Infinity SM-115: The Forgotten Studio Monitor from a Golden Era

Name: Infinity SM-115
Brand: Infinity

In the late 1970s, when high-fidelity audio was undergoing a revolution in both consumer and professional circles, Infinity Systems—founded by audio pioneers Cary Christie and Arnold Nudell—was at the forefront of loudspeaker innovation. Known for their groundbreaking servo-controlled subwoofers and advanced driver designs, Infinity carved a niche in the audio world by blending cutting-edge engineering with audiophile-grade sound. Among their lesser-known but highly significant offerings was the Infinity SM-115, a passive full-range studio monitor that bridged the gap between professional broadcast environments and high-end home audio systems.

Though not as widely celebrated as the legendary Servo-Statik series or the IRSV tower systems, the SM-115 holds a quiet distinction as a no-nonsense, high-performance monitor designed for accuracy, durability, and sonic neutrality. Originally marketed to recording studios, radio stations, and discerning audiophiles, the SM-115 exemplifies Infinity’s commitment to technical excellence during a pivotal era in audio history.

Technical Specifications

The SM-115 was engineered as a two-way passive monitor with a focus on extended frequency response and low distortion—critical traits for studio monitoring. It featured a relatively compact cabinet for its time, making it suitable for near-field applications, yet it delivered a surprisingly full-bodied sound thanks to its carefully tuned crossover and high-quality drivers.

Specification	Detail
Model	SM-115
Manufacturer	Infinity Systems
Year Introduced	1978
Type	Passive, two-way studio monitor
Driver Configuration	15" woofer, 1" dome tweeter
Frequency Response	45 Hz – 20 kHz (±3 dB)
Sensitivity	91 dB (1W/1m)
Nominal Impedance	8 ohms
Recommended Amp Power	25–150 watts
Crossover Frequency	2.2 kHz
Cabinet Material	Medium-density fiberboard (MDF) with wood veneer
Dimensions (H×W×D)	28" × 16" × 14" (71 × 40.5 × 35.5 cm)
Weight	62 lbs (28 kg) per speaker
Finish Options	Walnut, black ash, rosewood veneer

The heart of the SM-115 was its 15-inch polypropylene-cone woofer, a large driver for a studio monitor at the time, which allowed for deep bass extension without the need for a separate subwoofer in many setups. Paired with a soft-dome tweeter—a departure from the piezoelectric or horn-loaded tweeters common in earlier professional monitors—the SM-115 delivered smooth, extended highs with minimal harshness.

The crossover network was designed with audiophile-grade components, including air-core inductors and metal-film capacitors, reflecting Infinity’s attention to detail even in their more utilitarian models. The cabinet was heavily braced and internally damped to reduce coloration, and the front baffle was slightly time-aligned to improve phase coherence between drivers.

Sound Characteristics and Performance

The Infinity SM-115 was not designed to be “euphonic” or “warm”—it was built for accuracy. In studio environments, this meant engineers could trust what they were hearing. In home settings, it meant a transparent, uncolored window into the recording.

With its 15-inch woofer, the SM-115 offered exceptional bass authority for a passive two-way, reaching down to 45 Hz with authority and low distortion. This made it suitable not only for music production but also for film and broadcast monitoring, where low-frequency content needed to be evaluated critically.

The midrange was remarkably clear and neutral, with excellent vocal presence—ideal for voiceover work and vocal tracking. The tweeter contributed a smooth, non-fatiguing top end, avoiding the “ice pick” highs that plagued many monitors of the era. While not as extended as modern ribbon or AMT tweeters, the SM-115’s dome tweeter held up well under prolonged listening sessions.

Sensitivity was a healthy 91 dB, meaning the SM-115 could play loudly with moderate amplifier power. However, due to the large woofer and relatively high moving mass, it benefited from a robust amplifier with strong current delivery—ideally 50 watts or more per channel.

Notable Features and Innovations

While the SM-115 wasn’t as technically radical as Infinity’s servo-driven subwoofers, it incorporated several forward-thinking design elements:

Time-Aligned Crossover: The tweeter was mounted slightly behind the woofer on a sloped baffle, helping to align the acoustic centers of the drivers for improved off-axis response and imaging.
Low-Resonance Cabinet: The thick MDF construction, internal bracing, and constrained-layer damping reduced cabinet coloration—a rarity in budget-conscious studio monitors of the time.
Professional-Grade Binding Posts: The rear panel featured heavy-duty, gold-plated five-way binding posts, supporting banana plugs, spades, or bare wire—uncommon in consumer speakers of the era.
Modular Design: The drivers and crossover were designed for serviceability, allowing studios to replace components without sending the entire unit back to the factory.

These features made the SM-115 a favorite among broadcast engineers and mastering studios, particularly in the late 1970s and early 1980s, when large-diaphragm dynamic drivers were still preferred over the smaller, more compact designs that would dominate in the digital era.

Common Issues and Maintenance

Like all vintage speakers, the Infinity SM-115 is now over four decades old, and certain components may require attention:

Surround Rot: The foam surrounds on the 15-inch woofer are prone to drying out and crumbling. Re-foaming or reconing is often necessary to restore bass performance.
Capacitor Aging: The crossover capacitors, while high-quality for their time, may have drifted in value or developed leakage. A full recapping is recommended for optimal performance.
Tweeter Fatigue: Though durable, the soft dome tweeters can suffer from oxidation or voice coil misalignment if overdriven. Gentle use and proper amplification help prevent damage.
Cabinet Wear: The wood veneer, while beautiful, can show scratches or finish lifting. Gentle polishing and careful handling can preserve its appearance.

Despite these age-related concerns, the SM-115 is generally robust and well-built. Replacement parts are occasionally available through vintage audio specialists, and the driver specifications are well-documented, making restoration feasible.

Current Market Value and Collectibility

The Infinity SM-115 is not a common sight on the vintage audio market, and when it does appear, it often flies under the radar. Unlike the more famous Infinity models, it lacks the cult following that drives up prices. However, this makes it a hidden gem for collectors and restorers.

In original, working condition, a pair of SM-115s typically sells for $300–$600, depending on cosmetic condition and whether they’ve been refurbished. Fully restored pairs with reconed woofers and recapped crossovers can command $800 or more, especially if paired with matching stands or original grilles.

While not a “blue chip” vintage speaker like the AR-3a or JBL L100, the SM-115 offers exceptional value for those seeking a large, accurate monitor with historical significance. Its blend of studio-grade performance and home-audio refinement makes it a compelling choice for vintage studio monitor enthusiasts.

Conclusion and Legacy

The Infinity SM-115 may not have the fame of its more exotic siblings, but it stands as a testament to Infinity’s engineering philosophy: accuracy, durability, and innovation. Designed for professionals but embraced by audiophiles, the SM-115 delivered a rare combination of deep bass, neutral tonality, and robust construction.

Today, it remains a rewarding project for vintage audio lovers—offering both sonic satisfaction and a tangible connection to a transformative era in audio history. For those willing to restore and appreciate it, the SM-115 is more than just a speaker: it’s a piece of audio heritage, quietly waiting to be rediscovered.