The Foster biodynamic headphone family is arguably one of the most popular OEM audiophile headphone models out there. What we have here today even just from a glimpse one can see how similar they are. Basically you have the same chasis, frame and headband. The differences are 1. driver 2. type of wood used in back enclosure 3. damping scheme inside and 4. ear pads.
But do they sound alike too? Again let’s pull out the graphs.
Tonal balance: Slightly U-shaped but the mids aren’t as withdrawn as on poorly executed V-shaped headphones. Vocals are instead quite forward, although the upper treble peak at around 12khz does colour some female vocals depdnding on how the track is mastered. Bass is sinfully addictive, tastefully elevated that it is punchy but not boomy or muddy at the same time.
Other qualities (technicality etc.): Soundstage is not particularly wide, probably due to the small inner opening of the ear pads that does not allow much volume of air when fitted. Layering and separation are fantastic, clearly better than most mid-fi headphones. Macrodynamic qualities are EXCELLENT, arguably one of the best out there even when faced with much more expensive headphones. These outclass many planars and electrostats at dynamics.
Pads: Pads dictate the majority of the sound. When switched to TH900 pads or old Denon pads the tonal balance change significantly. More on this in a follow-up article.
Fostex TH900 MKII Sapphire Blue Limited Edition, frequency response measurement
Tonal balance: VERY V-shaped kind of sound. Mids are a lot more withdrawn than the Teak. Vocals can sound artificially bright or even piercing on some tracks. Two huge spikes in the sibbilance region (9db boost at 5.8khz and 11khz) make these even brighter than the Sennheiser HD800, a well-known bright reference. In fact, the two resemble quite a lot in terms of FR, where the TH900SB is like a bassier but even brigher HD800. In their stock form, I can only listen to accoustic/classical music with the TH900SB (essentially the same tuning of all limited edition TH900MK2 except for the emerald version, according to Fostex. This ‘anniversary tuning’ claims to have dialed in bass and treble to make it less V-shaped than the regular TH900MK2… but as you can see, it’s still got a lot of bass and treble. Comparing to Crinacle’s measurement of the OG TH900, the Ltd Edition is less scooped in the mids). It does bass amazingly though, even better than the Teak one could say. There is noticeable more extension, resulting in more slam.
Other qualities (technicality etc.): Soundstage is a LOT wider than the Teak. The ear pads of the TH900SB is a lot more roomy. Layering and separation are top-tier, about on par with the HD800/T1/Utopia. Macrodynamic qualities are magnificent, arguably one of the best out there period. So these are really amazing in their technical performance. If one has to fault these then maybe the sense of transparency and natrualness is the only potential weakpoint when comparing to top-tier open backs.
Pads: With E-MU Teak pads or the Accessories House Denon pads, tonal balance improve by a LOT. They become as neutral as the Teaks, while you do lose some performance on qualities like soundstage and air. More on pad rolling and other pad options in a follow-up article.
TL; DR: both the TH900MK2 limited edition and E-MU Teak are excellent choice for closed-back dynamic headphones. They excel in certain qualities like punch and slam while the overall tonal balance is quite nice too.
Also, you can pad swap or even chamber swap using custom wooden cups from E-MU or Lawton Audio. I’ve ordered some from E-MU and will update measurements when they arrive.
the unheard of headphone lab 
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