Koss • Pro4S Headphones

The latest model in Koss's long Pro4 line.

by Rad Bennett | August 4, 2015

ohn C. Koss founded his company in Milwaukee in 1958 and soon began manufacturing headphones, marketing them as a companion listening device for a portable turntable he was selling. Until the time the Koss brand entered the market, headphones had been used for conversation in military and broadcasting situations. Koss proved that headphones could also reproduce music in high fidelity and stereo, riding a wave of success into the 1970s and becoming as recognizable as JBL or Marantz. But as the company diversified, it got into financial trouble, dropping out of everyday awareness in the late 1980s. The 1990s saw it begin a comeback that has continued to the present. The Koss name has once again become known to a younger generation from its appearance as one of the ad accounts on Mad Men.

The first Koss model was the SP3; shortly after, the Pro4 series began. This series has been tweaked over time, most notably with the introduction of the Pro4AA and the Pro4AAA models. The latter was retired in the 1980s; the former was retired as well, but came back by popular demand along with a tweaked titanium version, the Pro4AAT. Both the Pro4AA and Pro4AAT are still available at an unbelievably low price of $99.99 each. All of the Pro4 models are over-the-ear, closed-back designs first and foremost aimed at those who record music and need headphones that have a neutral sound without leaking any sound that would get into the music being recorded or mastered. Even with this emphasis on professional use, many of the Pro4 models were popular with discerning music listeners and hi-fi buffs.

Given this rich history, imagine my delight at getting hands-on experience with Koss’s latest addition to the line, the Pro4S, selling at a mere $149.99. With the Pro4S, Koss has come into the 21st century by offering a design that is light (only 6.9 ounces) and fashioned in a more contemporary style. Koss has made these concessions to overall modern taste while continuing to focus on sound quality.

The Pro4S headphones come in a fairly unassuming outer wrap, but when I got through that I discovered a solid zippered carrying case made of thermoset and found that everything I needed was inside: the headphones, the 4 1/2’ coiled cable with 3.5mm plugs at the ends, an adapter so I can plug the cable into 6.35mm (1/4") jacks, and literature describing the headphones and their unique lifetime warranty. I’ve been through so many headphone packages that had me scurrying around opening little plastic bags (and misplacing them!) to find the Koss packaging refreshing. It’s ready to go. I could just tuck in under my arm and be off, knowing I had everything I need.

The headphone cups are rotated flat to fit in the provided case, but the unit also folds at the arms to give even smaller headphones you could throw in a backpack or gym bag. The design is simple and neat: black with silver trim, a padded headband, and generously padded ear cups utilizing Koss’s D-profile -- recessions literally in the shape of a "D" -- which fit the ear far better than a round ear cup. The ear cups adjust securely with click stops.

The driver is the company’s new SLX40 40mm unit made up of a neodymium magnet, a Mylar diaphragm and a copper voice coil. Again according to Koss, it has been painstakingly tuned to provide the flat response needed for recording and monitoring, and Koss spent two years developing it.

The detachable cord can be plugged into either the left or right ear cup. When plugged into one it renders the other as an output, allowing pass-through or daisy-chain listening. A Koss promo video made it clear how this works. There were three guys sitting at a console. The first was plugged into the board, the second plugged into the free output of the first’s ear cup, and the third plugged into the free output of the second’s ear cup. Looking at this situation, it became quite clear why Koss decided on a coiled cord. It stays neat and out of the way.

Frequency response rated by Koss is 10Hz-25kHz. The Pro4S has an impedance of 35 ohms and a sensitivity of 99dB, which means it can easily be powered by a cell phone or other portable music device without additional amplification. The construction is solid, with both plastic and brushed-aluminum parts. They make the plastic look so much like metal these days that it’s hard to know where one ends and the other starts. Koss states that the all-important hinges are metal; I’ve operated them a lot and can’t see that they’ve become any less stable than they were out of the box.

Putting the Pro4S into use proved a joy. The headphones fit my head perfectly. They were snug enough that I could wear them for physical activity at the gym, yet it they never got too hot, even when I perspired a bit. Like its predecessors, the closed-back Pro4S not only keeps sound from leaking out, but it also helps to keep it from leaking in. As passive noise-reduction headphones, then, the Pro4S is quite successful. The very loudest sounds did get through faintly, but I had to take the headphones off to be able to understand conversation.

I listened using both the coiled cord that Koss provides and a special-length cord I had made to use at the gym. I found the sound from cable to cable did not vary, so I used them both as needed, primarily depending on the Koss cord. All of that said, once the headphones were securely on my head, I got to the business of listening at home, because that’s what it’s really about.

have most of my music collection saved as files -- CD quality on up to 24 bits/96kHz -- which I stream through a Squeezebox Touch to my main system. My first thought was to hear the Koss 'phones through the headphone jack of my receiver/preamp, but I discovered that the Squeezebox Touch has an almost hidden-away direct headphone output that has enough power to produce a good output, so I used that instead -- a good thing, as I like the DAC in the Squeezebox Touch better. Its quality has been hotly debated on the Internet, the conclusion being that it is the equivalent of most outboard DACs costing a lot more money.

We all have our favorite test discs (or files); some of mine cross over with those of TAB’s Richard Freed, as we both were around to experience the birth of the stereo LP. The one disc we’d both surely agree on is España, recorded by Decca with Ataulfo Argenta conducting the London Symphony Orchestra (an ALAC file of High Definition Tape Transfer files made using a London commercial four-track analog tape). The sessions were perfect. This is music-making so wonderful that it defies criticism. Suffice it to say that the Argenta-led performances of Chabrier’s España Rhapsody and Rimsky-Korsakov’s Capriccio Espagnol are still to this day the versions to which we must compare any new recordings.

And Decca did not shirk in the engineering department. Stereo was new in January 1957, when the recording was made, and Decca had fashioned an array of microphones called "the tree." It was a soundbar arrangement bearing five Neumann omnidirectional microphones, three in a "T" formation at the center and two on outrigger booms. For orchestral recordings it was suspended in the air, right above the conductor. Variations on it are still used today.

Because I went on to be a conductor for a while, I loved the sound of this system. It provided a conductor’s point of view. And now, it seems as if it were made for good headphone listening, at least as far as classical music is concerned. At the beginning of the Chabrier, after a moment of reassuring tape hiss to let us know it’s an analog recording, the string pizzicatos start, violins left, cellos and basses right. The trumpet enters left center, then the harp in the right channel, then everyone in a glorious melee of sound topped by the first trumpet, soon countered by a theme in the right channel by the horns.

The Koss headphones reproduced this track as it should sound -- rich but detailed and exciting. The horns and cellos practically glowed with warmth, the woodwinds were mellow in the center of the stage, crisp violins and percussion gave the music bite. Toward the end of the piece are some bass drum solos where I could actually hear the attack and the resulting tone with equal measure.

Some have complained online of a small soundstage with the Pro4S. It’s not the widest I’ve heard, but it is surely not narrow. Stereo effects in the Chabrier were quite good -- in fact, as good as I’ve ever heard them. The HDTT tape master helps by providing wider separation than vinyl versions.

I found the Pro4S fully up to the challenges of opera when turning to another Decca recording, Puccini’s La Bohême, the stereo version with Renata Tebaldi and Carlo Bergonzi (ALAC file [Decca 425 534]). At the beginning of the first act, Marcello (Ettore Bastianini) and Rudolfo (Bergonzi) are commiserating and bragging about their woes and successes. Because this was the era during which Decca engineers tried to mimic stage action, the two singers crisscross across the stage several times, an incredibly exciting and pleasing experience through the Koss Pro4S, which imaged the singers’ stage wanderings with precision, while perfectly reproducing the static orchestra. Moreover, the Pro4S caught all the nuances in the different styles of the singers. Though this recording is warm overall and the voices sounded particularly three-dimensional, it also has plenty of detail, as in the beginning of Act III, and the Pro4S revealed every bit of it.

How would the Pro4S deal with modern high-definition recordings? I threw some 24-bit/94kHz files at them to see. First up was Rachmaninoff’s Symphonic Dances with Eiji Oue conducting the Minnesota Orchestra [Reference Recordings RR-96]. The performance is romantic and lively, lush but also lean, with singularly clear textures. The recording matches, being warm yet loaded with a lot of detail. Through the Pro4S, various woodwind solos (including saxophone) emerged with great clarity, with the piano and chimes sounding clearer than ever. The full-orchestra passages had enough weight but were also very lithe. The accuracy of the Pro4S seemed to catch the idea that this was an HD recording, a little subtler than a CD.

I introduced the Pro4S to Acousence, a relatively new audiophile label that has produced some outstanding recordings with Jonathan Darlington conducting the Duisburg Philharmonic. I listened to Debussy’s La Mer (24-bit/96kHz FLAC download [Acousence Living Concert Series No. 17]).The composition is one of endlessly shifting orchestral color that is fully realized by the Duisburg players and was perfectly reproduced by the Pro4S. Harps glissando here and there, woodwinds skitter in and out of the orchestral fabric, cymbals and orchestral bells provide accents. Debussy did a masterful job at describing in music the constant shifting of ocean waves. The Pro4S gave such a detailed image that I could almost feel the salt spray on my face.

Turning away from classical music and going back to 16 bits/41.1 kHz, the next thing I did to check out the accuracy of the Pro4S was to play a cut or two from three different jazz albums featuring a vocalist with a big band. The albums were Mel Tormé Swings Shubert Alley (ALAC file [Verve CD 821 581 2]), Swinging at the Blue Moon Bar & Grill with Steve March Tormé (ALAC file [Frozen Rope CD SM 11970]), and the stereo version of June Christy’s Something Cool (ALAC file [Capitol CDP 7 96329 2]). The vocals were as clear as I could wish for on all of them, but the instrumental contributions were unalike. All three recordings were effective yet so very different, something the Pro4S made very clear.

Then, to give the Pro4SA a populist spin, I turned to items I have on my Nano 6, which I wear as a watch. It’s ready to go music wherever I am. I can plug wired headphones in or put in a Sony Bluetooth dongle and listen wirelessly. I keep mostly pop music there, all in CD quality. I’ve never gone down to MP3. The quality of the sound is just as important when I’m on the go as it is when I am seated comfortably in my listening room.

The Pro4S did full justice to one of my favorite pop acts -- Vampire Weekend, all three albums. I listened mostly to cuts from Modern Vampires of the City (ALAC file [XL Recordings no catalog number]) and they really blew me away. Vampire Weekend has won many awards for this album, but in my opinion the recording engineers should have shared those accolades, because Vampire Weekend is probably the best-recorded rock band in the land. The Pro4S made these points absolutely clear. The band revels in the use all sorts of big and small percussion, often reminiscent of African drumming or island music. Ezra Koenig’s lead vocals are free and unfettered in the high range, and the keyboards of Rostam Batmanglij provide rich filling and well-defined texture. There’s very solid bass with good definition, and once in a while there’s some subterranean synthesizer sound that you feel as much as you hear. The Koss Pro4S caught it all, right down to Koenig’s smallest intake of breath and the lowest synthesized growl. What a wonderfully full yet totally transparent sound!

I then plugged the Pro4S into Bruce Springsteen’s Born in the USA (ALAC file [Columbia CK 38653]), expecting good results. What a disappointment, especially after Vampire Weekend. Strident, with ill-defined bass, splintering drums and distorted cymbals, it was barely tolerable, just short of ugly. Was the Pro4S failing the test?

In a word, no! I made that file from an ancient CD, one from the bad old early days of CD. The Pro4S was just reproducing what was there on that old disc. I then realized something that I'd been hearing all along. The Pro4S had sounded different with every album I played. Koss had done what it set out to do: create a headphone that is neutral -- able to sound like whatever you play through it. This means it can sound like one of the best headphones around (España, Modern Vampires of the City) or not so good (Born in the USA).

ere comes the bottom line. The Koss Pro4S has superbly balanced sound from lows to highs. It has a warm sound with lots of detail, or cool and lean, depending on the recording. If I played garbage, I heard garbage. But this is the perfect headphone for me, because I like to set things flat and trust -- or not -- in the recording. That might not be for all listeners. Some might prefer headphones that emphasize a favorite type of music or a particular frequency range. The Pro4S won’t -- unless it’s part of the recording.

For me, the Pro4S is a headphone that can travel anywhere. I can use it for the most serious listening or monitoring, I can pack it in its hard case and use it as a commuter headphone, or I can even wear it to the gym. It’s a headphone for all seasons, so to speak. It’s amazing enough to find a headphone of this quality at the very low price of $149.95, but if you purchase it you’ll also have one of the most incredible warranties in the business. If something goes wrong, even if you might have caused it, you can send the headphones back to Koss with $9.00 and in about a week, they’ll send you new ones.

So in addition to true-to-the-recording sound, the Koss Pro4S virtually defines great value for the dollar. I’m very happy to have made its acquaintance.