This is a review and detailed measurements of the Sennheiser HD650 open back headphone. I purchase mine back in 2017. It costs US $400 with free shipping as of this writing from Amazon. My sample is near new because I don't use it everyday (due to high noise level, I have to wear closed back headphones).
The HD650 feels rather plastically although it has been robust in the face my rough handling at times:
Here is a close shot of the driver in mine just in case there are variations in production that are visible such:
My history with this class of Sennheiser headphones goes back to 1999 or so. We (my team at Microsoft) had just developed our audio codec and wanted the labels to use it for online music distribution. I was invited to a number of major music labels (Sony, Warner, Universal, etc.) for codec shootouts. I asked around and "the" headphones of the time at the labels was the Sennheiser HD series (560?) , Stax and Sony MDRV-6 (?). I promptly bought all and used them all the time in testing our codecs. Happy to say we did well with all the major labels and got our codec approved for distribution.
I donated my old Sennheiser to my son years ago and don't know where it is. But somehow I remember the velour pads to be softer and more luxurious than the current set which I find a bit stiff.
Anyway, being one of the most popular headphones out there, the HD650 has been reviewed to death. The purpose here then is to not provide yet another review but provide a good baseline for our measurements and testing of other headphones since so many people are familiar with it.
The measurements you are about to see are made using a standardized Gras 45C. Calibration level is 94 dBSPL @ 425 Hz.
Sennheiser HD650 Measurements
There is nothing more important than frequency response of the headphone as each is seemingly different and that difference leaves a very distinct character:
Focusing on 100 Hz to 7000 Hz, we see that the response of the HD650 comes very close to our reference curve in dashed blue. This is interesting since the HD650 predates the research that led to the blue preference curve. No wonder then that HD650 is so popular.
Below 100 Hz, we fall short and short a lot when it comes to bass and sub bass region. This is likely to make the headphone sound colder and perhaps brighter.
Showing the difference between the preference curve and left channel we get:
You can see the shortfall in bass and matching above that much better now. Note that above a few Kilohertz, there are reflections and other inaccuracies so care must be taken in interpreting that region. So don't be alarm about the large variations reaching 10 kHz and above.
We can take a quick look at the "Group delay" and see the warning signs of where to be careful about:
Those massive phase variations indicated by sharp dips or peaks indicate mixing of reflections usually which can create troughs in the frequency response.
There is some kind of resonance between 200 and 300 Hz which we can (barely) see in the frequency response as well.
Distortion measurement shows that there is excessive amount of compared to the (few) headphones we have measured in bass region:
Looking at this as absolute level we get:
The HD650 has variable and high impedance:
The high impedance means that it likes to have "high" drive voltage (at least 2 volts in my experience). The variable impedance means that if you drive it with a headphone amplifier with high impedance, e.g. a tube amplifier, it will modify its frequency response. That modification may be to your liking, or not. Better rely on equalization as I will show shortly instead of a haphazard one using a tube amp.
Sennheiser HD650 Listening Tests and Equalization
The HD650 sounds like it measures. That is, it has a very good sound and tonality out of the box -- better than any other headphone I have tested. No wonder that I am often surprised how good it is when I go back to testing it for a few seconds after using another for a long time. You can however do much, much better with just a bit of equalization:
Dual shelf filters work together to lift the bass up. There is a shortfall around 8 kHz. And some resonance peaking around 13 kHz that needed to be knocked off. I tried other filters but their effect was not conclusive. This set transforms the HD650 into a superb sounding headphone. I am listening to it right now and can't put it down! Try this wonderful clip from Nils Frahm for example (track #2):
The Tidal version is far better but even the youtube one sounds wonderful. Let it keep laying and it lays very good low frequency tones which the HD650 reproduces with little difficulty. Its reproduction with equalization is better than vast majority of speakers in rooms.
Where is not so good is the spatial qualities. There is a nice open circle that the sound sources emanate from. It starts from your ears and circles forward. There is no sensation of space beyond that. Nor that nice layering that the Sennheiser HD800S which I recently reviewed brings to the table. I always get a feeling of the sound closing up in space when I wear the HD650 after wearing others.
If you are wondering about effect of equalization on distortion, here it is:
Edit: the original measurement was in error. This is an updated one with slightly more optimization than distortion graph earlier. Will sync the two later.
As expected, my large increase in bass level naturally increases the distortion there. But listen to this: without EQ you hardly hear those notes down in 20 to 30 Hz. No sense in having a clean signal you can't perceive! With equalization, those notes now play as if you have 20 Hz to 20 kHz reference quality speaker over your head. Don't miss on this opportunity to hear the full spectrum of the recording.
As noted on the graph, extra distortion in bass is not always perceived as such. Instead, harmonics actually help fill out the bass region giving one the impression of more bass, not necessarily more distortion.
Conclusions
There are a lot of sexier headphones out there than this old design but the HD650 not only holds its own still but with equalization produces an excellent high fidelity experience. If you are anti-equalization, you can still enjoy a lot of what this headphone has to offer.
Happy to recommend the Sennheiser HD650 headphone.
------------
As always, questions, comments, recommendations, etc. are welcome.
Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
The HD650 feels rather plastically although it has been robust in the face my rough handling at times:
Here is a close shot of the driver in mine just in case there are variations in production that are visible such:
My history with this class of Sennheiser headphones goes back to 1999 or so. We (my team at Microsoft) had just developed our audio codec and wanted the labels to use it for online music distribution. I was invited to a number of major music labels (Sony, Warner, Universal, etc.) for codec shootouts. I asked around and "the" headphones of the time at the labels was the Sennheiser HD series (560?) , Stax and Sony MDRV-6 (?). I promptly bought all and used them all the time in testing our codecs. Happy to say we did well with all the major labels and got our codec approved for distribution.
I donated my old Sennheiser to my son years ago and don't know where it is. But somehow I remember the velour pads to be softer and more luxurious than the current set which I find a bit stiff.
Anyway, being one of the most popular headphones out there, the HD650 has been reviewed to death. The purpose here then is to not provide yet another review but provide a good baseline for our measurements and testing of other headphones since so many people are familiar with it.
The measurements you are about to see are made using a standardized Gras 45C. Calibration level is 94 dBSPL @ 425 Hz.
Sennheiser HD650 Measurements
There is nothing more important than frequency response of the headphone as each is seemingly different and that difference leaves a very distinct character:
Focusing on 100 Hz to 7000 Hz, we see that the response of the HD650 comes very close to our reference curve in dashed blue. This is interesting since the HD650 predates the research that led to the blue preference curve. No wonder then that HD650 is so popular.
Below 100 Hz, we fall short and short a lot when it comes to bass and sub bass region. This is likely to make the headphone sound colder and perhaps brighter.
Showing the difference between the preference curve and left channel we get:
You can see the shortfall in bass and matching above that much better now. Note that above a few Kilohertz, there are reflections and other inaccuracies so care must be taken in interpreting that region. So don't be alarm about the large variations reaching 10 kHz and above.
We can take a quick look at the "Group delay" and see the warning signs of where to be careful about:
Those massive phase variations indicated by sharp dips or peaks indicate mixing of reflections usually which can create troughs in the frequency response.
There is some kind of resonance between 200 and 300 Hz which we can (barely) see in the frequency response as well.
Distortion measurement shows that there is excessive amount of compared to the (few) headphones we have measured in bass region:
Looking at this as absolute level we get:
The HD650 has variable and high impedance:
The high impedance means that it likes to have "high" drive voltage (at least 2 volts in my experience). The variable impedance means that if you drive it with a headphone amplifier with high impedance, e.g. a tube amplifier, it will modify its frequency response. That modification may be to your liking, or not. Better rely on equalization as I will show shortly instead of a haphazard one using a tube amp.
Sennheiser HD650 Listening Tests and Equalization
The HD650 sounds like it measures. That is, it has a very good sound and tonality out of the box -- better than any other headphone I have tested. No wonder that I am often surprised how good it is when I go back to testing it for a few seconds after using another for a long time. You can however do much, much better with just a bit of equalization:
Dual shelf filters work together to lift the bass up. There is a shortfall around 8 kHz. And some resonance peaking around 13 kHz that needed to be knocked off. I tried other filters but their effect was not conclusive. This set transforms the HD650 into a superb sounding headphone. I am listening to it right now and can't put it down! Try this wonderful clip from Nils Frahm for example (track #2):
The Tidal version is far better but even the youtube one sounds wonderful. Let it keep laying and it lays very good low frequency tones which the HD650 reproduces with little difficulty. Its reproduction with equalization is better than vast majority of speakers in rooms.
Where is not so good is the spatial qualities. There is a nice open circle that the sound sources emanate from. It starts from your ears and circles forward. There is no sensation of space beyond that. Nor that nice layering that the Sennheiser HD800S which I recently reviewed brings to the table. I always get a feeling of the sound closing up in space when I wear the HD650 after wearing others.
If you are wondering about effect of equalization on distortion, here it is:
Edit: the original measurement was in error. This is an updated one with slightly more optimization than distortion graph earlier. Will sync the two later.
As expected, my large increase in bass level naturally increases the distortion there. But listen to this: without EQ you hardly hear those notes down in 20 to 30 Hz. No sense in having a clean signal you can't perceive! With equalization, those notes now play as if you have 20 Hz to 20 kHz reference quality speaker over your head. Don't miss on this opportunity to hear the full spectrum of the recording.
As noted on the graph, extra distortion in bass is not always perceived as such. Instead, harmonics actually help fill out the bass region giving one the impression of more bass, not necessarily more distortion.
Conclusions
There are a lot of sexier headphones out there than this old design but the HD650 not only holds its own still but with equalization produces an excellent high fidelity experience. If you are anti-equalization, you can still enjoy a lot of what this headphone has to offer.
Happy to recommend the Sennheiser HD650 headphone.
------------
As always, questions, comments, recommendations, etc. are welcome.
Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
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