[quote author="Michael Krusch"]By the way: Even the Manley Massive Passive uses opamp inputs (beside the Farnell inductors :green: ).
I don't see the problem. If you like it clean or one tube is enough colouration, why not using an opamp for balanced input.[/quote]
You certainly have a point here - if the signal runs thru a transistor mixing desk later anyway, then why not putting it thru opamps even in the "tube" compressor / preamp / whatever. And nevertheless it makes me cringe every time I see a tube circuit, or even a discrete transistor circuit, with opamp inputs and outputs.
I'm not entirely sure why this is. But I'm quite convinced that tubes are not the only devices that make some coloration, and building a tube device may not only mean getting "tube coloration" (whatever that is in that particular case), but might also mean avoiding something else, namely "opamp coloration" (again: whatever that is in that particular case). And building a single box following one design philosophy may be a good thing even when technologies are mixed in the whole recording process, but then at least the building blocks are available independently, so you can choose which part of your system (FX devices, desk, tape machine, ...) are made of which technology. If you have a mix of opamps and tubes in one box, you don't have them available as separate building blocks anymore.
*Of course* it's perfectly ok to create devices whose philosophy is a certain mix of technologies! In a way, the choice to have tubes, but no transformers, plus opamps, makes up a whole new category of its own, which has a right to exist just like anything else. And it will certainly have a special sonic character of its own.
However, in classic tube circuits, much of the sonic character comes from transformers as much as, or even more than from tubes. And transformer balanced inputs and outputs show a quite different behaviour in terms of common mode range that elecronically balanced inputs and outputs. (Not to speak of the multitude of so-called-symmetrical outputs which emulate a center-grounded transformer instead of a real floating transformer!)
So as these generally (and I know there was a lot of generalising in the above!) have quite different sonic idiosyncrasies, it's quite misleading to adress both categories with the same words, such as "class A tube circuits".
JH.