Originally Posted by alphaman
Sorry for just getting around to replying to your reply ;)
The amps ain't THAT different in that both are "vacuum tube and discrete solid state hybrid headphone amplifier[s]"
About that diode ... my bad on stating that being the ONLY part needed for a drop-in type CCS upgrade for the MSSH -- and YOUR bad for not noticing that :)
Specifically ... if you look at the the Borbely ckt you'll note two addit. transistors and three Rs (Q4, Q5,R11/12/13 )., sorta similar to MiniMax CCS topology, but the borbely uses one MOSFET and one BJT
While it is true this amplifier uses vacuum tubes and solid state devices, the WAY they are used is certainly different. Otherwise one might conclude that all hybrid amplifiers are very similar simply because they use the same basic components...
For starters, the input stage above looks like differential pair with D1A and D1B acting as a tail CCS; a topology similar to the SOHA II. On the other hand the SSMH is a simple common cathode triode voltage amplifier.
The circuit around Q4 and Q5 is a CCS acting as source load for the output MOSFET stage, not for the tube stage. On the SSMH it is the tube heater that acts as the MOSFET load.
Finally, you can see that in the above diagram both the tube and MOSFET as well as the output load and MOSFET are directly coupled, thanks to the use of a bipolar power supply, whereas the SSMH uses a simple +48V supply and coupling caps to isolate it's two stages as well as the headphone load.