Today with more and more emphasis on quality in the signal path
and with the rise of digital recording there has been a huge
interest in the stand alone mic preamp particularly as quite
often you only need to be recording one thing at a time such
as vocals. So now you have a huge choice from small budget boxes
that do a basic job of getting your mic signal into your DAW
to very expensive esoteric designs that have to be heard to
be believed! and for some people the choice of mic preamp has
become the most important element of the recording path.
But at the bottom line selecting the right mic preamp for a
given microphone, or conversely selecting the right mic for
a given preamp, involves two major things (and a few other smaller
ones):
Input headroom -- Do you have enough?
Loud sounds can distort the input.
Noise -- Will the mic preamp add noise to your microphone?
First off it must have
low self noise at high amplification levels. Some microphones
really need turning up to 11 so it’s no use if the mic
preamp adds a load of noise to the signal. In fact it's relatively
easy to get gain electronically but to get smooth flat gain
with no noise requires some really good design and technically
the signal to noise ratio should be at least 90 db and be free
from radio frequency (RF) interference.
After that things get more subjective…. Most mic preamps
are valued for their transparency and clarity and we bought
a Focusrite isa 428 to use with our AEA ribbon mics because
we wanted 80 db of clean transparent gain. However there are
a raft of expensive mic preamps on the market that make a virtue
of adding Character to the sound usually by having
a valve stage which can be driven to thicken or warm up the
sound. Going back to headroom, any mic preamp can be driven
into distortion whether valve or solid state design and too
much signal either from the mic or some other point in the circuit
can exceed the ability of the tube or transistor to operate
linearly and cause it to clip, creating distortion. In a solid
state preamp this overloading tends to emphasise the odd harmonics
(3rd 5th etc) which are generally perceived by the ear to be
harsh or metallic (BAD) while conversely in a mic preamp
with a valve input stage, the overdrive creates an even harmonic
distortion which we perceive as adding body or warmth to the
sound (GOOD)
So a really a good quality mic
preamp should be able to amplify a microphone up to line level
without adding noise and be transparent and flat over the entire
audio spectrum but there are hundreds to choose from and much
is made of the relative design pros and cons of a particular
preamp. It’s generally agreed that transformer designs
produce the best sounding mic
preamp and the famous Neve 1073 is a great example
designed and build by audio Guru Rupert Neve in the early 70s.
If you want one of these today you need deep pockets as even
the reissue models cost over a thousand pounds and there are
other models even more expensive.
BUT, before you rush out and mug
grandma remember that the mic preamp is just one part of the
signal path and there are many factors that go to making a good
recording...The choice of mic, the performance of the player
and the room, the AD converters and even the cables all have
a part to play. It's so easy to get caught up in the idea that
you can't make a decent recording unless you spend thousands
of pounds on gear but lets get real. Most modern mic preamps
built into budget desks such as a Mackie are perfectly good
and you are far more likely to get an improvement by choosing
a different mic or moving it a few inches and getting a decent
level than spending £3000 on an esoteric mic preamp. It's
the old story.... use your ears and hone your recording techniques
until you are sure that you can make a small improvement with
a new mic preamp. Then hire a few in and choose the one that
works for you.
