on the bench: microphone preamps
TUTORIAL
ON THE BENCH:
MICROPHONE
PREAMPS
We looked at microphones
back in Issue 58, now what
about the preamps that turn
them up? Let’s investigate.
Text: Rob Squire
Lifting myself off the bench this year has
proven more difficult than anyone could
have imagined, so finally, let’s pick up from
where we left off in Issue 58, when I looked
at microphones. It seems appropriate now to
look at the next item in the chain: microphone
preamps. I know, I’m skipping straight over
cables… but let’s save them for a rainy day
shall we?
You’d think the job of a microphone preamp
would be pretty straightforward, and, given
all the years of technological development, all
preamps would be pretty much the same by
now. Yet, this certainly doesn’t appear to be the
case, given the number of preamps released each
year and the time engineers devote to choosing
the right one for each job.
THE INGREDIENTS
When you look through the wide variety of
tasks a preamp must perform, it soon becomes
obvious why they differ from one another so
greatly.
The fundamental requirement of a preamp, of
course, is gain. A preamp is required to amplify
the low-level signal from a microphone to match
the line levels required by other processing or
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recording units. These levels may vary from
anywhere between virtually no amplification
(on snare drums and guitar amps) to gain levels
of 70dB or more (for ambient mics on acoustic
guitars and whispered vocals etc). This is not
trivial to achieve as it represents a range of
amplification of over 3000 to one!
One way that many early preamps dealt with
this was to build an amplifier module with a
fixed gain, say 70dB, and the amplifier was
commissioned into a fixed installation – usually
a radio station – with the ultimate gain required
for its role adjusted by adding pads or attenuator
networks to the input. This can work well
when the amplifier is used with the same mic
capturing an experienced announcer day-in and
day-out, but it’s pretty inflexible in situations as
varied as recording studios. One upshot of this
approach is that the noise floor of the fixed gain
preamp is a constant; i.e., the noise level won’t
change substantially as the input pad is varied
to effect a change of overall gain. So, unless the
preamp itself is particularly quiet it can be hard
to achieve a high quality signal-to-noise ratio.
A later approach, particularly taken up by
Neve, was to use a number of amplifier stages
that could each have their gain adjusted over
a small range, and, within that range, meet
the designer’s technical aims for noise and
distortion. Using this approach, the total gain
of the preamp is achieved through combining
the number of amplifier modules switched
into the signal path, the gain of each module
and the use of attenuation pads. This carefully
orchestrated combination preserves headroom
through all amplifying stages and keeps noise
and distortion at a minimum. When a low gain
setting is required, a whole amplifier unit can be
switched out of the signal path, thus minimising
the amount of electronics the signal is required
to travel through. To achieve a smoothly stepped
range of gain settings with a single gain control
requires a well-engineered multipole switch.
This explains why the gain control switch of
a Neve 1073 is so physically large and complex
looking… there’s a going on (see the photo
of a ’70s Neve preamp gain switch above).
Well implemented, this is a very good way of
controlling gain.
LATE GAINS
Later methods of gain control arose as single
amplifiers were developed that could produce
a large range of gain, while maintaining
satisfactory technical performance throughout.
Typical examples of these are the API 2520
and the Quad Eight AM10 amplifiers. These
are examples of discrete operational amplifiers
(op-amps) – by far the most common amplifier
topology used in audio equipment today. An
examination of operational amplifiers could
occupy an article within itself; indeed I have
whole books that deal with the topic. For
our purposes here, it’s sufficient to note that
operational amplifiers have two inputs and one
output. One input is in phase with the output
and the other input is out of phase with the
output. The amplifier itself has a very high
gain, typically in the order of 100dB. When this
amplifier is implemented as a preamp, some of
the output is fed back to the ‘out of phase’ input.
This feedback reduces the gain of the amplifier,
and if the amount of this feedback is made
variable, say through a potentiometer, then
we have a variable gain amplifier. Typically, a
range of gain of 20 to 60dB is easily achieved.
The added benefit of this preamp design is that,
as the feedback is increased and the output is
reduced, the noise floor is lowered along with
it. At low gain settings the noise floor is lower,
and as the gain is increased the noise floor rises
proportionally along with it.
Probably the most common microphone preamp
topology found today utilises a simple pair of
transistors implemented with variable gain
feeding an IC operational amplifier. With good
low-noise transistors running with variable
gain in Class-A, and an Integrated Circuit (IC)
op-amp, a microphone preamp can be built for
a couple of dollars. You’ll find preamps along
these lines in most consoles and other units with
integrated mic preamps.
HEADROOM CHARACTERISTICS
Gain control and the basic principles of
‘headroom’ go hand in hand. There are plenty
of ways to get all the gain required in a preamp,
but fewer ways to do it without compromising
headroom or noise performance. ‘Headroom’ is
tied, in part, to the voltage supply rails the unit
employs, but wherever more than one amplifier
stage is employed, careful consideration needs to
be given to the gain and noise relationships of
each amplifier stage. Given the dynamic range
of many sources, a poorly designed preamp can
still overload on peaks while the gain control is
set to give the average desired recording level.
Just as with a chain of outboard, it’s easy to
accidentally overload a unit in the middle of the
chain, while still maintaining a suitable level at
the end. The same mechanism can occur within
a preamp itself. However, within a preamp
there’s no opportunity to get your hands on the
middle of the chain and make the required gain
adjustments to clean things up. This dynamic
headroom is often one of the characteristics that
sorts the wheat from the chaff.
TRANSFORM IT
In Issue 55, ‘On The Bench’ looked specifically
at transformers, and without doubt one of
the most distinguishing characteristics of
microphone preamps is their use (or otherwise)
of an input transformer. Many designers would
agree that the use of an input transformer
contributes significantly to the sonic qualities
of the preamp as a whole. Transformers add
harmonic distortion with a unique frequency
dependence; less distortion at high frequencies
and more at low frequencies. Transformers will
also naturally roll off very high or supersonic
frequencies.
Transformers also serve perhaps one of the most
important functions required in a microphone
preamp – input balancing. A microphone that
requires lots of amplification is a good candidate
for picking up extraneous noise – in the cabling,
mains frequency hum and radio frequency
interference. This interference is picked up
equally in the hot and cold conductors of good
audio cable (oops… here come cables…) and we
require the balanced inputs of the preamp to
amplify the difference on the hot and cold cables
(the microphone signal) and reject the common
mode signal (the cable-induced interference).
Transformers are great at this. Their limited
supersonic and characteristic response filters out
radio frequencies, meanwhile intrinsic balancing
nulls out the hum. Best of all, this all happens
before the signal gets into any amplifier stages.
Transformerless preamps, on the other hand,
can be designed to have very good common
mode rejection but do require very well
balanced sources (microphones) and cabling to
achieve their best rejection of common mode
interference.
Although a transformerless preamp inevitably
saves on costs and space – transformers are
expensive and large – it can still offer a very
high level of technical performance – both low
noise over the full gain control range and very
wide frequency response. Frequency responses
out to 100kHz are not uncommon, so recording
for bats is now possible!
This extended frequency response aspect is an
interesting one. Perceptions run along two basic
lines: that preamps which provide a sense of ‘air’
and ‘openness’ are generally transformerless,
while ‘warmer’ or ‘thicker’ sounding preamps
generally involve transformers in the circuit (and
a response of say –3dB @ 30kHz)
“
The 1176, with a maximum of
+40dB available between the
input and output level controls,
can easily provide sufficient
gain.
”
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OTHER PREAMP OPTIONS
Of course, there are few other things we usually
require on a microphone preamp besides a
gain control. Pad, phase reverse and phantom
power are other common ingredients. Most
transformer-based mic preamps feature a pad,
typically providing 20dB of attenuation before
the signal hits the transformer. Unless the input
transformer is specifically designed to handle
very large signals – and this would also require
it to be physically large – the pad is employed to
attenuate the signal first, to prevent saturation
and gross distortion within the transformer
itself. Most transformerless preamps with a
minimum gain of around 20dB also offer a pad
to deal with particularly hot signals. Indeed, the
simple maths show that if we engage a 20dB pad
and then dial up 20dB of gain we are achieving
overall unity gain, or a gain of 0dB, and we
could reasonably expect to be able to inject a
line-level signal into the preamp. This illustrates
how ‘hot’ microphone signals can, in fact, be.
It’s quite possible to approach near line-level
signals from a condenser mic on a loud source,
and being aware of this presents an opportunity
to create another mic preamp option in your
arsenal. Many line-level outboard devices have
enough gain available to provide sufficient gain
to function as a mic preamp with the right mic
on loud sources. So next time you put a tube
condenser or dynamic mic on the kick drum, try
running it straight into an Urei or UA 1176, for
example. The 1176, with a maximum of +40dB
available between the input and output level
controls, can easily provide sufficient gain. And
you will still be able to tickle the compressor’s
gain reduction with judicious balancing of the
input and output level controls.
PHANTOM MENACE
You’d think phantom power would be
straightforward, whereby 48V is fed to
microphones requiring it. However, it’s
surprising how many manufacturers skimp in
this area and provide an inadequate phantom
supply that buckles under load. Some condenser
mics draw close to the theoretical maximum
of 14mA available to a microphone through
the phantom supply resistors. Under this kind
of load, the phantom voltage drops out of
regulation and becomes full of hum and buzz.
The provision for phantom power also places a
limitation on the maximum input impedance
of the preamp. In recent times, many designers
have begun to take advantage of the subtle tonal
changes available by offering a variable input
impedance.
This input impedance is all about the load that
the microphone ‘sees’ and how that load affects
the output of the microphone. This affect is
most significant with microphones that employ
an output transformer. Phantom power is
delivered to the microphone through the agreed
standard of 6.9kΩ resistors, one feeding the hot
and one the cold of the preamp’s input socket.
These resistors are present regardless of whether
the phantom power is turned on or off and place
a load on the microphone, thus determining
the highest possible input impedance that the
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preamp can achieve. These resistors on their
own would yield an input impedance of 13.6kΩ.
In practice, it will be much lower than this when
other circuit impedances are included. However,
there’s good evidence for the sonic benefits
of quite high input impedances for ribbon
microphones. The input impedance of most
preamps is around the 2kΩ mark. A variable
input impedance that ranges higher and lower
than this can take a mic from open and airy to
dark and warm – a neat trick implemented with
a minimum of passive parts.
HITTING THE HIGH-PASS
High-pass filters are a very useful tool, whether
available as part of a mix level EQ process, or
incorporated into a mic preamp. Cleaning up
low frequency garbage (noise, room rumble,
trucks on the highway) can significantly assist in
gaining clarity in a final mix. By placing a high
pass filter within the preamp itself this ‘cleaning
up’ can be done in the tracking phase: less to
think about later, and as they say, commitment
is a good thing! There is wide variation in the
facility provided in high pass filtering, ranging
from simple passive filters at a fixed frequency
to a continuously variable frequency with a steep
filter slope. To achieve a rate of cutoff or slope
greater than 6dB per-octave – typically 12- or
18dB per-octave – additional amplifier circuits
are required to provide active filtering. A good
filter design will allow complete bypassing of
these additional circuits when the high-pass
filter is de-selected. Using a simple combination
of a resistor and capacitor (and no additional
amplifier circuitry), a 6dB per-octave slope can
be achieved. Although the cutoff rate of this
design is low, the sonic impact is minimal in the
pass band.
A quirk of some of the sonically interesting
high-pass filters is that they create a small
amount of boost in frequencies just above
the cut-off frequency. This has the perceived
effect of restoring a sense of ‘weight’ back into
the sound while still achieving low-frequency
attenuation. One aspect of high-pass filters to
always keep in mind is that, like all analogue
filtering or equalisation, not only are the
volumes of your chosen frequencies altered,
they’re also phase shifted. With any high-pass
filtering there will be small amounts of phase
shift occurring into the pass band above the
cut-off frequency, and different filter designs
will induce different amounts of this phase shift.
You have been warned.
While it’s no longer difficult, nor particularly
expensive, to build a high quality microphone
preamp from a technical perspective, there is
still plenty of scope for a designer to explore
the nuances. In selecting a mic preamp for a
particular job we’re all seeking that successful
marriage of mic to preamp in order to yield
the desired representation of the sound we’re
capturing.
Happy hunting!
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