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Customer Review

59 of 65 people found the following review helpful
5.0 out of 5 stars A review of these legendary, yet misunderstood headphones, March 21, 2012
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This review is from: Sennheiser HD 650 Open Back Professional Headphone (Electronics)
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 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).

Elaborating a bit further on the previous statement, 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 a nice distinctive metallic sound 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.
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Showing 1-6 of 6 posts in this discussion
Initial post: Apr 3, 2012 9:53:15 AM PDT
Rocket Man says:
The problem with this Headphone is not with frequency response or distortion. Even properly amp.
It's just that some times the Bass is not fast enough. One note keep on going and going for too long and resonning.
When this sounding effect appear lots of arrangements and details are missed.
As a bass player I can tell you that the bass here is not natural.
Most of the people like a sound with A LOT of Bass and Sennheiser clearly knew that when they designed the HD 650.
You have to try a pair of Grado 325 I or AKG K 702 to compare and you will forget about this one.
The positive point of the HD 650 is the soundstage but their Bass presentation is definitevely not musical.

In reply to an earlier post on Jul 14, 2012 4:21:01 PM PDT
Dennito says:
Thank you for the detailed and technical review. This is the first place I've been able to find the size of the driver. Great review.

In reply to an earlier post on Sep 2, 2012 7:34:22 AM PDT
D. Pyle says:
This was the best headphone review I have read! Thank you for the time you took to do the tests, and for reporting on the objective aspects of a product that ultimately will be enjoyed (or not) subjectively. If there were a prize for best review ever, I'd nominate you!

Did you ever compare the HD650 with the HD600? I own the HD600 and they sound fantastic. Far better than the HD598 that you did compare. I'm considering buying a 2nd pair of the HD600, but if the price drops on the HD650, I may be tempted to buy and see if their slightly better specifications translate into better sound. Have you tested the HD600 for comparison with the 650?

In reply to an earlier post on Sep 2, 2012 8:00:53 PM PDT
Thank you for your comment. I have not heard the HD600s. Measurements for the 600s show a little more treble response while the 650s have more bass extension. Overall, both headphones are actually very linear sounding in frequency response. But the 650s have less distortion, even better driver matching, and a different plastic for the ear cups for a near resonance-free chassis. The HD650 (introduced July '03) evolved from the HD600 (introduced fall '96), which essentially is an HD580 (introduced spring '93) with a better grille and driver matching. It just goes to show that audio gear, when properly designed, will still provide a high-quality experience long after they are initially released to the market.

In reply to an earlier post on Sep 2, 2012 8:14:35 PM PDT
[Deleted by the author on Sep 2, 2012 8:14:46 PM PDT]

Posted on Sep 22, 2015 12:25:36 PM PDT
Gabriel H. says:
This is a very well written review! It's been a while since you wrote it, but I'd like to thank you Mr. Miceli for taking the time and posting your findings. As a result, I'm now a happy owner of the HD-650 (which I chose over a range of options, including it's sibling the HD-600 and the AKG Q701/K701). I pair it with an Objective2 amp and a Fiio X1 DAP and couldn't be happier.
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