A Rare Issue with Sennheiser HD6-- Headphones

First and foremost, I'd like to thank @ext1 for taking the time to chat and walk me through a bit of an annoying troubleshooting process. Hope things are well on your end, dude, and that growth is steady and sustainable.

For a bit of background, I'm using a Frankenstein-ed pair of HD600s at the moment because I'd accidentally dented the one diaphragm trying to clean some dust of its surface. Nearly everything about the pair currently sitting on my head as I type this is a stock marbled 2014 variant of the headphone save for the actual driver units themselves, which were produced in 2019. I believe I got them from @joch a few years ago-- they arrived in a Dynamat-modded baffle, and much as I liked em I just felt I preferred the older set somehow even after removing the Dynamat.

Did a bit of a comparison and even with plain FR, the differences were very much obvious (same pads and grilles used):

2014 HD600 (GREEN); 2019 HD600 (BLUE)

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MiniDSP EARS measurements are far from perfect, but the above results were repeatable, and more importantly they tracked with what I heard; the older HD600 pair just had more bass annd a smoother treble response overall. I swapped some bits around and basically came to the conclusion that the baffles were largely responsible for the low end on the older HD600 while the mid-upper treble prominence (at almost precisely 12kHz) was inherent to the drivers.

Eventually, I settled on using the 2019 diaphragms in the 2014 baffles (RED trace below), and according to Jupiter this is somehow similar to a stock black screen HD600, but with more prominent bass (no wonder I liked the silly thing so much):

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Things were fine for quite a few years afterwards, but given how cats have a penchant for messing with cables and knocking things off desks, it was only a matter of time before something happened and the Senns would make the acquaintance of a hardwood floor... several times. Things were fine for the most part, but one thing I'm coming to realise now is how imaging precision on the Magni Piety and MCTH aren't particularly pin-point precise.

I got a Magni Unity in a few weeks ago, and immediately upon plugging things in I felt things were very slightly off-center. This coincided with yet another episode of Sennheiser-meets-floor after one of my cats would get snagged on the cable in the middle of running around and drag things down with her.

At first I think that it's the amplifier exhibiting channel imbalance, because when I go back to both the Piety and the MCTH, it's nowhere near as much of a bother. To my surprise though, running FR sweeps (this time with a handheld dynamic microphone because my constant tinkering seems to have rendered the MiniDSP EARS unusable) showed consistent behaviour (please note the labels on the bottom; I've isolated a single pair below to make it a bit easier to see):

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Naturally, I'm chagrined and I panic. I go over everything with a watch loupe and test to make sure everything is fit into place. The foam behind the one magnet seems to be a mite thinner than the other, but that seemed unlikely. On @spoony 's advice I tried adding some micropore tape to dampen the greater excursion on the hotter left channel, but that mainly seemed to skew the whole response slightly counter-clockwise, not nearly enough to alleviate the midrange imbalance.

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After a while of this bothering me on and off (usually coinciding with my moving to the Magni Unity when the treble on the Piety gets on my nerves) I swallow my pride and message Jupiter directly. This may come as somewhat of a surprise but I actually hate being a bother to people so seldom message unless I feel it's a big frickin deal. I send over some photos and videos, and he very kindly helps walk me through some things I could do to test things out. Eventually I'm asked to try pushing the diaphragm in by the voice coil, and that's when I notice that it felt like the left diaphragm could be displaced a bit more before making contact with the magnet:





So maybe the diaphragms produced within the last few years (settling in issues in the Romanian factory?) aren't quite as robust as older ones. I recall @MLegend had a driver failure on their pair after about two years, and that'd be around the same generation as the "new" HD600 drivers I'm rocking here.

Here's the real kicker though, according to Jupiter, who has possibly handled more Sennheiser headphones than anyone else outside of Sennheiser factory personnel, this is exceedingly rare:


I feel fortunate to be part of this community because I have been the recipient of much generosity and kindness, but between this being a rare failure mode and the Campfire's Solaris apparently not liking my ears (thin ADLC drivers possibly getting perforated) it really does feel like what luck I have is trying very hard to keep me in check.

So there's a possibility that, over time, the diaphragms in Sennheiser HD600s/HD650s/HD6XXes may deform with use. At least in my case it's an extreme outlier in that I'm mishmashing parts together-- the 2014 diaphragms take more effort to snap into the 2019 baffles than the 2019 diaphragms do, which makes sense since they weren't made to fit one another, though I don't necessarily see how slightly more pressure on the outer perimiter of the diaphragm housing could accelerate this in one channel more than the other unless the left housing was just slightly softer than the right one.

This doesn't seem to affect technical performance in any meaningful way; other than imaging being slightly panned to the left, all the expected aspects of HD600 (with pancaked pads) performance like beautiful tonal balance, great detail retrieval with standout microdynamics, intimate headstage with good depth, and surprisingly satisfying bass with correct upstream are all intact. Out of the Piety and MCTH it's not much a bother at all, though it's certainly evident now when I'm really looking for it on certain tracks, mainly stuff with a lot of lower mid information like close-miced vocals.

I got these headphones from a good friend so I'm keeping them, just maybe going to be begrudgingly looking for a used pair to have on hand as a channel balance sanity check when circumstances permit another stupid expense, if not moving to other "main use" headphones altogether once able. orz

 

Have you tried swapping L and R pads or drivers around? As you realized the baffles affect the response quite a bit, maybe one got some damage? Have you tried swapping the cables around?

Might also take new pads. Or maybe it's like you said and it's indeed a driver mismatch. Sometimes swapping things around you can find a config that produces less of a channel imbalance.

Unfortunately I have an imbalance with my HD800 nowadays, too. One side consistently has less bass and treble extension. Not much I can do in that case, though.

 

So to be clear I don't think pads are going to be as much of an issue because this is the microphone i'm using to measure things, again because the MiniDSP EARS are on the fritz. No way to get a seal, I just shoved the mic right up against where the driver was:

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I did go over the headphones a ton and Jupiter likewise lent a second pair of eyes as best he could via my photos and videos, but things all seemed fine with respect to the baffle and capsules themselves.

I did try swapping the capsules around. These are older measurements from right before the EARS died, can retake with the dynamic mic when I have the time. First is how things currently are (definitely already some weirdness here, but this wasn't as bothersome to me really and I can't be sure it's not the amp I used having a bit of imbalance at whatever volume position I was in here):​

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And this is what happens when I swap the capsules between baffles. Very much weird stuff with the "left" diaphragm (currently in right baffle) still producing a much hotter signal:​

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Honestly the FR from the cardioid mic shoved into the capsule looks sort of accurate. More so than I got with my cardioid mic, heh. Most impressive!

So yeah, looks to be the driver :/

 

Haha, other than the low bass and high treble rolloff you mean? Yeah I'm very fond of this mic (pedantics: it's a supercardioid) because other than a bit of upper midrange hash (easily offset by placing it off-axis which'll help with plosives either way) and pretty strong proximity effect that'll end up leaving it a muddy sounding mess if I get too close to the mic (my HD600s are nowhere near as bassy as the more recent FR graphs might suggest), it's solid.

Manufacturer-provided FR of the mic (audiotestkitchen doesn't have this one) and a quick vocal sample for fun, so you can maybe see how it influenced the 3rd, 4th, and 5th HD600 FR measurements above, keeping in mind that the mic was pressed right up against the baffle grille for consistency's sake. [VOCAL SAMPLE HERE].

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@Lyander So, to be clear and to update, my pair definitely have some channel imbalance, more than the normal amount of driver variance, but the absolute driver failure I was hearing and seeing with measurements is completely gone, for now.
If I can replicate what I originally saw when I first measured my pair with the Midgard, I will post the data here.

I originally noticed the issue when I was testing and using the Schiit Midgard for the first few days. My pair sounded as if one side had a little more bass and was more full sounding. The center image was slightly shifted to the right, but bass sweeps sounded mostly centered, which made me think it was just an ear pad position/seal issue or my own hearing. This didn't really bother me and other than that, it sounded normal enough to keep listening.

When I first took measurements, the difference between the left and right channels was strange and baffling. Compared to the right driver, the left response had similar treble behavior, but the bass started rolling off around 800hz-1kz and the whole response was around 5db quieter. I was a dummy and didn't save the measurements, simply thinking that the driver was toast and sharing the data would serve no purpose. However, when I actually listened to music and ran sweeps in the left channel, it didn't sound that bad, so I thought either my hearing was going or my brain simply adjusted to it. I took apart the capsules and cleaned them as well as the foam discs to check if any stray hairs were causing the issue or if the driver had somehow become loose from its position. Measuring it again resulted in the same response, so I thought the driver was dying.

Since I couldn't find replacement drivers to buy anywhere, I just listened to them without thinking about the measurement issue because they sound usable enough to not make me want to purchase another pair.

I wanted to see if I could measure the difference I was hearing between single-ended and XLR out of the Midgard with waterfall and spectrogram plots, so I measured my pair again recently and the abysmal response I was getting out of the left channel was gone. I have no idea what caused the original problem. I couldn't replicate the issue no matter how many times I adjusted positioning and re-measured.

@Lyander A couple tests I use for channel balance that work for me and me might be useful to you are these.
https://www.audiocheck.net/audiotests_stereo.php
https://www.audiocheck.net/audiotests_polaritycheck.php

I mainly use the in-phase guitar sample to check channel balance nowadays when I hear off centered imaging while listening to music.

 

@MLegend Thanks! I use audiocheck for a lot of stuff but I mainly use REW pink noise to check for balance whenever I suspect something's off. I don't often have reason to check components for balance since my rigs tend to remain static for long periods of time. Never thought of using the absolute polarity check for that sorta thing, though, nice.

Ahh gotcha, the behaviour you were experiencing was different from mine where the imbalance is more just a midrange thing in how it manifests. Earlier bass rolloff seems to be more common with the Senns, and even Solderdude's measurements of an HD600 exhibited the same, though not to as great a degree as yours. With how much minutiae can affect headphone response it's a miracle we can ever get usable results out of these things.

Whatever the case may be, I do hope that your Sennheiseirs don't exhibit this issue moving forward. Academic curiosity notwithstanding, it's a pain in the neck to have to deal with.

 

@Lyander I should record a small vocal snippet like yours of the two cardioid mics I use (don't have a super cardioid, though (yet!)).
One is quite neutral and the other one has a bit of that K-47 upper mid to lower treble emphasis that I love.

It's not pleasant, but for anything and everything that requires me to check for channel imbalance or FR issues I like to use sine sweeps, specifically this website:
https://www.szynalski.com/tone-generator/

There's a large slider to adjust the frequency and smaller ones for volume level and channel balance.

 

I voted "DAMNIT, LYANDER" but not because I feel that way, I don’t think I've ever felt that way (well maybe a bit when you made me freak out about Bandcamp FLAC's ) I voted for that because it is just clearly the best answer.

 

x2