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

In the late 1970s, high-fidelity audio was changing fast in both consumer and professional circles. Infinity Systems was founded by Cary Christie and Arnold Nudell, and it was out front in loudspeaker design. The company was known for its servo-controlled subwoofers and advanced driver designs. One of its lesser-known but important offerings was the Infinity SM-115, a passive full-range studio monitor that sat between professional broadcast use and high-end home audio.

The SM-115 never got the attention of the Servo-Statik series or the IRSV tower systems. It was a no-nonsense monitor built for accuracy, durability and neutral sound. It was originally marketed to recording studios, radio stations and discerning audiophiles.

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 a studio, that meant engineers could trust what they were hearing. At home, it meant a transparent, uncolored view of the recording.

The SM-115 has a 15-inch woofer. For a passive two-way it had exceptional bass authority, reaching down to 45 Hz with low distortion. That made it suitable for music production and also for film and broadcast monitoring, where low-frequency content needed to be evaluated critically.

The midrange was clear and neutral, with excellent vocal presence, which suited voiceover work and vocal tracking. The tweeter had a smooth, non-fatiguing top end and avoided the "ice pick" highs that plagued many monitors of the era. It is not as extended as modern ribbon or AMT tweeters, but the SM-115's dome tweeter held up well under prolonged listening sessions.

Sensitivity was a healthy 91 dB, so the SM-115 could play loudly with moderate amplifier power. The large woofer has a fairly high moving mass, though, and it does best with a sturdy amplifier that has strong current delivery - ideally 50 watts or more per channel.

Notable Features and Innovations

The SM-115 wasn't as technically radical as Infinity's servo-driven subwoofers, but it had 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. Large-diaphragm dynamic drivers were still preferred then 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 some components may need 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 its age, the SM-115 is generally well-built. Replacement parts are occasionally available through vintage audio specialists, and the driver specifications are well-documented, so restoration is feasible.

Current Market Value and Collectibility

The Infinity SM-115 is not common on the vintage audio market, and when one turns up it often goes unnoticed. Unlike the more famous Infinity models, it has no cult following to drive up prices. That 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 with matching stands or original grilles.

It is not a "blue chip" vintage speaker like the AR-3a or JBL L100. It does offer good value for anyone who wants a large, accurate monitor with some history behind it.

Conclusion and Legacy

The Infinity SM-115 may not have the fame of its more exotic siblings. It was designed for professionals but embraced by audiophiles, with deep bass, neutral tonality and sturdy 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.

Market Value

┌──────────────────────────────────────────────────────────────────────────┐
│ INFINITY SM-115                                                 Apr 2026 │
├──────────────────────────────────────────────────────────────────────────┤
│ High      $48                                     Latest             $45 │
│ Median    $37                                     Trend         ▶ Stable │
│ Low       $12                                     Volume           ~8/mo │
├──────────────────────────────────────────────────────────────────────────┤
│    $50 ┤                                                                 │
│        │                                             .             .     │
│        │.                                 .                              │
│    $29 ┤                                                                 │
│        │                                             .                  .│
│        │    .                                                            │
│        │                  .                                              │
│     $0 ┤                                                                 │
│        ├─────────────────────────────────────────────────────────────────│
│    vol │▄   ▄             ▄               ▄          █             ▄    ▄│
│        └─────────────────────────────────────────────────────────────────│
│         2026-03                      2026-03                      2026-04│
└──────────────────────────────────────────────────────────────────────────┘

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