Noise and Stuff in Consoles:
More About Specifications
by John Roberts
Last time I suggested how to interpret power-amplifier specifications.
This issue, I'd like to delve into mixers. Since mixers are
usually marketed by features rather than specifications, there
is a little less hype-but still enough to shed some light on.
Before I get into the actual discussion of mixer specifications,
I'd like to review what a mixer does, and what, from a design
point of view, the hard parts are.
The most common requirement of a mixer is combining multiple
input signals into one or one pair of usable output signals.
An actual mixer (desk, console, etc.) will do many more functions,
but combining is number one. Functional requirement number
two is conditioning signals so they can be combined. This usually
requires amplifying low-level signals from microphones or musical-instrument
sources up to some nominal level. The second most common conditioning
performed on signals is equalization. Depending upon the specific
application, there may be several different reasons for frequency-response
shaping, but in general, it's to correct for errors and/or
make the signal sound better (that's what it's all about).
Finally, the mixer provides signal routing. In the case of
recording, you may want to route multiple signals to and from
a tape recorder. In a live-performance environment, you will
probably want to send additional mixes to monitors. In both
applications, you will probably want to send signals to and
from effects devices to add reverb, etc.
The goal of any properly designed mixer is to provide all
these functions with a minimum of effort by the operator and
with no loss of signal fidelity.
To better understand console specs, let's figure out what the
hard parts are and concentrate there. In general, the most
apparent sonic artifact is noise. Even the best designed mixer
has noise. To better understand this, let's look at what noise
is and where it comes from. There are probably three significant
sources of noise in consoles: mike preamps, summing buses,
and cumulative channel noise.
Without getting too theoretical, you have thermal noise (a.k.a. "Johnson" noise)
and electronic (semiconductor) noise. Thermal noise is caused
by (you guessed it) heat. Heat causes electrons to bounce around.
Electrons bouncing around in a resistor generate small, random
voltages. This thermally induced current is characterized as
a power that varies as a function of temperature. The larger
the resistance, the more voltage, even though the power is
constant. The voltages are very small, typically measured in
nanovolts, or a decimal point with 9 zeros. Thermal noise can
generally be ignored when dealing with line-level signals.
However, when dealing with low-impedance microphones, output
signals can fall in the microvolt to millivolt range (3 to
6 zeros after the decimal point), so thermal noise is important.
But before you decide to run out and put your microphones and
console in a refrigerator, you need to understand that thermal
activity is relative to absolute zero Kelvin (-273° K), and
room temperature is in the 300s. Therefore, dropping even tens
of degrees will not make a significant difference. The important
thing to deduce from all this is that a perfect, noiseless
microphone preamp, if such a thing were possible, would amplify
the noise of the microphone's resistance (normally 150200
ohms). A lower-impedance microphone, while having less noise
voltage, would also have less signal and require more gain
... I think you see where this is going.
The second type of noise is electronic noise. There are several
causes, but these are usually lumped together for analysis.
Electronic noise is usually referenced to the input of a circuit
or device and stated as a noise voltage in series with the
input of a perfect noiseless amplifier and a noise current
in parallel with the input dumping into the input and feedback
network's impedance. For analysis, this is converted to a voltage
and multiplied by the circuit's gain.
THE MICROPHONE PREAMP
Now, using these new analytical tools, let's first look at
microphone preamps. Because we are dealing with relatively
low-source impedances (150200 ohms), feedback resistors are
usually kept low. Note: a design tradeoff may occur when you
make the feedback network very small to keep noise down, because
it requires a very large-series capacitor to deliver full low-frequency
response. In fact, one of our "friendly" competitors cut corners
there and has microphone preamps that are -3 dB down (half
power) at higher than 50 Hz. This is only measurable on a mike
preamp set for full gain, so they're getting away with it,
so far.
While many manufacturers (Peavey included) specify their preamps
in EIN (Equivalent Input Noise), which is computed as output
noise divided by gain, I prefer a specification call NF (Noise
Figure). Noise Figure states how much more noise you have than
a theoretically perfect preamp. My preference for NF is that
it's easier to interpret. Over the years I've seen many specifications
of EIN that were lower than theoretically possible! Just for
the record, the theoretical minimum is something like -132
dBm. Some of these may be honest mistakes, as theoretical noise
levels are defined in power, and conversion to voltage leaves
some room for interpretation as to what the actual impedance
is at the input of a preamp, etc. The important point is that
the state of the "Peavey" art right now in mike preamps is
running around 12 dB NF. That's important, because if you
could find a quieter preamp, even for a hundred times the price,
you probably couldn't hear the difference. We beat our closest
competitors by about a dB; however, you'll be hard pressed
to hear that. Look for an EIN in the -130s but not more than
-132, or better yet, a noise figure less than 3 dB. More likely,
you will find such sonic shortcomings in our competitors' products
as premature roll-off at high gain/low frequency and lack of
headroom (e.g., the gain can't be turned down low enough to
accept high output microphones direct, when miking a mortar
for the 1812 Overture).
THE SUMMING BUS
The other noise problem in large console structures involves
the summing amps. Here the problem is due to the insertion
loss and the need for compensatory gain, not because you're
applying a lot of gain to a small signal. Note: in virtual-earth
summing structures, the signal may appear to sum at unity gain
with no loss, but in fact the summing amplifiers' EIN is referenced
in series with its input and amplified by a "noise gain" term.
Without getting into the math or specific circuit designs,
the rule of thumb is that most summing structures have a noise
gain of N+1, where N is the number of channels being combined.
This is generally not a big problem in small mixers, but as
you get into tens of channels, the summing-amp noise can become
significant. For this reason, we actually use discrete low-noise
transistors in our Unity(TM) 4000 series summing amps, and
we do measure a few dB quieter than the other guy. The thing
to be careful about when looking at a summing-bus noise floor
is that a realistic number of channels are connected. There
are two (maybe more) design philosophies regarding summing
topologies: one "backgrounds" signals not assigned to the bus;
the other just connects them as called for. In the "backgrounded" system,
the noise gain and thus the noise floor is constant; backgrounding
facilitates a balanced bus for minimum crosstalk and hum pickup.
The non-backgrounded (open) bus appears quieter in listening
tests or measurements with only one or two channels punched
up. Don't be tricked. Measure your board the way you're going
to use it, with N channels assigned.
CHANNEL NOISE
The last and final noise source is a tricky one, because at
first glance it doesn't appear to be significant. Suppose your
well-designed (Peavey) console has an individual channel noise
of -96 dBu (obviously, the mike preamp is down, fader at unity,
no EQ, etc.). This sounds pretty good, until you add up 32
of these. This kind of noise addition is called incoherent
(that doesn't mean nobody understands). Incoherent noise sources
sum as the square root of the sum of the squares - easier to
do than say. To follow through with the example, 32 channels
at -96 dBu through an ideal noiseless summing amp combine to
-81 dBu. On the other hand, if the channels' noise floor were
coherent (such as an identical power-supply hum in each channel),
they would sum linearly, resulting in a final noise floor of
-65.9 dBu. Unless you have a poorly designed console or an
incredibly large structure (the biggest Peavey AMR(TM) board
has something like 120+ inputs to the 2 bus), this will be
swamped by even one channel of a theoretically perfect microphone
preamp at 60 dB of gain ( 132 dBm + 60 dB = -70 dBm).
When working with consoles, noise is a fact of life. Accept
what can't be eliminated (-132 dBm x mike gain), but don't
accept what can be designed out (noisy buses, poor bus structures,
hum, etc.). I hope I shed a little light on a confusing subject.
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