Sennheiser HD6## Closed Back Exploration / Work In Progress

Discussion in 'Modifications and Tweaks' started by E_Schaaf, Apr 15, 2022.

  1. E_Schaaf

    E_Schaaf MOT: E.T.A Headphones

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    Almost everybody wants a closed back equivalent to the HD6## series that doesn't suck, right? Well, this thread is going to be a change log / stream of consciousness in pursuit of such a goal. I'm going to lay out some self imposed restrictions:

    1. The modifications must be reversible
    2. The modifications must be simple enough for anyone with the right materials to implement
    3. The modifications must not create any annoying dips or peaks in the tonal response, and retain the general character of the stock headphone
    4. The modifications must not significantly worsen measured distortion, nor create any ringing in the treble exceeding 1ms at -35dB.
    5. All measurements are taken at the same volume setting on my output device.

    With those things in mind, lets go!

    Here are some measurements of the stock headphone, using a DIY FPC rig and B&K compensation. Take this as a point of relative comparison more than an objective or 'correct' representation of the tonality of the headphone itself. This is a 6xx with slightly worn pads, what might be considered a typical set out in the wild:

    [​IMG]

    The first thing I wanted to do was see how far I could get with the most basic back cup I could find. So I found this one on thingiverse as a platform to make changes from (thanks to whoever Sposhal is for figuring out fitment):

    [​IMG]

    The first thing I did was add a centered port of the same size as the magnet vent on the HD650 so I could play with vent filters and varying degrees of openness (this will make more sense later). Here's the print:

    [​IMG]

    Here are some measurements of what you get when you use these cups with no other adjustments to the headphone:

    [​IMG]

    White in the image above is what you get if you plug the center vent completely, more or less sealing the cup like the original Thingiverse model. Red is what you get with the hole uncovered. Obviously these are both far from ideal and not terribly similar to the stock sound of the headphone. Naturally, the next step was to find an in-between, so I simply covered the hole with some adhesive felt that allows a small amount of air to pass. I do want these to be as acoustically sealed as I can after all, otherwise there'd be no point in doing a closed mod to begin with -

    [​IMG]

    Here's the result in red, vs the fully open port in white:

    [​IMG]

    What I'm looking for here is a reduction in narrow dips as a result from port interactions. The felt filter achieved that to some degree despite the fact that the tone is still not ideal - but at least now the port dip is broader and shallower from the midbass into the low mids vs before. More cohesive. Moving on.

    The next thing to do is to find some different earpads. Often times lowered bass amplitude is due to a lack of pressurization on the ear side of the enclosure. Sennheiser stock earpads are breathable and allow air to pass through to the outside much moreso than leather or suede. When you don't have enough earside seal on a closed back, it often exaggerates parts of the FR that are being affected by the cup's inherent resonances and phasing interactions. I cycled through a bunch of earpads and found the best midrange linearity with these:

    [​IMG]

    They are perforated only in the ear cavity and not on the exterior of the earpad, so they do trap a bit more pressure without boosting the baffle response too much. Here's the effect on the FR:

    [​IMG]

    Red above is with the new pads (and no front baffle foam), vs the stock pads and foam in white, with the felt-covered cup port previously pictured. Next, I wanted to see if I can generally smooth some of those dips and bumps a bit, so I added some very simple damping - a 50mm blue foam round behind the driver, also centered in the cup over the felt filter. It's a low density polyurethane foam similar to 'Aquazone' sold on McMaster Carr. It does compress a bit against the back of the driver when the cups go on, but still fits snug.

    [​IMG]

    Here's what you get! I'll go ahead and overlay with the stock 6xx since this is as far as I've gotten for now. Red is the config as described above, white is stock 6xx:

    [​IMG]

    A few things to note:

    1. A bit more sub-bass amplitude vs before. Nice!
    2. No more bump in the upper midrange. Above 800hz, It's flatter than before!
    3. Not much change in the treble response, in fact such a small amount of change it could even be attributed to nothing more than positioning on the coupler.
    4. The fairly broad ~3db dip centered at 400hz is not terrible. The stock 6xx tends to have a fuzzy lower midrange due to high-ish bass distortion. The dip feels masked and I don't notice it much... though I am going to put some effort into flattening it further.

    So what's next?

    1. I'm going to scrap the thingiverse cup design aside from copying the edge fitment. The shape of the cups as-is obstructs external rotation against the headband, which is ergonomically annoying. And they look kind of boring.
    2. I plan to print the entire back face as a non-solid high-infill pattern, so that I can adjust airflow out of the back with more flexibility than with a simple centered port.
    3. Piggybacking on (2), I plan to use thin adhesive neoprene on the back wall of the cup. It's closed cell and does not allow air to pass through, and has great absorptive properties (or at least better than exposed plastic). With this material, I can do differently sized center port cutouts to further adjust the port tuning w/ the felt throttle.

    Anyway, not too bad for a couple days of effort! Pardon the 58X headband on these 6xx capsules, it's just what I had lying around (all my Senns are in parts).

    [​IMG]

    My longer-term plan for this mod is to offer the STL files for free for anyone with access to a printer to make themselves. I'll of course link all the materials used for the mod so anyone can DIY. Or, if you're not inclined to 3DP and source your own materials, I plan to sell low cost 'kits' with the cups and just as much materials as you need to do it yourself. Of course that's a ways off - still much to do on the sound and the look.

    Thanks for reading and I'm open to questions too - I can always take more 'what if' type measurements as well. I'm not paying too much attention to CSD and distortion post-mods just yet until I dial in the tone a bit more, but I'll get to it soon :)
     
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    Last edited: Apr 15, 2022
  2. Armaegis

    Armaegis Friend

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    You gotta change the thread title to HD6## or something, because the HD6 is an entirely different headphone.
     
  3. E_Schaaf

    E_Schaaf MOT: E.T.A Headphones

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    Good point, I dunno if I have privileges to change thread titles, paging @rhythmdevils for help
     
  4. rhythmdevils

    rhythmdevils MOT: rhythmdevils audio

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    Nice work @E_Schaaf ! Very interesting mods. As always ;)

    I don't know if HD6## is what you wanted, but I changed the thread title.
     
  5. yotacowboy

    yotacowboy McRibs Kind of Guy

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    This is so cool. I've often wondered how much the Thiele/Small parameters of a headphone driver and cup volume of a headphone, and lossyness of a semi-open baffle like with the HDXXX series interact/affect the response. Similar to the lossy driver/baffle cabinet drawings @k4rstar posted in the good sound club thread, i think.
     
  6. bobboxbody

    bobboxbody Friend

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    How is the isolation from outside sound on these, compared to say Fostex TH series?
     
  7. E_Schaaf

    E_Schaaf MOT: E.T.A Headphones

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    Interesting thought... honestly I have never tuned a headphone 'by the book' using the TS specs of a driver (and probably never will). The proportions are wildly off compared to speaker drivers in cabinets in rooms, and there's very little development space for any given wavelength since the earside space of a headphone is similar to or even smaller than the rear cup volume (and often bleeds over a little or a lot like you pointed out with purposefully leaky baffles)... I assumed - maybe wrongfully - these things would just throw everything off. I tend to take the approach of trying tons of iterations until it more or less clicks into place, though maybe using the specs could be a shortcut of sorts. Someone smarter than me can figure it out. I'm here for the chaos and adventure as much as the destination ;)

    Haven't measured isolation on them, but as a guess, probably pretty similar vs the Fostex TH series. Certainly not like a helicopter headset (unfortunately)
     
  8. Ti_Leo

    Ti_Leo Almost "Made"

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    That port reminds me of Sennheiser HD265! HD265 was closed back and there's a hole in the middle of the cup, and there's a sharp plastic thing that covered most of the hole, with 2 small one port opened.

    IIRC, HD265 had foam in the cups, and the pokey thing poked though the foam and reached to the back of the driver. I can't remember whether there's foam in the back of the drivers, probably not, but I'm sure if you swap drivers from, say hd650, the plastic thing would poke though the foam at the back of hd650.

    The plastic thing looked like some air guide to me, like injecting air directly to the back of the diaphragm.

    I still have that pair of HD265, they sounded ok-ish, I didn't try to review them so didn't have much impression. But I'm afraid I've lost both of the pokey things, sigh.
     

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