Sennheiser Consumer AudioHD 650 - Audiophile Hi-Res Open Back Dynamic Headphone, Titan

In this technical review I will explain and gives a few examples why the HD650 are the excellent headphones that they are. I'll also dispel a few misconceptions about its performance. Initially, I was unsure whether it was really worth buying these expensive (unfortunately now even more expensive) headphones, since the HD650 has earned a special reputation on certain large forum sites on the Internet. Apparently the HD650 is great, but at the same time is "veiled", with a "dark sound signature"; it's also "hard to drive", "picky about amps, scaling with more expensive amps into the >$1000 territory". It created confusion in me and I thought to myself if all that really makes any sense. So I first researched electro-acoustic principles on an informative blog and learned some truth that made logical sense. Then I finally purchased it.Cosmetically, the HD650 has a nice clear-coat finish over the titanium-silver (actually pearl grey) color scheme, where the small multi-colored sparkly flecks of paint can be seen around sunlight. Even the outer side of the grille gets the sparkly treatment, while the inner side is bare stainless steel. The stock cable (which is just fine BTW!) is thick and very supple. It only has subtle low frequency microphonics when tapping it with fingers, and none when it's rubbed against clothing. The fit of the headphones is secure, and stays put during left/right and angled up/down head movements. The somewhat strong initial clamping force lessens over time. The modular construction of the headphones makes disassembly easy. For instance, tilting the ear cups downward a bit past the resistance as if trying to lay them flat unsnaps the upper half of the ring from the central hubs. Once the upper half of the ring is then pulled out, place the ear cup in your lap with the cable entrance side facing you, and using your fingers in that area, firmly push away the lower trim piece which simultaneously causes the grille to come off as well! Replacing the headband pad is simple. (Remove the capsules beforehand.) I managed to get the edge of my thumb just behind the Made in Ireland plastic stopper and pushed it out quickly and forcefully. Then extend the metal band until it comes out. Repeat for the other side. Now slide the pad out. It is not glued on. To reassemble, first carefully feed the edge of the pad all the way through the narrow groove of the plastic arc. Then put the left and right bands through the second groove behind the pad and snap back in the stoppers.Now, with all that out of the way, the best and easiest way to evaluate headphone performance is with 20Hz to 20kHz sine wave sweeps (download the logarithmic one at audiocheck.net) and pink noise. If it sounds even and clean, then that would carry over into normal use, since sine waves are the basic building blocks of sound reproduction. Similarly, a TV display's performance is seen with gray-scale linearity, RGB coordinates for gamut accuracy close to BT. 709, white balance targeted to 6500K, etc. If it performs well there, such a display would be accurate for normal program material. For audio, the sine wave sweep should not contain frequency sections that get your attention in terms of peaks, valleys, ringing or other anomalies. I'm happy to report that the HD650 has a remarkably even and linear response during the sweep! The so-called "mid-bass hump" isn't even perceived as being louder than the 500Hz area. A critical area (especially for female voice and higher notes from strings and wind instruments) is from around 700Hz to 3kHz. It is totally even there! The HD650 does a fantastic job in that portion of the spectrum as it's incredibly natural and transparent sounding. It's also very easy to hear the treble frequencies become finer sounding during the upward progression. Pink noise is smooth and balanced. It's virtually impossible to discern any imbalance between the higher frequency portion of the noise with the lower frequency portion. I would like to point out that the compensated frequency response graph at Inner Fidelity shows a 15dB notch around 5 to 6kHz compared to 1kHz. But actually there is no audible recession of 15 decibels in that area when listening to the sweep! It is flat when listening by ear. That notch is responsible for these headphones not imparting harshness and sibilance. There is a slight audible dip at about 7kHz and another at 10kHz. But after that the high treble is actually nicely extended and present, especially in the 12 to 16kHz region. At the other end of the spectrum, the deep bass lacks a few dB of rumble in the lowest octave. Overall, the HD650 has a substantially flat response and a lifelike timbre. And the HD650's wonderful transient response can be attributed to its high frequency extension, high damping factor from the amplifier for the lower frequencies, and the low mass aluminum coil. Everything is critically damped.I also conducted a comparison with the HD598 and HD428. The HD598 lacks the bass response, bass weight, fuller body of the low mids to the start of the upper mids (200Hz to 750Hz) of 650. But the 598 gets into big trouble with the lower treble and beyond. I can detect a bit of a dip after 1kHz and then there are a couple of sharp audible peaks and a bit of shrillness from around 2kHz to 5kHz with an indistinctness over all the high frequencies afterwards. Its pink noise has some emphasis in that area (the 'tssshhh' sound), with the lower frequencies being slightly quieter. Hence, its sonic character is lean and bright in the upper mids and transition to the low treble. It's easy to notice that female vocals don't have a natural timbre and cymbals lack detail compared to the HD650. Onto the HD428. A little bit of distortion and looseness is heard in the lower bass area but progresses quite evenly until around 3 to 4kHz where it has quite a big noticeable drop, and comes back up to about 10kHz and then gently rolls off the rest of the highs. Its pink noise is tilted towards warmth with the lower frequencies a bit louder than the higher ones. Though Headroom's graph shows a weak bass response for the HD428, it does not take into account how the human ear interprets low frequency resonance inside the closed ear cups. (A quick sweep or 3 second chirp also reveals an echo-knock inside the right cup). Bass is quite tight, but lacks real impact or weight (except for the fabric material tickling the ears), even though it gives the impression of a little more bass than the 650 due to resonance (but that resonance affects the perception of low frequency transients). But at least the 428 does not have that harsh low and mid treble of the 598! It seems more like a 650 with a subdued, mellow or recessed response from the low treble and beyond that, but it's a surprisingly clean sounding can for the price! Its treble is more articulate than the 598s. I think it represents a superb value along with the HD448 or HD449 and is pleasant for long listening sessions too as it doesn't offend my ears like the more strident HD598.So now that we know the HD650 is actually very balanced in frequency response (which also relates to good dynamics), it's on to distortion. It can be summed up briefly: This headphone's distortion is inaudible! That means pure, clean, smooth, full-bodied sound with natural harmonics beings heard leading to accurate timbre, high resolving ability and detail retrieval since there's no discernible garbage being introduced by the transducer. There is a clear and precise differentiation of all frequencies and octaves. Headroom's graph for the 500Hz sine wave distortion (tested at 90dB) shows the HD650 has vanishingly low levels, lower than some other high-end headphones more expensive than it. It has a really small second harmonic and that's about it! Higher order distortion is virtually non-existent. The HD650 is so linear that even when turning the volume up, it still doesn't seem like it's loud enough! The sound never gets thick or harsh. And there is no grain at all in the sound. The HD598 has some graininess in the mids and highs that is ever-present indicating odd order harmonic distortion. The HD428 isn't grainy, but has a bit of a smoothed over thickness or lushness in the mids and lacks articulation, indicating even-order harmonic distortion. But generally, the even-order harmonics are not as offensive as the odd ones. Headroom's graphs back up what I heard from all three. The HD650 is a real winner here.Since harmonic distortion will necessarily produce intermodulation distortion when multiples tones are present, IMD is never nice to hear. I can back this up with an example. Corona's Rhythm of the Night R.B.X. Euro Mix is a well-mastered Eurodance song that has a strong grinding bass. Once the main bassline starts at 0:36, and particularly at 0:42, the HD598 outputs a significant amount of IMD at very high volume. The bass and treble become garbled and the song becomes an unenjoyable mess. The HD428 chokes too. Occasional pops can be heard since the smaller driver doesn't have enough excursion capability. The HD650 is totally unfazed by the high demands, rendering the controlled bass with clarity, impact, and weight with appreciable bone conduction as slight vibrations are also felt through the front teeth when both rows are touching. I do not recommend listening at such high volumes for any extended length of time for obvious reasons. In fact, I hear more nuances in the music at more moderate to low volumes. It may not be as stimulating that way, but it's better quality listening.I also found three easy examples of frequency response differences among all three cans! In the CD Maxi Single of DJ BoBo's Somebody Dance With Me, at the female vocal section "I've got this feeling, somebody dance with me!", the words "feeling" and "me" are emphasized. With the HD428, they are recessed in amplitude and thus a bit dull and lacking in dynamics. With the HD598, that pitch is where a narrow peak occurs, so it's shrill and bright sounding there. And with the HD650, the response is perfectly balanced with controlled dynamics. Likewise, with 2 Unlimited's No Limits, the female vocal part "There's no limit!" the "limit" is a similar pitch to the above example and will exhibit the same thing. Again, the HD650 has such a wonderful and natural timbre in the critical high mids. And an easy example of the HD598's emphasis in the upper mid to low treble transition is Sonic the Hedgehog 3's spin-dash sound effect. It is very loud and strident. On the HD650, it's so clean, balanced, and it's easy to hear the real clarity and "fineness" of that sound. Now which headphone is more veiled? ;) There's no hype in the highs with the HD650. Hyped-up highs seem to be the new audiophile Flavor of the Times.In terms of source material requirements, lossless files or 320kbps MP3s would provide optimal sound quality. 128kbps MP3s sound thinner, particularly in the bass and midrange, due to the missing data. Also, songs whose average volume range from about -12dB to -18dB FS will showcase the excellent micro and macro dynamics and effortless sound projection of these headphones. The majority of pop, rock, and dance music up to the mid-1990s hover in that range, whereas nowadays most music is unfortunately compressed to a loud overall level with hardly any dynamic range.Next up is the physical inspection of the transducers. The HD650's 40mm Duofol diaphragm (part number 600129 minisys) has a large dome with a noticeably spherical or rounded surface area. Although the dome's diameter is slightly larger than the HD598's, the latter's dome is flatter and deflated looking in comparison to the 650's. The HD428 has a very tiny dome and voice coil (the latter seems to be only about 10mm), which will naturally limit its bass performance.The last section is the awesome spatial sound. With the HD650 the stereo image is psycho-acoustically perceived as occurring in a free and open space. It is instantly recognized by the auditory system like so. I get no sense of the ear cups creating any sort of boundary to the sound. Whether it's real-life acoustic or synthesized music, I can sense the soundstage size of all the different songs I have in a realistic way that's absolutely natural and also true to the source.Don't get too caught up in Sennheiser's marketing when they say 45 degree angled baffles by themselves automatically create a speaker-like sound as that may create a sighted or expectation bias. There's a near negligible difference in the radiating point of a mono signal when I listened to angled (HD598) and straight (HD650) baffle design headphones. Yet the HD650 easily beats the HD598 in width, depth, height, and separation even though it doesn't have angled drivers. I'll explain why the spatial sound is superb. The 650 has close-tolerance drivers (max. 1 dB difference) meaning both left and right channels have nearly identical phase (time) and frequency (amplitude) response, which are required for good imaging. So in that regard, the HD650's transducers are reproducing the electrical signal (i.e. the source) with much greater accuracy. Pink noise is perfectly centered without the slight shift towards the left like on the other two headphones. The sine sweep is also centered all the way from 20Hz to 20kHz and does not pan to the left and right with the upper mids and treble like the lower-end models. And compared to the other models, during stereo playback the HD650 has a solid phantom center that is present (thanks to the constantly centered sine sweep), while the left and right channels are simultaneously heard separately just outside the pinna (instead of in the ear). Remember that "stereo" is derived from the Greek word for "solid." And what's solid is what's in the middle.Elaborating a bit further on the previous statements, the center portion of the mix is *distinct* and is experienced as *separate* from other sounds that are placed towards the left and right while they are *simultaneously* occurring. The asterisks denote the differences between the HD650, which has precision-manufactured drivers, and the HD598, which does not. For example, in Madonna's "Dress You Up", the beginning part where she says "you've got style" there are reverb tails of "style" at the left and right that are separate from the main center with the HD650, but not with the HD598. Any slight pan or shift during the sweep will result in lack of stability in stereo imaging performance. For instance, in Phil Collins' Another Day in Paradise, during the chorus, "Cause it's another day for you, you and me in paradise" the HD428 shifts the "me" to the left of center compared to "you", since the former is an upper mid sound while the latter is a lower mid sound. The HD650 has no flaws here. And phase information in the music (e.g. reverberations from the instruments in the hall in classical music) seem wide open. And the totally open chassis design further allows the sound to radiate out without contributing its own reflections.The HD650's earcups utilize a polyamide 6 and glass-fiber construction (PA6 GF) that is quiet and solid when the gloss black portion adjacent to the earpads is lightly tapped with a fingernail. Likewise, only that nice distinctive stainless steel clang can be heard when tapping on the grille when the headphones are worn or held. Doing the same thing to the HD598 (which utilizes conventional ABS plastic) reveals a hollow plastic sound. Indeed, this attribute (lack of resonances in the housing) along with the aforementioned driver matching contributes to the sense of transparency in the HD650's stereo imaging, in that a pure audio image that consists only of time and level differences is formed in the mind.It's also finally worth mentioning that the HD650, since its distortion is so low, will reveal the relatively high distortion from tube amps. So this "scaling with expensive amps" mentioned in the opening paragraph essentially means listening to the various amounts of second harmonic distortions of tube amps. I personally would recommend pairing the HD650 with a nice solid-state amp. It doesn't matter how much that amp costs, but about getting one that measures well (flat response, no distortion or noise), has near zero output impedance, and provides enough voltage (anything in the 2 to 3 volts RMS range is absolutely sufficient) for a powerful and exciting dynamic range. In the audio industry, higher priced gear does not always translate into better performance. This industry has some strange things going on now, like ridiculous price differences between the high-end models. Considering I bought the HD428, HD598, and HD650 new from Amazon last year for $50, $180, and $350 respectively, it seems a bit odd for the next two models to cost $1000 and $1500.The HD650 has a full and well-balanced frequency response, extremely low distortion with its clean, pure, and smoothly detailed sound even at very high listening levels. It has no discernible diaphragm or earcup resonance which would otherwise produce a peak in the mid-range or treble response and has a wonderful, naturally open spatial sound. Sennheiser may never voice a reference headphone this nicely ever again or at least for a long time to come. What more is there to ask? The HD650 easily satisfies the criteria for true high-fidelity reproduction.

(If you're wondering how these compare to the Sennheiser HD 600 Open Back Professional Headphone, please see the bonus section that follows this review.)These are, bar none, the most wonderful headphones I've ever heard. For perspective, I have or have had: Audio-Technica ATH-M50 (of course) and M40x, Fostex T50RP MK3, Oppo PM-3, Philips Fidelio M1MKII and L2, and both the Sennheiser HD598SE and HD600. None of these but the HD600 come close. If you're considering the HD598 as a lower-priced alternative, it's a great headphone in its own right (especially if you don't want to go down the rabbit hole of choosing a suitable amp), but there isn't as much family resemblance in the sound as you might think.Still, it was through buying the 598 that I decided to take the plunge on the 650—Sennheiser's 5xx series is clearly a gateway drug to the 6xx series, which fortunately is so good there's little reason to step up to the 700 and 800 besides bragging rights. There's a lot of warped machismo in the world of audiophilia, and to the greatest extent possible I'd like to avoid that in this review.Simply put, the 650 is about the best all-around headphone you can buy for its current Amazon price of $315.99. If you already have an amp and/or a dedicated listening setup and you've never heard these, you owe it to yourself to try them. Worst-case scenario, you don't like them and you're out $6-7 for a return label. Best-case scenario, you find out just how good (and yes, also how bad) recorded music can sound. These are very much the BMW 5-series of the headphone world: there are bigger, faster, and more expensive options, but these remain the perennial gold standard. Maybe they're not your style, but there's no denying their refinement and artistry.I think they sound absolutely perfect. Some complain they're too "laid back," even "veiled." I suspect those people just don't hear very well, because if you have sensitive hearing, "exciting" headphones are a euphemism for "death by treble." I have dog-like hearing, and most of the time it's more curse than blessing. Even with the 650, the treble can be brain-piercing with the wrong amp.As for amp recommendations, this is a contentious and fiercely debated topic. As a general rule, you should spend the most on the final device in your playback chain (speakers/headphones), and progressively less on the "upstream" components: amp, DAC, media player. Given a budget of no more than $300 for an amp, you could buy the excellent Aune B1 Class A Portable Phone/Headphone Amplifier and still have enough left over to pay for a year of streaming music. Of course, if you want to pair the 650 with "endgame" gear, by all means do! The 650 has a reputation for "scalability," meaning its sound will (allegedly) continue to improve with better amps, DACs, recordings, etc. My own gear is fairly modest, so I can't speak to how this sounds plugged into a $5,000 amp or rewired with $700 cables. I will say that it REALLY comes to life with a tube amp (I have a Darkvoice 336SE), but I completely understand if that's a can of worms you'd rather not open.All I know is that the 650 sounds good enough to be the end of the headphone rainbow for me. The audio hobby is usually a desperate, futile struggle against diminishing returns, and as far as I'm concerned, the 650 IS that threshold in the headphone world. In other words, you can spend ten times more, but it won't sound ten times better.Given that, you might be wondering whether you even need an amp with these. "Need" is a strong word, and in the context of expensive headphones it's pretty hard to maintain a sense of perspective. I've done the unthinkable and plugged these directly into my iPhone's headphone jack, and you know what? They STILL sound phenomenal, but only for easy music. What's easy music? Pop, rock, basically anything recorded to be uniformly loud. Without an amp, the 650 can fall apart on classical and jazz recordings with a wide range between loud and quiet sections—there simply isn't enough power on tap to move the diaphragm as nimbly as this sort of music demands. If you never listen to instrumental music, you might be able to get away with not using an amp, but I can't recommend it. Buying such marvelous headphones and not amping them properly would be like buying a beautiful painting and not framing it. You can do it, but for a little more money you might as well get the full effect.Let me end with some totally subjective words that describe the sound of the 650: rich, natural, musical, effortless, clean. Now some words that could just as well be applied to a fine wine (the combination of status anxiety, groupthink, and reckless cognitive bias unites both hobbies): broad-shouldered, velvety, well-mannered, savory, harmonious. In either case, a smooth finish. :)Bonus section: HD650 vs HD600Obviously if you've read this far, you're deep enough into the audio hobby to have realized an important psychological truth about yourself: you can't leave well enough alone. Neither can I. That's why, even after falling head over heels in love with the sound of the HD650, I ordered the HD600. I just had to know.The verdict? It's almost too close to call, but for my preferences the 650 is the better headphone. The two have been compared to death online, but if you have the capacity to think for yourself, audio review sites and forums are usually an unsavory if fascinating combination of shilling, self-justification, and "follow the leader" parroting of received opinions. Many self-proclaimed "audiophiles" have strong opinions about equipment they've never actually heard, which I can't accept.Given that, I decided the only way to compare the two headphones honestly was to listen to both myself for hours, going back and forth on a wide variety of recordings. The difference between a good recording and a bad recording dwarfs the difference between lossy and lossless, and the HD600 (more so than the HD650) may end up changing your taste in music because it makes good recordings sound SO GOOD and bad recordings sound SO BAD. In other words, it reveals the "truth" of the recording, and sometimes the truth hurts—it's really hard to enjoy The Killers now.The fact is that the 600 and 650 are ultimately more alike than different, and the popular insistence that they sound completely different has more to do with what Freud called "the narcissism of small differences" than the headphones themselves. If you Google some variation on "HD650 vs HD600," you'll hear over and over that the 650 is "bassier" or "darker." Not really. There's actually very little difference across this parameter. Piano music presents an exception, but this is rarely what people think of as a bassy genre.There are two significant differences. The first is soundstage. The HD650's soundstage is wider, but that doesn't mean it can make a cramped recording sound airy. The HD600's soundstage is more intimate; there's less space between left and right channels, but it's not a huge difference, and I could see people preferring (or at least not minding) the HD600's tighter spacing.The most striking difference between the two headphones is in what I'd call "smoothness." The best analogy I can think of comes from digital photography. As an image sensor increases its sensitivity, it also increases its noise because you can't amplify a signal without also amplifying noise. There are two rival philosophies for dealing with noise: leave it as "grain" or smooth it out. Grain has its devotees, and the advantage of grain is that it preserves fine detail. The disadvantage is that past a certain point it becomes a distraction. The advantage of smoothness, meanwhile, is that it looks superficially nicer. The disadvantage is that when you look closely you won't see all the fine details. Like all things, it's a continuum of compromise. The 600 is the headphone to get if you value detail at the cost of an occasionally unpleasant grainy and even metallic quality to the sound. The 650 is the one to get if you care more about music than sound and don't care if your headphones sand down the sharp edges of your music a bit.Going back and forth between the two, it quickly becomes clear that the 650 is the stronger all-around performer: it sounds nicer on more recordings and across more genres. The 600, meanwhile, is the champion of a particular niche: good recordings with lots of micro detail. Want to be able to count how many times the skin of a drum reverberates after it's struck? Want to hear such fine vocal gradations you'll know how a singer felt during recording? The 600 is for you.Make no mistake: at its best, the 600 produces the most astonishingly detailed sound I've ever heard. Unfortunately, at its worst it sounds grainy, jumbled, and not particularly musical. I'm sorry to report that piano music sounds particularly off on the 600: low frequencies disappear and take the fullness of the keys with them. Vocals can sound oddly recessed, even far away. In general, the 600 seems to fare better with female vocals than male ones, and with strings over pianos. It's absolutely glorious for acoustic guitar, but then, so is the 650.The 650's great advantage lies in its ability to bring out vocals like a spotlight. They stand out so clearly and powerfully from the instrumentation that you'll feel like you're hearing your favorite songs for the first time. The effect is really quite incredible: it's like there's a special sonic column reserved for vocals and unpolluted by other sounds. Based on Sennheiser's own specs, the 650 has lower distortion than the 600 and it's clearly a more refined driver unit. Whatever the technical reason, the 650's background is pitch black: sounds rise from and fall back into a sea of silence. Be forewarned: a well-recorded vocal track through the 650 may bring tears to your eyes, and that's why I kept the 650 and returned the 600.