SkullCandy Hesh-2 Bluetooth Headphones Mod thread
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Starting a thread to document  some info/mod on the SkullCandy Hesh-2 Bluetooth phones. Lets see if the sound quality can be improved.
 
My reasons for choosing the Hesh-2...it is the over-the-ear type, with pleather pads, should provide good noise isolation, and more importantly, to me at least, it can be had for relatively little $, the refurbished sets are less than $40 sometimes.
 
impressions from the stock phones (~10 minutes listening):
*Bass quantity is good, but very much one-note, no much bass detail/texture.
* very lay-back presentation;
* Mids are actually quite ok, good voice; but not much highs.
* Resolution is actually ok enough for back ground music listening.
* over all, doesn't sound super bad for a pair of sub-$40 phones.
** my main complain at this point, is that the ear cups are small, barely covers my ears...not very comfy. 
 
then it was time to take it apart:
 
the ear pads pop right off...

 
....and take out the four little philips screws holding the cup. Left cup holds the battery and charging circuit....
 

 
 
...big 50mm drivers. no wonder the bass was big...
 

 
 
.... right cup holds the main circuit board. The headphone amp stage uses a SGM4812 chip.....
 

 
 
... drivers are press-fitted into the baffle, simply use a flat screw-driver to pry them out....
 

 
DC resistance of these drivers, is around 14 ohms, so they are probably 16-ohm nominal impedance.
 
Next step: lets replace stock drivers with some Koss KTXPro-1/KSC55 drivers and see how it goes. Stay tuned....
 
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I'm curious how this goes, and I'm sure drivers are the biggest factor, but I would have to think the amp and DAC are going to really hold things back and the cup designs. Still, great to try this and I'm hoping that you get some solid improvement. Cheers and good luck.
 
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SGM4812: the rated output power at 3v supply voltage, is 34mW into 16ohm and 27mW into 32 ohm, that is with 0.1% distortion. Certainly powerful enough. All I need is 10mW of clean power.
 
Since this chip is supposedly designed for tablet/phone use, I suspect the signal quality in the Hesh-BT is about as good as we can get from the headphone jack of a low/mid end tablet/smart-phone.
 
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  SGM4812: the rated output power at 3v supply voltage, is 34mW into 16ohm and 27mW into 32 ohm, that is with 0.1% distortion. Certainly powerful enough. All I need is 10mW of clean power.
 
Since this chip is supposedly designed for tablet/phone use, I suspect the signal quality in the Hesh-BT is about as good as we can get from the headphone jack of a low/mid end tablet/smart-phone.
Good to know, any idea about the DAC section? Really cool project by the way with lots of potential.
 
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I'm curious how this goes, and I'm sure drivers are the biggest factor, but I would have to think the amp and DAC are going to really hold things back and the cup designs. Still, great to try this and I'm hoping that you get some solid improvement. Cheers and good luck.
I wouldn't worry too much about it. Bluetooth is much better now than a few years ago, even without APTX. The sad part is the quality of recorded music has declined so much in all of the popular genres. The Koss drivers are very good but tend to be very forgiving too. The cup design could be an issue since these are designed as open drivers. Can't wait ro see how they turn out.
 
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I wouldn't worry too much about it. Bluetooth is much better now than a few years ago, even without APTX. The sad part is the quality of recorded music has declined so much in all of the popular genres. The Koss drivers are very good but tend to be very forgiving too. The cup design could be an issue since these are designed as open drivers. Can't wait ro see how they turn out.
I have no Bluetooth fears, I love Bluetooth and for portable I sold my Oppo PM3 driven balanced with the DP-X1 in favour of Apt x between my phone and a B&O H7. Since then I have been using Bluetooth exclusively for on the go and at home with my Desktop speaker system from Kanto. Love Bluetooth.
 
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  Good to know, any idea about the DAC section? Really cool project by the way with lots of potential.

looks like the bluetooth function is done in the CSR8635 "bluecore" chip, it is some kind of "all-in-one" solution, supports bluetooth 4.1, DAC can handle up to 96kHz sampling rate.
 
There is nothing on the back side of the board, except a pair of 220uf/6.3v capacitors, they are the output caps for SGM4812.
These caps can be upgraded, but the available space is only 8mm, so either the replacement cap is no more than 8mm height, or no more than 8mm in diameter.
 

 
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The Koss drivers are very good but tend to be very forgiving too. The cup design could be an issue since these are designed as open drivers. Can't wait ro see how they turn out.
 
not sure if the Koss drivers are designed specifically for a certain type of housing. The R10 uses very similar (if not the same) driver as the KSC35.  R10 is fully closed, KSC35 fully open. I suspect they just grab whatever they can make cheaply and try it in a housing....
 
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Driver test #1, KSC55 drivers.
-- this is the driver in KSC55 and KTXpro1, I suspect UR40 has the same drivers as well.
-- 60 ohm, "titanium-coated'
-- the sound signature is somewhere between KSC75 (brighter) and KSC35 (warmer).
 
The only obstacle is the driver size, it is a bit smaller than the 50mm stock drivers
 

 
 
.... no worries, a little bluetack fixes everything (well, ok enough for test purpose)....
 

 
 
....put everything back togather.......hmm, which pair have the KSC drivers? Take a listen and you can tell right away.
 

 
With KSC55 drivers:
-- sound level is a little lower than stock driver (about two clicks lower).
-- better clarity, the sound has realism. Much better imaging.
-- bass quality/texture improved a lot. No more "one-note boom-boom" bass; bass quantity is reduced a bit, but still quite a lot, border-line bass-heavy.
-- a bit brighter, some "shimmer" in the highs.
-- presentation is still kind of lay-back, but not as distant as with stock drivers.
-- no obvious improvement in the mids. The stock drivers mid range is quite ok, especially the voice playback.
-- quite good for rock and acoustic stuff, especially the live concert recordings (actually give that "live" feel, surprising).
-- the sound is a little echoey, probably need to add stuffing material into the cups. But I suspect the sound might become boarder-line bass-lean, after adding stuffing and take care of the echo problem.
 
Overall, doesn't sound bad at all. Better than the KSC55 driven by the headphone jack of my cheap LG android.
(this has more to do with the super poor signal quality from the LG's headphone jack than anything else. The phone jack connects directly to the DAC chip in that LG, they didn't even bother to put in an amp stage. On the other hand, the LG phone only cost me $10 so it was built on a super low budget.)
 
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You could try a piece of felt or cotton cosmetic pad in the back of the cup. If it isnt against the driver, it shouldnt impact the bass much. If it does, a small hole for a bass.port might bring it back. Check out the mods at http://diyah.boards.net/board/6/headphones
 
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noise isolation is what I need, if possible I'd rather not add holes to the cup.
 
Added about 1g of Acousta-stuf into each cup to deal with the echo problem.
https://www.parts-express.com/acousta-stuf-polyfill-speaker-cabinet-sound-damping-material-1-lb-bag--260-317
 
Bass did reduce a little, the sound is a little "harder", but no more "echoy" feel. And once the sound is cleaned up, I start to realize just how low the bass extends, this thing can produce low low rumbles quite ok! The deeper bass probably has more to do with the higher impedance of KSC drivers than anything else:
 
with the stock 16 ohm driver, and stock 220uf output caps, the bass roll off -3dB point is 45Hz; so despite the stock driver produced a lot of bass, the bottom end actually did not go deep.
 
with the KSC 60 ohm drivers and stock 220uf output caps, the -3db point is now 12Hz (roll off starts at 17hz); considering my files are converted from my CD's, which only do 20~20kHz anyway, this 12Hz roll-off is more than deep enough.
 
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How are you arriving at the rolloff values? This is an area I know little about so I ask out of ignorance as opposed to questioning what you are saying.
 
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https://en.wikipedia.org/wiki/High-pass_filter
 
The output cap and driver form a high pass filter. The #'s I wrote are simplified theoretic values (since the driver is not a resistor, its impedance varies across the audio frequencies). Treat them as ball-park figures.
 
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  https://en.wikipedia.org/wiki/High-pass_filter
 
The output cap and driver form a high pass filter. The #'s I wrote are simplified theoretic values (since the driver is not a resistor, its impedance varies across the audio frequencies). Treat them as ball-park figures.
My mind is too old for all the school work
 and don't feel like doing a spreadsheet 
 
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