Mic Pre-Amp is the first part in the recording
chain’s path to the destination recorder and
there are a number of aspects related to it, and
to some others further down the chain. This could
be with a stand alone Mic Pre, a Mixer or a
Recorder with its own mic amps.
Lets start with the Gain Range available from the
front end IC, such as the Analogue Devices
SSM2019. This can be a stepped gain arrange or a
continuously variable one. Due to the wide range
of sound sources, from a quiet acoustic guitar to
a group of percussionists, there needs to be at
least a 60dB range of adjustment.
primary purpose of the Gain range is to have it
set it to avoid clipping of the mic pre-amp, with
a sensible working position of the faders, sending
on the signal to the recorder. There is a ‘trade
off’ factor, involving the headroom versus the
basic noise floor, which will be discussed later.
factor is the range of output levels of different
mics. You have the low output levels from ribbons
and moving coil mics, and the higher differing
levels from capacitor mics and now also with the
newer ribbons, with their on-board ICs producing
higher output levels, giving lower noise floors.
There is a case for Linear faders, as opposed to
Logarithmic types, for use to control the levels
sent to the destination recorder. With a linear
fader you have a more ‘delicate’ control over
any trimming of levels, should un-anticipated
peaks occur. In the wide dynamic range world of
classical music recording, it proves to be much
easier, to in-audibly trim the levels.
is another factor in the choice of faders,
especially for us DIY constructors of our
recording kits. This is the use of ‘stereo’
faders, with the two tracks wired in parallel. The
reliability is very considerable enhanced!
All Mic-Pre / Mixers should have a built-in
line-up oscillator to properly relate their
outputs to the destination recorder’s
controllable line level inputs. The idea is that
the overall noise floor and headroom of the mic
pre-amp can be sensibly utilised, as mentioned
is indeed a trade off between the two aspects. For
instance, should the recorder have its input too
low, you could be using too high a gain setting
with the mic pre-amp/mixer, and therefore increase
the chance of ‘clipping’ the mic amp’s input
and reducing the head room available. In the
opposite direction you could have the recorder’s
input too high and the noise floor extracted from
the mic pre-amp/mixer’s output will be higher..
solution is to have a 1kHz output directly
available from the mic-pre/mixer output, activated
via push button, installed out of easy reach. In
the writer’s DIY case, this is intended to be
received at neg. 12dBfs by the recorders, getting
the best compromise between headroom and noise
floor. A 90dB noise floor can be obtained.
There is a case for the metering on recorders to
have the neg.12dBfs point at half way on its
scaling. The main thing we are interested in is
avoiding signals going OTT. The two particular
Olympus LS10 SD card recorders, shown in Pic: 1,
do at least have their neg.12dBfs point half way
across the small space that they have available.
the writer does currently judge levels with a HHB
MDP500 recorder (Pic.
One), due to its much wider metering
display, and its neg.12 dBfs point is indeed half
way. With the same line-up tone, the SD card
recorders are set a couple of dBs lower with the
same tone. The MDP500 could also be a third
backup, as it has balanced mic inputs, should the
DIY kit’s mic amps ‘implode’!
it happens the MPD500 is also used for playback as
you can quickly jump between tracks, which show
the Take Number you have called. The writer also
lists in the Session Log, the last three digits of
the automatically selected Wav File numbers, shown
on the LS10 recorders.
With headphone monitoring, and also in cases where
there is a feed to loudspeakers, there should be
mono buttons available to produce a mono signal.
This allows both monitoring systems to be checked
for centre signal position accuracy. This is also
useful if the speakers happen to be out of phase,
however this should be noticeable with the music
a final point, at least two or even three
headphone sockets should be provided on any mixer
and even some recorders, so that ‘the talent’
can come to the control room and be able to listen
at the same time and more quickly make common
judgements, as to how they think they are getting