you look at the Gain-Bandwidth Product of any amplifying device you will see
that it has a linear roll-off with rising frequency. It is impossible for
the "over-kill" low noise device to have a bandwidth approaching 150,000 Hz,
and gain! However, as such phono stages are on offer, you may think that
many designers are plainly ignorant. On the contrary, this is exactly where
the fad for bandwidth limiting stems from.
musical instruments timbre (the way it sounds) is dependant on all its
harmonics being present and ordered correctly in time. Bandwidth limiting
therefore alters the sound. A fact that seems to be lost on some members of
the Hi-Fi press.
one time there were no one-box-does-all phono stages. Magnetic phonograph
cartridges were generally all moving magnet types. Preamplifiers were inside
amplifiers or "stereograms". Although moving coil existed in those days,
they were rather bulky and gave a similar output to that of today's moving
magnets. The moving coil fad started in earnest at a much later date.
Originally, to make the usual (low output) variety of moving coil cartridges
work into the phono stage you had to use a step-up transformer. Not until
recently had technology advanced sufficiently for semiconductors to replace
is now possible to make gain stages that are low noise and have the required
real bandwidth to amplify the tiny outputs of moving coils, and thus
dispense with the step-up transformer. But as yet, it is still impossible to
optimise a circuit to perform both the functions of MM and MC amplification
without sacrificing the performance of one or the other.
There is one more factor in the one-box compromise. It is mathematically
impossible for the frequency equalisation to be identical for both the gain
ratios required. Get it right for MM, and the bass will be a good decibel up
for MC. You can get round this by making the EQ passive, placing it between
"flat" amplifier stages, and some commercial designs do exactly that. But
there's an even bigger drawback with this method called clipping of high
frequencies. This is what is described as grain and by some extreme of
imagination, detail! It is in fact distortion.
one-box phono stage designer is in a no win situation, but such products are
on sale, somehow get good reviews, and are very lucrative for their
manufacturers. However, the compromises explained here will at least throw
some light on why our no-compromise products score so highly in direct A-B
our phono stages are fixed. Either as Moving Magnet or (soon to be
introduced) Moving Coil stages. The only way to have both options is to have
one of each, or to use a step-up transformer with your MM stage. Better
still, the Elevator EXP electronic step-up transformer.