i found a post on usenet from David Bock that gave some interesting infos
" To say an amplifier has a HF
rolloff without qualifying and comparing is like assuming it is a
black box with simply a certain function and no side effects, but we
know from real world experience that it's not a black box and the HF
rolloff is achieved through some means, and every means has it's owm
unique set of effects and side effects which should not be ignored for
a thourough discussion. Looking at a single parameter and making a
judgement ignores the reality of the mic system as a whole.
For example, yes you are correct that the U87 achieves HF rollof
though frequency selective negative feedback through the capsule to
the FET. But what does that really mean? Well for one thing you now
have the "sound" of negative feedback (as compared to an amplifier
with no negative feedback, which will sound different). You also have
changed the frequency spectra of the noise floor as well as the
overload charachteristics and harmonic content. There are other ways
to get HF rolloff, like the U67/M269's feedback through the capsule &
transformer- different again than just running through the capsule to
the gain elemant. And then there's anopther way, using a shunt
capacitor to ground on the output of the gain device, or using the
shunt capacitance of the output transformer. All have different
effects and side effects.
So to really compare Neumann FET mics to tube mics based on frequency
response alone isn't really a scienfic approach, there's far too many
unequalized variables. But the comparison can be done, if you have a
U87 and and a U67 any or all of the feedback elements can be removed
and flat amps could be produced, at which time the age old question of
tubes vs. transistors in MICROPHONES could finnaly be answered.
But, you do raise a point people might not often remember, is that
most of these amplifiers are effectively bandpass filters, and that's
probably a really good thing when you think abvout where alot of mics
get used nowdays, not in real purpose built large studios! A mic with
reall flat low end would too often be "rumbly". Neumann was trying
(around the U87 time0 to build something that sonded good on alot of
applications, and made various copmpromises (which all engineering is)
to successfully achieve that goal.
Does this help? "
i hope it's not a bad thing to bring this quote here outside of the long thread it came from
francois