• Serviceable construction: yes
• Primary applications: mixing, mastering and critical monitoring