Infinity RS-1A: The Pioneering Servo-Controlled Loudspeaker System

In the late 1970s, most loudspeakers used passive crossovers and conventional cone drivers. Infinity Systems went another way with the RS-1A, a hybrid active loudspeaker system built around servo-controlled bass. It came out in 1978. It is not as well known as some of Infinity's later models, but it was one of the first commercially available speakers to use servo feedback in the bass driver, for accuracy and low distortion.

Historical Context and Design Philosophy

Infinity was founded in 1968 by Arnie Nudell and Cary Christie. It quickly gained a reputation for electromagnetic ribbon tweeters and servo-driven subwoofers. The RS-1A built on the success of earlier servo-woofers like those in the Servo Statik series. A conventional speaker relies solely on the mechanical properties of the driver and enclosure to control cone movement. The RS-1A used a feedback loop to monitor and correct bass driver motion in real time, which is control theory applied to a loudspeaker.

The RS-1A was marketed as a “reference system,” aimed at serious audiophiles and studio engineers who wanted ultra-low distortion and tight, articulate bass. It was part of a broader trend in the late '70s and early '80s, when high-end brands like Acoustic Research, KLH, and Advent explored active and servo-assisted designs. The RS-1A was a hybrid. The bass was actively servo-controlled, while the midrange and high frequencies were handled by conventional passive drivers.

Technical Specifications

The RS-1A’s most groundbreaking feature was its servo-controlled 12-inch woofer. A sensor (typically a voice coil velocity sensor or optical pickup) monitored the actual movement of the cone and compared it to the input signal. Any deviation—caused by inertia, back-EMF, or enclosure resonance—was corrected by the internal servo amplifier, drastically reducing distortion and improving transient response. This allowed the woofer to achieve deep bass extension with remarkable control, even in moderately sized enclosures.

The midrange and tweeter were fed via a passive crossover, powered by the user’s external amplifier. This hybrid design reduced complexity and cost compared to fully active systems while still delivering the benefits of servo control where it mattered most: the bass.

Sound Characteristics and Performance

The RS-1A had a precise, dynamic sound. The servo woofer produced bass that was exceptionally tight and articulate, without the "one-note" boom often associated with ported enclosures. Transient response was outstanding. Percussive attacks like kick drums and plucked bass strings came through with startling realism.

The midrange was neutral and detailed, though some listeners noted a slight forwardness, which was common in designs of that era aiming for clarity. The tweeter gave smooth high-frequency extension without harshness, and the overall tonal balance was even.

Because the bass was actively driven and servo-corrected, the RS-1A depended less on room placement than many passive speakers. It could deliver consistent low-frequency performance even in less-than-ideal rooms, a major advantage for both home and semi-professional use.

Notable Features and Innovations

• Servo-Controlled Bass: The defining feature, reducing distortion and improving bass accuracy.
• Hybrid Design: Combined the benefits of active control with the simplicity of passive midrange/tweeter operation.
• Built-In Servo Amplifier: Each RS-1A cabinet housed its own dedicated amplifier for the woofer, eliminating the need for an external sub amp.
• Modular Construction: Allowed for easier servicing and component replacement—a rarity in high-end speakers of the time.
• Professional-Grade Components: Used heavy-duty internal wiring, robust terminal cups, and high-quality crossover parts.

The industrial design is distinctive: a tall, narrow cabinet with a slightly angled front baffle, with the drivers in a vertical line. It is utilitarian and puts function over form, in keeping with Infinity's engineering-first approach.

Common Issues and Maintenance

Because of their age and complexity, surviving RS-1A units often need attention. The most common issues include:

• Failed Servo Amplifiers: The internal amp boards, often based on discrete transistor designs, can suffer from capacitor leakage, resistor drift, or transistor failure. Electrolytic capacitors should be replaced as a matter of course.
• Degraded Wiring and Connectors: Internal wiring can become brittle, and solder joints may crack over time.
• Woofer Surround Disintegration: The foam or rubber surrounds on the 12" woofer may have perished. Re-foaming or reconing is typically necessary.
• Crossover Component Drift: Passive crossover components (capacitors, inductors) may have aged, affecting tonal balance.

Restoration is feasible but takes technical skill. Replacement parts are scarce, though some vintage audio specialists offer reconditioning services. The servo feedback loop has to be calibrated properly. Misalignment can lead to instability or poor bass performance.

Current Market Value and Collectibility

The Infinity RS-1A is a rare find today. It never reached the commercial success of models like the IRS V or the Beta series, and production numbers were limited. Working examples get the attention of collectors of vintage active and servo-driven speakers.

In good condition, a pair of RS-1As can fetch $800 - $1,500, depending on functionality and cosmetic state. Fully restored units with upgraded capacitors and reconed woofers may sell for more, especially with original documentation or accessories. The RS-1A is prized for its historical significance and innovative engineering.

Conclusion and Legacy

The RS-1A combined control theory and high-fidelity audio ahead of its time. It didn't revolutionize the consumer market overnight, but it laid the groundwork for later active and powered speakers. Modern systems from brands like KEF (with LS50 Wireless), Genelec, and even high-end DIY communities owe a conceptual debt to early servo designs like the RS-1A.

Today, the RS-1A stands as a testament to Infinity’s fearless innovation. It’s not just a speaker—it’s a piece of audio history, a working prototype of ideas that would take decades to mature. For the vintage audio enthusiast, restoring and listening to a pair of RS-1As is not merely a technical challenge; it’s an opportunity to experience the sound of progress.

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