Audeze MM-100 & Sennheiser HD 660S2 Two Affordable Open-Back Headphones!

Audeze MM-100

The latest product from the Californian headphone specialists at Audeze is the $399 MM-100, a full-sized, over-ear, open-back planar magnetic headphone designed both for music professionals and for audiophiles who want a neutral “pro monitor” sound signature. With this dual purpose in mind, the headphones were given a double debut at the recent Axpona high-end audio show and the NAMM (National Association of Music Merchants) pro audio show, both of which took place in mid-April, 2023. The “MM” in MM-100 stands for Manny Marroquin, the 17-time Grammy-winning engineer/producer who collaborated with Audeze on the headphone’s sonic performance. Audeze previously worked with Marroquin on the more expensive MM-500 Professional Headphones ($1699).

Although the MM-100 is substantially less expensive than Audeze’s flagship models (the LCD-5 planar magnetic headphones and CRBN electrostatic headphones both sell for $4500), this new headphone is still built and tested in the company’s Southern California workshop. Audeze says that despite the lower price, the MM-100 is “crafted to stand up to the rigors of life, whether it takes you from the studio to the road, from home to school, or from the office to live performance.” The lightweight, premium chassis is reportedly built for both comfort and durability, featuring aluminum yokes, magnesium grilles, and a spring-steel headband. The ear pads are made from supple, gel-filled leather, and the adjustable suspension strap feels “feather-light for hours,” providing “effortless comfort for even your longest sessions.” The single-sided cable can attach to either ear cup.

The MM-100 uses all-new planar magnetic drivers designed for “class-leading detail and low distortion,” according to Audeze. The drivers use Neodymium N50 magnets and a 90mm transducer made from Audeze’s proprietary Ultra-Thin Uniforce diaphragm material. The headphones also feature the company’s unique Fluxor magnet array and Fazor phase management system. The MM-100 has an impedance rating of 18 ohms and a sensitivity of 98 dB/1mW (at Drum Reference Point). Audeze recommends at least 250mW of power, but the headphone can be driven by anything between 100mW and 5W RMS. 

Sennheiser HD 660S2

The Sennheiser HD 660S2 ($599.95) is a follow-up to the HD 660, which launched in 2017. One of Sennheiser’s most neutral headphones, the HD 660 gained popularity with audiophiles and audio professionals alike. The new HD 660S2 aims to retain the “extremely precise yet non-fatiguing sound” of the original, but with more bass impact and a more extended, slightly elevated sub-bass. The goal was an intimate sound with great timbre, but with low bass that sounds deep and accurate while “expanding” the entire sound of the music. Sennheiser promises a sound that is “natural and balanced, never harsh or brash,” but with “breathtaking resolution and precision to take you closer than ever to instruments and vocalists.” Compared to the original HD 660, the HD 660S2 offers double the sound pressure at the lowest frequencies, according to Sennheiser, but the sound still sounds balanced “throughout the sub-bass, mid-bass, and beyond.” The extension at the low end simply creates “more space for the sound,” providing a deeper backdrop against which the high frequencies feel “more brilliant.” The HD 660S2 also offers a deeper soundstage, according to Sennheiser. Some audiophiles may be wary of too much sub-bass (we all remember the sound of those early Beats by Dre headphones), but Sennheiser says this is not the “sloppy or boomy” kind of sub-bass that becomes a distraction from the music.

The bass boost was achieved by a combination of small changes. The HD 660S2’s dynamic driver uses an ultra-light aluminum voice coil with 300-ohm impedance for excellent impulse response, and the new surround is more compliant than before, dropping the resonant frequency to 70 Hz (down from 110 Hz in the original). Sennheiser also made an effort to better optimize the airflow around the transducer — even the mesh grilles on the outsides of the ear cups were reportedly tweaked to manage airflow at the ideal impedance. The magnet’s air gap works more efficiently as well, providing greater sensitivity over the entire frequency spectrum. So the HD 660S2 offers more than just higher sound pressure at the lowest frequencies; it also offers “greater precision, speed, and resolution” than its predecessor, according to Sennheiser.

The new HD 660S2 offers listeners what they requested most from the headphones’ predecessor. With precision and power like no other and new sensitivity across all frequencies, listeners will hear details they’ve never heard before, especially at the lower end of the spectrum.

— Jermo Koehnke, Sennheiser Product Manager