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

In the mid-1970s most high-fidelity loudspeakers used conventional dynamic drivers. Infinity Systems (later known as Infinity Audio) went a different way with the QLS-2, a hybrid that sat between electrostatic precision and dynamic driver practicality. It came out in 1976 as part of Infinity's push into the high-end audio market. It wasn't a full electrostatic like the Quad ESL-57 or KLH Model Nine. The QLS-2 used a quasi-electrostatic design that delivered many of the sonic benefits of electrostatics while avoiding two key limitations, low output and narrow dispersion.

The QLS-2 combined a large planar midrange section, a high-frequency ribbon tweeter and a conventional dynamic woofer. Infinity got clarity, speed and transparency from the electrostatic side of the design. The woofer supplied the bass extension and efficiency needed for real-world listening rooms.

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 for a transparent, detailed and highly dynamic sound that stood apart from the cone-driver speakers of its era. The planar midrange used a thin, conductive diaphragm suspended in an electrostatic field. That gave it very low mass and near-instantaneous transient response. The midrange was clear, with pinpoint imaging, and audiophiles of the time described it as "seeing through a window into the recording."

The high frequencies came from a pure ribbon tweeter, a rarity in consumer speakers at the time. Dome tweeters can suffer from breakup modes and time-smearing. The ribbon design offered ultra-fast response and a smooth, extended top end. Highs were sparkling without harshness, and cymbals, strings and vocal sibilants sounded natural and uncolored.

Bass came from a 12-inch dynamic woofer in a long transmission line enclosure, which helped extend low-frequency response down to 35 Hz. The woofer wasn't as fast or articulate as the upper drivers, but it was well integrated and gave solid, room-filling bass, especially with high-current amplification. The transmission line design also reduced cabinet resonance, which helped the overall clarity.

One of the QLS-2's most praised attributes was its soundstage and imaging precision. The planar midrange and ribbon tweeter were aligned vertically, which created a coherent wavefront. The result was a tall, stable soundstage with excellent depth and layering. Listeners often reported that instruments appeared to float in space.

Notable Features and Innovations

The QLS-2 packed in a lot 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

The QLS-2 was well built, but its design brings 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.

Use high-quality, current-capable amplifiers and avoid clipping. 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 development 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.

The QLS-2 is not as widely recognized as the later Infinity IRS or the original EM/ES series. It was a transitional design that brought high-end electrostatic concepts into practical home audio. Its rarity, design and midrange performance make it a good find for a collector.

Conclusion and Legacy

At a time when high fidelity was dominated by incremental improvements in cone drivers and boxy cabinets, Infinity explored new ground in transducer design. The QLS-2's hybrid approach paired electrostatic-like clarity with dynamic bass. It paved the way for later work, including the IRS V and the 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.