Infinity QLS-4: The Electrostatic Pioneer of the Late 1970s
The QLS-4 is a hybrid electrostatic loudspeaker from Infinity Systems, from the late 1970s. It pairs an electrostatic panel with dynamic woofers for the bass. The name stands for "Quasi-Logarithmic Source No. 4." Later models like the IRS series got more attention, but the QLS-4 remains a cult favorite among vintage audio enthusiasts.
A Glimpse into History
Infinity launched the QLS-4 in 1978. Infinity was founded by Arnie Nudell and Cary Christie, and at the time most high-end speakers used dynamic cone drivers only. Infinity was working with advanced materials and new transducer designs. The QLS-4 came out of a lineage that included the Servo-Statik series and led toward the IRS (Infinity Reference Standard) models.
The hybrid layout is what sets it apart. A large electrostatic panel handles the midrange and treble, and a bass module with conventional dynamic woofers handles the low end. The idea was to get the speed, detail and low distortion of electrostatics while fixing their usual weakness - limited bass extension and output.
The QLS-4 was not a mass-market product. It was hand-built, complex and expensive, aimed at serious audiophiles who put sonic accuracy first. It is tall and narrow, with a translucent or white fabric-covered panel.
Technical Specifications
| Specification | Detail |
| Type | Hybrid electrostatic / dynamic speaker |
| Frequency Response | 35 Hz – 20 kHz (±3 dB) |
| Crossover Frequency | ~300 Hz (electrostatic to bass module) |
| Electrostatic Panel | 6' x 1.5' curved stator design |
| Bass Module | Dual 12" long-throw woofers (sealed) |
| Sensitivity | 86 dB (2.83V @ 1m) |
| Nominal Impedance | 4 ohms (electrostatic section) |
| Recommended Amplifier | 50–200 watts, high-current capable |
| Crossover Type | Passive, internal |
| Dimensions (H×W×D) | 78" × 18" × 20" (with base) |
| Weight | 120 lbs per speaker |
| Inputs | Dual binding posts (bi-wire capable) |
The electrostatic panel used a curved stator design, which Infinity claimed improved dispersion and reduced edge diffraction. Unlike flat panels, the gentle curve helped maintain a coherent wavefront across a wider listening area. The panel was driven by a high-voltage signal derived from the audio input, requiring no external power supply—a hallmark of true electrostatics.
The bass section housed two 12-inch woofers in a sealed (acoustic suspension) enclosure, tuned for tight, accurate low-end response. The crossover network was passive but sophisticated, designed to seamlessly blend the dynamic bass with the electrostatic upper registers.
Sound Characteristics and Performance
Listeners often describe the midrange as lifelike, transparent, and free of coloration, which is what electrostatics are known for. Vocals are rendered with uncanny realism, as if the singer were in the room. The treble is smooth and extended, without the harshness that can plague dynamic tweeters.
The bass is not as deep or powerful as a modern subwoofer, but it is tight and well integrated. The crossover point is around 300 Hz (~300 Hz). With a proper setup the transition is generally smooth, though some early reviewers noted a slight discontinuity in certain rooms or with mismatched amplification.
Imaging and soundstage are exceptional. The QLS-4 produces a wide, deep, and precisely layered soundstage, with pinpoint instrument placement. Its dipole radiation pattern gives a spacious, open sound that many find more natural than conventional box speakers.
The QLS-4 is demanding. It needs a large room, ideally with at least 8 feet of clearance from the rear wall so the electrostatic panel can work efficiently. It also needs high-quality, high-current amplification to drive the 4-ohm load and control the bass modules.
Notable Features and Innovations
- Hybrid Electrostatic Design: One of the first commercially available hybrids to combine electrostatic clarity with dynamic bass.
- Curved Stator Panel: Improved dispersion and reduced diffraction for more consistent off-axis response.
- Modular Construction: The bass module could be separated from the electrostatic panel, allowing for flexible placement (though rarely used in practice).
- No External Power Supply: Unlike some electrostatics (e.g., MartinLogan), the QLS-4’s panel is driven directly by the amplifier, simplifying setup.
- Bi-Wire Ready: Allowed for improved signal separation between bass and upper frequencies.
Common Issues and Maintenance
Owning a pair of QLS-4s today takes work. These speakers are rare and aging, and several issues commonly come up:
- Panel Degradation: Over time, the electrostatic diaphragm can become brittle or develop weak spots, leading to arcing or reduced output. Re-venting or re-lacing the panel may be necessary.
- Capacitor Aging: The internal crossover capacitors, especially in the high-voltage section, may dry out or fail, requiring replacement with modern equivalents.
- Bass Driver Wear: The 12" woofers may suffer from surround deterioration or voice coil misalignment after decades of use.
- Fragility: The tall, narrow design makes the speakers top-heavy and prone to tipping if not properly secured.
Restoration is possible but takes specialized knowledge. A few vintage audio technicians and DIY communities know how to service these speakers. Replacement parts are scarce, but can sometimes be found through forums or salvage.
Current Market Value and Collectibility
The Infinity QLS-4 is highly collectible. Complete, working pairs in good condition can fetch $2,000 to $4,000, depending on restoration quality and provenance. Non-working or project pairs may sell for under $1,000.
Part of the value is historical significance. The QLS-4 sits between the experimental electrostatics of the 1970s and the refined hybrids of the 1980s and 1990s.
Conclusion: A Legacy of Innovation
It may not have matched the commercial success of more conventional speakers. It did show that electrostatic clarity could be paired with practical bass performance, and later hybrids from Infinity and others followed.
For the modern audiophile, the QLS-4 offers more than just sound—it offers a connection to a pivotal moment in audio history. With proper care and the right system, it can still deliver a listening experience that feels startlingly modern: transparent, immersive, and deeply emotional. In the world of vintage audio, the QLS-4 isn’t just a speaker. It’s a statement.
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