Infinity RS-IIA: The Forgotten Pioneer of Passive Radiator Design

In the late 1970s, Infinity Systems was willing to challenge conventional speaker design. One of its lesser-known products was the Infinity RS-IIA, a compact, high-efficiency speaker system that used passive radiator technology to get deep bass from a relatively small enclosure. It was not as famous as the company's flagship IRS series or the mass-market EM series. It was an early Infinity attempt at extended low-frequency reproduction without the drawbacks of ported cabinets.

Released in 1977, the RS-IIA was positioned as a premium bookshelf or desktop monitor for audiophiles who wanted accurate, dynamic sound in smaller rooms. Most compact speakers of the time had thin bass and limited output. Infinity used a passive radiator - a sealed box with a non-powered diaphragm that resonates in response to internal air pressure - and that let the RS-IIA reach bass extension down to around 29 Hz, impressive for its size and era.

The RS-IIA was designed during the tenure of co-founders Arnold Nudell and Cary Christie. It shares the engineering-first approach that would later define the IRS V. Though modest in size, it showed Infinity's commitment to time-aligned drivers, low-distortion motor systems, and advanced cabinet construction.

Technical Specifications

Specification	Detail
Model	RS-IIA
Manufacturer	Infinity Systems, Inc.
Production Years	1977–1981 (approximate)
Type	2-way, passive radiator, bookshelf
Driver Configuration	1 x 8" active woofer, 1 x 1" dome tweeter, 1 x 8" passive radiator
Frequency Response	45 Hz – 20 kHz (±3 dB)
Sensitivity	91 dB (1W/1m)
Nominal Impedance	4 ohms (requires amplifiers stable into lower impedances)
Crossover Frequency	approximately 3 kHz (reported)
Enclosure Type	Sealed with passive radiator
Dimensions (H×W×D)	18" × 10" × 10.5" (45.7 × 25.4 × 26.7 cm)
Weight	28 lbs (12.7 kg) per cabinet
Finish Options	Walnut or black ash veneer

Sound Characteristics and Performance

The Infinity RS-IIA was engineered for clarity, efficiency, and surprising bass depth. Its sensitivity is 91 dB, so lower-powered amplifiers can drive it effectively, including vintage tube gear. That was a major selling point when many audiophiles favored Marantz, McIntosh, and Fisher amplifiers.

The 8-inch woofer has a high-compliance surround and a strong magnet structure, which allows good excursion and low distortion. It is paired with a passive radiator of identical size. The system's bass response rivaled much larger ported designs, without the time-smearing and chuffing noise often associated with ported enclosures. The sealed cabinet gave tight, controlled bass with excellent transient response, which suits jazz, classical, and acoustic recordings.

The dome tweeter is not as refined as later metal-dome designs, but it delivers smooth, non-fatiguing highs. Some early units used soft-dome tweeters with ferrofluid cooling, a relatively advanced feature for the late 1970s. The crossover network was carefully tuned to blend the drivers, and Infinity's attention to phase coherence gave the RS-IIA a stable, well-focused soundstage.

Listeners often describe the RS-IIA's sound as warm yet detailed, with a natural midrange and an engaging presentation. It is not as analytical as some contemporary electrostatics or planars. It does well at musicality and rhythm, and it was a favorite of those who valued emotional engagement over clinical precision.

Notable Features and Innovations

The RS-IIA's most significant innovation was its dual 8-inch configuration: one active driver and one passive radiator. This let Infinity tune the bass resonance precisely without the compromises of a port. The passive radiator acted like a "tuned mass," boosting low-frequency output while keeping the transient speed of a sealed box.

Other notable features included:

High-efficiency design for compatibility with low-wattage amplifiers
Minimalist crossover using high-quality components to reduce signal degradation
Rigid MDF construction (unusual for bookshelf speakers of the era) to reduce cabinet resonance
Flush-mounted drivers to minimize diffraction and improve imaging

It was not marketed as a studio monitor, but the RS-IIA found favor in some broadcast and production environments because of its flat response and reliability.

Common Issues and Maintenance

As with any vintage speaker, the Infinity RS-IIA needs careful inspection before purchase. Common issues include:

Aging foam surrounds: The original foam on the woofer and passive radiator often deteriorates after 40+ years, leading to flaking, loss of compliance, or complete failure. Re-foaming by a qualified technician is highly recommended.
Tweeter degradation: Some early dome tweeters exhibit reduced output or distortion due to aging adhesives or diaphragm fatigue. Replacement or reconing may be necessary.
Crossover component drift: Capacitors and inductors can degrade over time, altering frequency response. Upgrading with modern film capacitors can restore clarity.
Cabinet wear: The wood veneer, while durable, may show scratches or finish lift. Gentle cleaning and waxing can restore luster.

Despite these age-related concerns, the RS-IIA is generally sturdy. Its simple crossover and high-quality original components mean that with proper restoration, it can perform as well today as it did in the 1970s.

Current Market Value and Collectibility

The Infinity RS-IIA is not a mainstream collector's item like the IRS series, but it has a dedicated following among vintage audio enthusiasts. Well-maintained or professionally restored pairs typically sell for $300 - $600 on the secondary market, depending on condition and location.

Factors that increase value:

Original finish with no veneer damage
Fully restored drivers and surrounds
Matching serial numbers
Original packaging or documentation (rare)

It is not a high-value investment piece, but the RS-IIA offers excellent value for the performance-minded listener.

Conclusion and Legacy

The Infinity RS-IIA did not achieve the fame of its larger siblings. In an era dominated by ported bookshelf speakers with boomy, one-note bass, it offered a more refined, articulate alternative.

Its use of passive radiator technology foreshadowed trends that would become widespread decades later, from high-end studio monitors to modern Bluetooth speakers.

For the vintage audio enthusiast, the RS-IIA is more than just a pair of old speakers — it’s a piece of audio history that still has something meaningful to say. With a little care and restoration, it can continue to deliver engaging, lifelike sound for years to come.

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