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Reversible AKG K701 bass mod

Discussion in 'Headphones (full-size)' started by papamogl, Apr 18, 2013.
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  1. ThurstonX
     
    +1 re: those tiny screws.  I lost one inside once and ended up having to resolder the red and white cables.  Fortunately, none the worse for wear.
     
  2. bluemonkeyflyer
    The Reversible Bass Mod elevates the bass and slightly attenuates the upper mids and treble frequencies as described by many others in this thread. The sound quality changes are audible and an improvement, IMO.
     
    I'm going to try felt and other materials to partially open/block the bass ports and some ear side of baffle mods, too.
     
    My simple DIY measurement kit is shown and my methods described HERE.
     
    Here are my graphs. Take them FWIW. 
     
    ~BMF
     
     
    akgq701bassmodspectrogram.jpg akgq701stockspectrogram.jpg
     
     
    akgq701reversiblebassmodwaterfall.jpg akgq701stockwaterfall.jpg
     
     
    akgq701reversiblebassmod.jpg akgq701stock.jpg
     
     
    akgq701bassmod.jpg
     
     
    akgq701bassportmodfullyopenir.jpg     akgq701stockadhesiveir.jpg
     
     
     
    EDIT: VARIABLE Bass Port Mods and Measurements
     
    IMO:
    1. Stock = Sealed Bass Ports ---> Not enough bass
    2. Bass Port Mod = Fully Open Bass Ports after removing the Stock Tape Disk from each driver makes a 2.5 mm bass port ---> Too much bass
    3. Partially Open Bass Port Mod = 1.8 mm hole using Double-Sided Tape ---> Just right amount of bass
    4. Partially Open Bass Port Mod = 1.3 mm hole using Double-Sided Tape ---> A little more bass than Stock
    5. Remove ear side foam disks = Too much bass
     
    Compare to the graphs, above, and draw your own conclusions:
     
     
    akgq701bassportmodwith1.3mmholespectrogram.jpg akgq701bassportmodwith1.8mmholespectrogram.jpg
     
     
    akgq701bassportmodwith1.3mmholewaterfall.jpg akgq701bassportmodwith1.8mmholewaterfall.jpg
     
     
    akgq701bassportmodwith1.3mmhole.jpg akgq701bassportmodwith1.8mmhole.jpg
     
     
    akgq701variablebassportmods.jpg
     
     
    akgq701variablebassportmod1.3mmir.jpg    akgq701variablebassportmod1.8mmholeir.jpg
     
     
    EDIT: Here is a photo of double-sided tape with a 1.3 mm hole. 
     
    photo41.jpg
     
    To my ears, a 1.8 mm Variable Modified Bass Port falls in The Goldilocks Zone and sounds better than Fully Open (Stock Tape Disks Removed), Fully Closed (Stock Disks in Place), and 1.3 mm Variable Modified Bass Ports.
     
    I used a hole punch with 6 different sizes. The smallest size punch = 1.8 mm and makes a uniform hole in each double-sided tape disk. I usde a thumbtack and toothpick to make the 1.3 mm holes in disks shown in this picture and the holes are not uniformly round. A hole punch is recommended.
     
     
    turbobb's youtube video shows how to get to the bass ports...Well Done!
     
  3. Dafo
    Spectacular view and very comprehensive, nice work.
     
    bluemonkeyflyer likes this.
  4. bluemonkeyflyer
     
     
    Props to 'oldidi' on the German site and to you for sharing this information.
     
     
    After my Trials and Measurements, I agree with you.
     
    The Bass Port Mod is also Variable. The effect is "tunable" by varying the size of the bass port. 
     
    The Stock Bass Ports covered by the adhesive disks is ~ 2.5 mm in diameter (at the "tube," not at the flare at the top. Punching a 1.8 mm hole in the Stock Adhesive Disk and re-applying it over the Bass Port brings the bass to a level that sounds better to me than Fully Open.
     
    Cheers,
     
    ~BMF
     
  5. alphaman
    Yes, that wimpy, slamless bass is an important drawback of the k701/702. 
     
    I have not tried any version of the mod yet, but the concept is intriguing. WRT the stock unit, I wonder why incorporate a reflex port when it's to be covered up at the factory? Maybe the designers found an easy way to present two different sonic signatures during the prototype phase ... the no-port version got voted in 51/49 (out of 100) im Wien, Österreich HQ auf Akustische und Kino-Geräte Gesellschaft m.b.H., aka AKG aka HARMAN holdin' an AK47 ....but rather change the costly thermoplastic cast/die, just cover it with an adhesive tape "bodge" :wink:
    Me, I would've opted for a postage stamp or kids sticker ...
      il_340x270.607517497_l07r.jpg
    ...or better ...
    smiley-sticker-16699545.jpg
     
    LOL!!!!!!!!!!!!!!!!!!!!!!!
     
    The tunable aspect is also appealing. BTW... Is that stock adhesive disk porous?
     
    FYI: I've owned a pair of k701s since 2006 and long-term use has been an interesting evolution. These cans take quite a while to break/burn in ... but the resulting increase in SQ is worth the wait. 
    Another "easy" mod is to replace the cable (with one of better quality and shorter). It does not have to be $$ Cardas. I'm using JSC industrial Multi-Conductor Foil Shield 75°C, 300 V, SR PVC Insulation.
     
  6. Dafo
    "FYI: I've owned a pair of k701s since 2006 and long-term use has been an interesting evolution. These cans take quite a while to break/burn in ... but the resulting increase in SQ is worth the wait. "

    The little round piece of tape on the back of the driver is soft and strechy.
    What if this little piece of tape just gets softer and more responsive to the movement of the driver when used over time ("burn in")?
    That increase in sound quality over time may very well be close to just removing the tape as suggested in this mod, and you are right, that sound is indeed an increase in SQ :wink:
     
  7. alphaman
    Hmm...and only a smaller diameter of it covers the actual port hole.
     
    In dynamic loudspeakers, a type of similar implementation is called a passive radiator (sort of a dummy driver) :
    Basics15.gif
     
    Anyone with a zest to experiment care to use some other material?? .... say rubber-balloon latex.
     
    Or create a tuned/damped system, again borrowing from speaker design....
    qp_all.jpg
    ... using, maybe, the filter from a cigarette?
     
    A transmission-line system is used in some "tuned" loudspeaker designs, like classic TDL...
    Transmission-line.png
     
    ... so, for the k701s ... maybe use drinking straw ...the kind with accordion flex part ....
     
    .... but, no plastic straws for me, boyz  ....now where did I put my crack-pipe???
     
  8. alphaman
    While the change in elasticity (compliance) of that tape stuff may contribute minorly to burn-in improvement, it's removal (partial or full) seems to perform an entirely different acoustic function: increase bass. The burn-in for the k701, IME, qualitatively reduces glare/brightness and improves image focus. Those contributions come from changes in mechanical compliance of the main drivers as well as, perhaps, the "breaking in" of the wiring/coils/electricals.
     
  9. Dafo
    I find all this very interesting. If I was to design a closed back headphone, there is no doubt I would experiment with bass ports, the diameter, tubes and the length of these tubes into the chamber. I also find designs like the cone-shaped tube, the b&w "nautilus design" behind the driver very intriguing.

     
  10. Dafo
    Just a thought really. "Burning in" that piece of tape is probably not all.
    Could be fun though, then we could change the name of the mod to "The AKG 5 minute burn-in!" :wink:
     
  11. alphaman
    In traditional loudspeaker design, acoustic suspension (i.e. non-reflex or non-port or no passive rad. or transmission line) gives tight but wimpy bass. Whereas, reflex/ported/etc. gives more powerful (deeper/punchier) but not-as-accurate bass.
    I don't know the specifics of the k701's actual driver -- but there is a topical h-f driver thread, with some good info and pics....
    AKGK701driverfrombehind.jpg
     
    Also note that way back in June 2012, in the 701 vs. 702 thread, MDR30 noted (about the above diver image):
     
  12. bluemonkeyflyer
    The Q701 Bass Port is actually flared similar to the B&W Nautilus Bass Port. The flare diameter is ~ 2.5 mm and the "tube" is ~ 2 mm.
     
    The stock self-adhesive disk covering the Q701 Bass Ports looks impermeable to me and the glue is very strong.
     
    I tried to modify the tube effect with thin craft felt, 3mm open cell foam, and punching various hole sizes in double-sided tape vs silicone furniture bumpers. The felt and open cell foam had no effect, functioning pretty much like the stock self-adhesive disk. Changing the hole size in silicone furniture bumpers seems to allow for a tunable bass port: smaller = less bass, larger = more bass.
     
    Angle Hair over the ear side of the driver sucks out the lower mids around 500 to 700 Hz by ~ 5 dB.
     
    Removing the stock foam ring on the ear side of the baffle causes even greater suckout by ~ 8 dB.
     
    The best sounding bass port mod to my ears (so far!) results from:
     
    1. Fully open bass port after removing the stock tape disk + add an equal size Arctic Cotton disk overlaying the stock foam disk + Paxmate on the underside of the plastic grille receptacle, only at the bottom, leaving all 4 "vents" fully open (2 small at the top, 2 large at the bottom).
     
    2. Partially open bass port after removing the stock tape disk + silicone disk with 1.5 mm hole placed over the open bass port + self-adhesive craft felt instead of Paxmate, as above.
     
    I have measurements and photos of each mod combination. I will post everything in a new thread this weekend and post a link in this thread for anyone interested.
     
  13. ThurstonX
     
    I've experimented with a few different materials and configurations on my Q701s.  Currently, and probably permanently, I'm using squares of porous spongy foam ~2-3mm thick over the port.  They are significantly wider than the original stickers I removed, but do not overhang the plastic piece in which the port is set.  For me, it's a nice compromise between closed and totally open, but the type of foam does make a difference.  I also tried some painter's tape over the ports with pin holes poked in them.  That was OK, but I went back to foam.
     
  14. bluemonkeyflyer
     
    I tried a 10x10x3 mm square of camera repair open cell foam over the ports. Pivoting the the cups in the vertical plane caused the edge of the foam near the bottom of the driver to "flap" upwards. This essentially opened the port, negating the desired effect of a partially open, "tuned" bass port. I placed double sided tape under the bottom edge of the foam and it stayed put with cup pivoting.
     
  15. alphaman
    It's important to note that while metrics (objective data/graphs) may, indeed, improve as a result of various mod(s), the actual subjective sound may not correlate to those changed metrics. E.g., flattening of the freq. response may not appeal to all ears (as the ever-controversial diffuse field vs. free field results indicate). 
    One particular subjective (controversial) quality is PRAT. I find that even 'phones that display a clean/impressive impulse response may not, in fact, sound as snappy/punchy one that graphically looks (measures) worse. Another unquantifiable is long-term listening -- this may yield subjective results (e.g. listening fatigue) that immediate sonics (or metrics) just can't correlate to. In this case, it's not that objective science is wrong ... rather, it's that currently known-to-human science is missing something ... and may be discovered/quantified by some future da Vinci or Galileo. 
    Unfortunately, for logical-fallacy reasons -- like sunk-cost fallacy, etc. -- various mod projects (audio forums are full of them!) gain a sort of "pop inertia" ... like some quaint pop celebrity ... and that may very well be the case with THIS thread. Indeed, as I noted a few posts back ... that AKG was probably aware of the port vs. no-port sonic issue .... and they SELECTED to cover it up after important/informed/well-founded decision metrics.
    Hence, with  proposed mod projects , I almost always (now) either ignore or treat them with extreme skepticism ... a lesson that was learned the hard way
     
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