 |
| Click to enlarge |
 |
| Click to enlarge |
Introduced in the early 1960s, the Shure SM33 was a refined evolution of earlier Shure ribbon models like the 330 and 333, which debuted in the 1950s. Shure’s ribbon microphone lineage began in earnest with the Model 300 and 315 in 1952, but the SM33 brought a modernized design tailored for television and radio studios. Its matte gray finish was specifically chosen to reduce glare under studio lights, a nod to its primary use in broadcast environments. The SM33 hit the market as a high-end, uni-directional ribbon microphone, competing with the likes of RCA and Western Electric, though Shure’s focus was on versatility for both voice and music applications.
Production of the SM33 continued through the 1970s, with some sources suggesting it lingered into the early 1980s. Its discontinuation likely stemmed from the broader industry shift toward dynamic and condenser microphones, which offered greater durability and lower maintenance for evolving recording and broadcast needs. The retirement of Shure’s specialized ribbon mic manufacturing team also played a role, as the company handed over repair responsibilities to experts like Dick Gardner, a harmonica player and ribbon mic specialist.
Notable Users
The SM33’s most famous association is with Johnny Carson, whose custom-engraved SM33 (inscribed “Johnny’s Mic…Not Ed’s…Not Fred’s”) sat on his desk during The Tonight Show for years. This microphone, auctioned for over $50,000 in 2005 after Carson’s passing, became an icon of late-night television. Beyond Carson, the SM33 found favor in recording studios for its ability to capture vocals, upright bass, horns, and even drum overheads with a warm, natural tone. Anecdotal evidence suggests artists like Muddy Waters may have used related Shure ribbon models (300 or 315), hinting at the SM33’s appeal in blues and jazz circles due to its proximity to Shure’s Evanston, Illinois headquarters. Its supercardioid pattern made it a go-to for live sound engineers needing noise rejection in critical applications.
 |
| Click to enlarge |
SM33 Technical Specifications
The SM33’s design prioritized studio-friendly features, combining a supercardioid polar pattern with a wide frequency response. Below are its key technical specifications, drawn from historical documentation and user reports:
- Frequency Response: 30 Hz to 15,000 Hz, offering a warm, smooth sound with a gentle roll-off in the high frequencies, ideal for both voice and music. The response is adjustable via a recessed rotary bass-rolloff switch, allowing users to tailor low-frequency characteristics.
Click to enlarge - Directional Pickup: Supercardioid, more directional than a standard cardioid, providing excellent rejection of off-axis noise and reverberation. This made it particularly effective in noisy studio environments or for isolating sound sources.
- Output Level: Typical for ribbon microphones, the SM33 has a relatively low output level, requiring a high-gain, low-noise preamp to achieve optimal performance. Exact figures vary, but ribbon mics generally produce output levels around -50 dBm or lower, necessitating careful preamp pairing.
Click to enlarge - Output Impedance: Variable, with a switchable impedance selector offering low-impedance settings (38Ω or 150Ω) to accommodate different amplifiers and long cable runs, a forward-thinking feature for its era.
 |
| Note the hole for adjusting the Voice/Music response, Click to enlarge |
Sound and Performance
In use, the SM33 delivers the classic ribbon mic warmth that enthusiasts crave. Its supercardioid pattern excels at isolating vocals or instruments in busy environments, making it a favorite for broadcast and live sound reinforcement. The wide frequency response captures the nuances of voices and acoustic instruments with a natural, un-hyped tone, contrasting with the brighter, more clinical sound of condenser mics. Users have praised its versatility, employing it on everything from jazz guitar amps to fiddles and upright bass. However, like all ribbon mics, the SM33 is sensitive to plosives and requires careful placement to avoid wind noise, and its low output demands a quality preamp.
Compared to competitors like the RCA 44 or Beyerdynamic ribbons, the SM33 holds its own but doesn’t quite match the RCA’s studio dominance. Its unique supercardioid design and matte finish gave it an edge in television, where aesthetics and noise rejection were paramount. For modern users, a well-maintained or re-ribboned SM33 (often by specialists like Stephen Sank or Dick Gardner) can still deliver exceptional results, particularly for those seeking a vintage vibe.
After the SM33
After the SM33’s discontinuation, Shure largely exited the ribbon microphone market for decades, focusing on dynamic and condenser designs like the SM57 and SM58. The decline in ribbon mic demand, coupled with the retirement of Shure’s ribbon manufacturing team, led to a hiatus until the 2000s. In 2010, Shure re-entered the ribbon market with the KSM313/NE and KSM353/ED, premium bi-directional ribbon microphones featuring Roswellite® ribbon material—a durable, high-tensile alloy replacing traditional foil ribbons.
The KSM313/NE, originally developed by Crowley and Tripp, offers a dual-voice design with distinct front and rear frequency responses: warm and full for amplifiers, or bright and flattering for vocals. It boasts a 146 dB SPL handling capability and a 30 Hz to 15,000 Hz frequency response, matching the SM33’s range but with higher output levels comparable to dynamic mics. The KSM353/ED, designed for studio and live performance, emphasizes pristine audio with a neutral response and similar durability. Both models leverage modern engineering to overcome traditional ribbon mic fragility, making them suitable for high-SPL environments like concert halls or guitar cabinets.
Unlike the SM33’s supercardioid pattern, these newer models are bi-directional, aligning with traditional ribbon mic designs. They represent Shure’s commitment to reviving ribbon technology for contemporary applications, from studio recording to live sound. As of 2025, the KSM313/NE and KSM353/ED remain Shure’s primary ribbon mic offerings, catering to professionals seeking world-class audio with modern reliability.
 |
| Click to enlarge |
EPILOGUE
The Shure SM33 ribbon microphone is a classic that embodies the warmth and character of vintage ribbon technology. Its history as a broadcast staple, notably on Johnny Carson’s desk, cements its cultural significance, while its supercardioid design and versatile sound make it a studio workhorse. Though discontinued by the 1980s, its legacy endures in the hands of collectors and engineers who value its unique tone. Shure’s modern KSM313/NE and KSM353/ED carry the ribbon mic torch forward, blending durability with the timeless sound that made the SM33 a legend. For those lucky enough to own a functioning SM33, it’s a piece of audio history that still delivers American craftsmanship and ribbon microphone sound.
Thanks for reading High on Technology, Good Music To You!
©April 2025 by Mark King, It's not ok to copy or quote without written permission
OUTPUT IMPEDANCE CHANGE
According to the Professional Products Data for the SM33 microphone, the mic came from the factory set to 150-ohm output impedance. This is changed by removing the output connector (XLR insert in the metal base), unsolder the black wire from pin 3 and solder the blue wire instead. Blue wire = 38Ω output impedance.
Here's another copy of the schematic provided by Shure®:
 |
| Click to enlarge |
ONE MORE THING - Correct impedance setting for Neve 1073spx
The Shure SM33, with selectable output impedances of 38 ohms or 150 ohms, requires careful pairing with the AMS Neve 1073SPX preamp (offering 300 ohm or 1200 ohm input settings) to optimize its warm, smooth sound (30 Hz–15 kHz). Shure’s documentation confirms the SM33 is factory-set to 150 ohms, with impedance changed by rewiring the 3-pin output connector (e.g., soldering the blue lead to pin 3 for 38 ohms). The Neve 1073SPX’s transformer-coupled design amplifies the SM33’s low output (~50 dBm) and impedance selection impacts tone, clarity, and voltage transfer.
Impedance Options -- Assuming a bridging impedance ratio (preamp input 5–10 times impedance of mic output) for ideal natural response.
- SM33 at 150 ohms (Factory Default):
- Neve at 1200 ohms: Ratio = 1200/150 = 8:1. This ideal bridging ratio ensures maximum voltage transfer, a clean, warm, and full sound. It preserves the SM33’s natural tone for vocals, instruments, or broadcast (e.g., Johnny Carson’s Tonight Show).
- Neve at 300 ohms: Ratio = 300/150 = 2:1. This lower ratio slightly loads the transformer, adding subtle warmth and saturation but potentially damping dynamics. It’s less transparent but suitable for colored, vintage tones (e.g., upright bass).
- SM33 at 38 ohms (Rewired per Shure Instructions):
- Neve at 1200 ohms: Ratio = 1200/38 = ~31.6:1. This high ratio maximizes voltage but may thin the low-frequencies, reducing the SM33’s characteristic warmth, making it less ideal for most applications.
- Neve at 300 ohms: Ratio = 300/38 = ~7.9:1. A good bridging ratio, providing clean amplification with minimal coloration, but the lower mic impedance may slightly reduce output level compared to 150 ohms.
Recommended Setting — Optimal Combination:
- SM33 at 150 ohms (factory default, no rewiring needed).
- Neve 1073SPX at 1200 ohms.
CONCLUSION
When you’re using the Shure SM33 with an AMS Neve 1073SPX, use the factory-default wiring (150-ohm) with the preamp at 1200 ohms for the best balance of clarity, warmth, and fidelity. This combination leverages the SM33’s professional-grade performance and classic ribbon sound.
