It is the immense size of the coupling cap at the output of many tube preamps that also limits the abilities of said preamp. When you are driving a 600 ohm load, the cap has to be at least 120uf (FWIW 30uf cuts off at 8 hz) in order to not have phase shift problems in the audio passband.
Look at what the output impedance of the preamp is at 20Hz and compare that to the 1000Hz figure and you will see what I mean! If you want the preamp to play bass without coloration, the two figures should be exactly the same.
By the time the cap gets over about 3-5uf, it is offering significant coloration at all frequencies due to inductive effects and increased dissipation factors due to its size.
In the old days, to drive a 600 ohm load with tubes, you needed an an output transformer in the preamp. Of course such transformers have problems of their own. But we developed a patent (we are an OTL manufacturer after all) that allows direct-coupling. Even if driving a 100Kohm load, getting rid of the output coupling cap in the preamp is an easily-heard increase in transparency.
Look at what the output impedance of the preamp is at 20Hz and compare that to the 1000Hz figure and you will see what I mean! If you want the preamp to play bass without coloration, the two figures should be exactly the same.
By the time the cap gets over about 3-5uf, it is offering significant coloration at all frequencies due to inductive effects and increased dissipation factors due to its size.
In the old days, to drive a 600 ohm load with tubes, you needed an an output transformer in the preamp. Of course such transformers have problems of their own. But we developed a patent (we are an OTL manufacturer after all) that allows direct-coupling. Even if driving a 100Kohm load, getting rid of the output coupling cap in the preamp is an easily-heard increase in transparency.