[Blaze182]

I'm looking to either buy, or build (or get built) a Dolby DTS Decoder at the size of the FIIO Alpen. 
 
It would have HDMI & Optical as Inputs, with a Headphone Jack as the only Output. 
 
The box would have volume control as mandatory, all other things secondary, if any. 
 
An example of the current market 'leader' http://www.astrogaming.com/mixamp%E2%84%A2-pro/3AM99-HBU9X-975.html#start=1
 
Has this been done before? Any advice? 

 

[nikongod]

  I'm looking to either buy, or build (or get built) a Dolby DTS Decoder at the size of the FIIO Alpen. 
 
It would have HDMI & Optical as Inputs, with a Headphone Jack as the only Output. 
 
The box would have volume control as mandatory, all other things secondary, if any. 
 
An example of the current market 'leader' http://www.astrogaming.com/mixamp%E2%84%A2-pro/3AM99-HBU9X-975.html#start=1
 
Has this been done before?  

 
There are dozens of commercial options, only a bit larger (2^3 times) than you are looking for. 
 
Building anything the size of the FIIO alpen is insanely hard. Or its just crap. Actually building anything in that form factor requires tons of design effort.
 

  Any advice? 

 
Buy the MIXAMP™ PRO. Or something like it. I am not familiar with these things. But you will not find a DIY solution even as small as the MIXAMP.