Infinity QLS-2: The Quasi-Electrostatic Revolution in Vintage Audio

Name: Infinity QLS-2
Brand: Infinity

In the mid-1970s, when most high-fidelity loudspeakers relied on conventional dynamic drivers, Infinity Systems (later known as Infinity Audio) made a bold departure from the norm with the introduction of the QLS-2—a hybrid loudspeaker that blurred the lines between electrostatic precision and dynamic driver practicality. Released in 1976, the QLS-2 was part of Infinity’s innovative push into the high-end audio market, establishing the company as a pioneer in advanced transducer technology. Though not a full electrostatic like the famed Quad ESL-57 or KLH Model Nine, the QLS-2 employed a quasi-electrostatic design that delivered many of the sonic benefits of electrostatics while overcoming key limitations such as low output and narrow dispersion.

The QLS-2 stood as a technological marvel of its time, combining a large planar midrange section with a high-frequency ribbon tweeter and a conventional dynamic woofer. This hybrid approach allowed Infinity to deliver a speaker with exceptional clarity, speed, and transparency—hallmarks of electrostatic design—while maintaining the bass extension and efficiency needed for real-world listening environments.

Technical Specifications

Specification	Detail
Type	3-way hybrid quasi-electrostatic
Driver Configuration	Planar midrange, ribbon tweeter, 12" dynamic woofer
Frequency Response	35 Hz – 20 kHz
Sensitivity	86 dB (1W/1m)
Nominal Impedance	8 ohms (compatible with most amplifiers)
Crossover Frequencies	500 Hz (woofer/midrange), 4 kHz (midrange/tweeter)
Enclosure Type	Transmission line with rear-firing port
Dimensions (H×W×D)	48" × 14" × 20" (122 cm × 35.5 cm × 51 cm)
Weight	78 lbs (35.4 kg) per cabinet
Recommended Amplifier	50–200 watts, high-current preferred

Sound Characteristics and Performance

The Infinity QLS-2 was engineered to deliver a transparent, detailed, and highly dynamic sound signature that stood apart from traditional cone-driver speakers of the era. Its planar midrange section—constructed using a thin, conductive diaphragm suspended in an electrostatic field—offered remarkably low mass and near-instantaneous transient response. This translated into a midrange that was exceptionally clear, with pinpoint imaging and a lifelike presence that audiophiles of the time described as “seeing through a window into the recording.”

The high-frequency reproduction was handled by a pure ribbon tweeter, a rarity in consumer speakers at the time. Unlike dome tweeters, which can suffer from breakup modes and time-smearing, the ribbon design offered ultra-fast response and a smooth, extended top end. The result was sparkling highs without harshness, making cymbals, strings, and vocal sibilants sound natural and uncolored.

Bass duties were assigned to a 12-inch dynamic woofer mounted in a long transmission line enclosure, which helped extend low-frequency response down to 35 Hz. While not as fast or articulate as the upper drivers, the woofer was well-integrated and provided solid, room-filling bass—especially when paired with high-current amplification. The transmission line design also reduced cabinet resonance, contributing to the speaker’s overall clarity.

One of the QLS-2’s most praised attributes was its soundstage and imaging precision. The vertical alignment of the planar midrange and ribbon tweeter created a coherent wavefront, resulting in a tall, stable soundstage with excellent depth and layering. Listeners often reported that instruments appeared to float in space with uncanny realism.

Notable Features and Innovations

The QLS-2 was a showcase of forward-thinking engineering:

Quasi-Electrostatic Midrange: Unlike full electrostatics that require high-voltage power supplies and special amplifiers, the QLS-2’s planar driver operated within standard amplifier parameters. This “quasi” approach made it far more user-friendly while preserving much of the electrostatic magic.

Ribbon High-Frequency Driver: One of the first consumer speakers to use a true ribbon tweeter, the QLS-2 set a new benchmark for high-frequency clarity and speed.

Transmission Line Cabinet: The complex internal labyrinth helped absorb rear wave energy, reducing distortion and enhancing bass control without the boominess often associated with ported designs.

Modular Design: The drivers and crossover were designed for serviceability—a rarity in high-end speakers of the era. This made repairs and upgrades more feasible, contributing to the speaker’s longevity.

Aesthetic Simplicity: Housed in a tall, narrow cabinet with a minimalist black grille, the QLS-2 had a modern, understated look that complemented its advanced technology.

Common Issues and Maintenance

While the QLS-2 was well-built, its advanced design comes with some age-related concerns:

Planar Diaphragm Degradation: Over decades, the thin mylar diaphragm in the midrange section can become brittle or develop static charges, leading to distortion or crackling sounds. Re-foaming or re-tensioning may be necessary.

Ribbon Tweeter Fragility: The delicate aluminum ribbon can be damaged by overdriving or physical contact. Repairs require specialized skills and materials.

Crossover Component Aging: Electrolytic capacitors and resistors in the passive crossover can degrade, affecting tonal balance. A full recapping by a qualified technician is often recommended for vintage units.

Transmission Line Damping Material: The internal damping fibers can settle or disintegrate over time, reducing bass performance. Re-stuffing the line may be needed for optimal response.

Owners are advised to use high-quality, current-capable amplifiers and avoid clipping, as the QLS-2’s low sensitivity and complex load can stress underpowered gear.

Current Market Value and Collectibility

Today, the Infinity QLS-2 is a sought-after collector’s item among vintage audio enthusiasts, particularly those interested in the evolution of planar and electrostatic technologies. Well-maintained pairs in good working condition typically sell in the $800–$1,500 range, depending on cosmetic condition, originality, and sonic performance.

While not as widely recognized as the later Infinity IRS or the original EM/ES series, the QLS-2 holds a special place as a transitional masterpiece—a speaker that brought high-end electrostatic concepts into the realm of practical home audio. Its rarity, innovative design, and superb midrange performance make it a rewarding find for the discerning collector.

Conclusion and Legacy

The Infinity QLS-2 was more than just a speaker—it was a statement of intent. At a time when high fidelity was dominated by incremental improvements in cone drivers and boxy cabinets, Infinity dared to explore new frontiers in transducer design. The QLS-2’s hybrid approach—marrying electrostatic-like clarity with dynamic bass—paved the way for future innovations, including the legendary IRS V and the later Genesis line developed by co-founder Bruce Thigpen.

Though overshadowed by its more famous siblings, the QLS-2 remains a hidden gem of 1970s audio engineering. For those who appreciate the marriage of scientific rigor and sonic beauty, the QLS-2 offers a listening experience that is still compelling today. Its transparent midrange, airy highs, and coherent imaging remind us that true innovation in audio isn’t always about loudness or size—it’s about revealing the soul of the music.