Infinity SM-155: Compact Innovation from a Golden Era of Speaker Design

In the mid-1970s, Infinity Systems was making a name in high-fidelity audio. The company was known for advanced driver technology and heavy engineering. Among its speakers was the Infinity SM-155, a compact bookshelf model with a clever design and a surprisingly full sound, especially in the bass.

The SM-155 is not as well documented as Infinity's flagship models like the IRS series or the Beta. It was part of a wave of compact designs that tried to deliver high performance in smaller enclosures, handy for apartments, dorm rooms or secondary listening spaces. The goal was deep, articulate bass without the large floor-standing cabinet.

Technical Specifications

Sound Characteristics and Performance

The most distinctive feature of the SM-155 was its passive radiator, a sealed-box alternative to the traditional bass reflex port. Instead of a ported tube, it used a secondary, unpowered 5.25" diaphragm mounted on the front baffle, below the active driver, to boost low-frequency output. That extended the bass without the port noise or chuffing that can plague small ported enclosures.

The result was a tight, controlled low end with good authority for its size. Bass rolled off gently around 55 Hz. That was enough for kick drums, bass guitar and lower piano notes, with more conviction than similarly sized contemporaries. It is no substitute for a subwoofer or a large floorstander, but it offered a balanced, full-range sound that was rare in compact speakers of the era.

The 1" soft-dome tweeter gave smooth, non-fatiguing highs. Some early metal dome tweeters could sound harsh or sibilant. The SM-155's tweeter had a refined top end instead. The crossover was not as sophisticated as those in Infinity's high-end models, but it did its job cleanly and blended the drivers with minimal overlap or phase issues.

Midrange clarity was another strength. Vocals and acoustic instruments came through with warmth and presence, which suited jazz, folk and classic rock, the dominant genres during its production era.

Notable Features and Innovations

The SM-155 looked modest, but it incorporated several design ideas that reflected Infinity's engineering approach:

• Passive Radiator Technology: At a time when most compact speakers used simple sealed or ported boxes, Infinity’s use of a passive radiator showed a commitment to maximizing bass efficiency without compromising transient response.

• Time-Aligned Crossover (Conceptual): Though not as rigorously implemented as in later models like the IRS V or the Beta series, the SM-155’s driver placement and crossover design hinted at Infinity’s growing interest in time-coherent sound reproduction.

• High-Quality Internal Wiring and Components: For a mid-tier model, the SM-155 used better-than-average internal cabling and crossover parts, contributing to its reliable performance over time.

• Stylish, Compact Cabinet: The walnut-veneer finish and clean front baffle gave the SM-155 a refined, professional look that fit well in both modern and traditional living spaces.

Common Issues and Maintenance

Like many vintage speakers from the 1970s, the SM-155 can suffer from age-related degradation, but it is generally sturdy if well maintained.

• Surround Rot: The foam surrounds on the 5.25" woofer and passive radiator are prone to drying out and crumbling after 40+ years. This is the most common failure point. Re-foaming or full reconing is often necessary for optimal performance.

• Tweeter Aging: While the soft dome is durable, some units may exhibit reduced high-frequency output due to oxidation or minor diaphragm fatigue. Cleaning the dome gently with isopropyl alcohol can help, but replacement may be needed in extreme cases.

• Crossover Capacitors: Electrolytic capacitors in the crossover network may degrade over time, leading to muffled sound or imbalance. Replacing these with modern film capacitors can significantly improve clarity and dynamics.

• Grille Attachment: The original cloth grilles were held by plastic clips that can become brittle. Replacement grilles or modern magnetic alternatives are available from specialty audio restoration vendors.

Inspect the speakers carefully before purchase. If they are not in original working condition, consider a professional refurbishment.

Current Market Value and Collectibility

The Infinity SM-155 is not a high-profile collector's item like the Beta or IRS models. It has a quiet following among vintage audio enthusiasts who like compact, well-engineered speakers from the 1970s.

In working condition, a pair of SM-155s typically sells for $150 - $300, depending on cosmetic condition and whether they've been restored. Refurbished pairs with new surrounds and updated crossovers often command the higher end of that range. Unrestored units may go for as little as $75 - $100.

They are particularly sought after by:

• Audiophiles with small listening rooms
• Vintage gear collectors focusing on 1970s American audio
• DIYers interested in speaker restoration projects

They are not rare, but original unmodified pairs in good condition are getting harder to find, especially with intact grilles and hardware.

Conclusion and Legacy

The Infinity SM-155 did not make headlines like its larger siblings. It was a well-thought-out answer to a common problem: how to get balanced sound from a compact enclosure.

At a time when "bigger is better" often ruled speaker design, the SM-155 delivered impressive performance in a modest footprint. Its passive radiator, clean crossover design and high-quality materials made it a standout among mid-tier bookshelf speakers of the 1970s.

Today the SM-155 works as both a functional audio component and a historical artifact from a time when American companies like Infinity were pushing affordable high-fidelity forward. With a little care and restoration, it can still hold its own in current listening environments.

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Awesome Infinity SM-155 3-Way Floor Speaker - 15" Woofers -

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