Impedances And Tuning; Un)Balanced Lines - Behringer EURODESK SX4882 User Manual

say, 15 dB of headroom (as is the case with DAT etc.). The PFL/
SOLO meter, on the other hand, looks only at the desk's analog
aux input level, if you hear distortion, but the meter says you're just
hitting 0 dB, then it must be coming from the aux send amp or the
FX unit. If PFL on the aux send reveals nothing amiss, turn down
the input on the FX unit, and turn up the desk's aux return.
99 times out of 100 distortion in the aux send > FX >
+
aux return loop will come from the FX unit (FX GAIN
TOO HIGH), and the same goes for a high noise level
(FX GAIN TOO LOW).
Noisy FX (or synth) returns can be greatly improved by
+
the addition of single-ended noise reduction between
FX output and aux (or channel) returns.
+ We found out that using analog single ended noise
reduction can help warm the sound of certain digital
reverbs which sound too cold/metallic, and also give
that "Echoplex" sound to digital delay decays.
+ Analog multitrack tape should be driven quite hard,
since its dynamic range (without noise reduction) is
likely to be 20 to 30 dB worse than other elements in the
recording chain. Try to record bright. You can always
mix back duller. Brightening up an off-tape signal will
bring up the level of tape noise.
When mixing or recording, keep the channel FADER lev-
+
els around or below 0 dB. If you do find the faders creep-
ing up or down, apply a suitable offset over all channel
faders, and try to control your bad habit in future!
11.

Impedances and tuning

Electronic inputs tend to have impedances measured in tens of
kiloOhms. Outputs, on the other hand, are generally two or three
orders of magnitude less. This is just as well, otherwise a signal
at an output might find that the line of least resistance is the limit
of the preceding unit.
In the patchbay section we recommended that you parallel the
MAIN MIX output of the EURODESK SX4882 into all 2-track
recording inputs. It would not do any harm to buffer each output
from the primary one (i.e. that feeding into your most expensive
DAT recorder or 1/2" mastering machine) with a 470 Ohm resis-
tor. Cassette, DAT and reel to reel recorders' input impedances
should be similar, but just in case they aren't, it is better to add
a fraction of a dB of thermal noise to the inputs of the secondary
recorders in the shape of a resistor, rather than having an unusually
low impedance input grabbing most of the signal. Another neat
idea is to parallel the Monitor L/R output via a 47 kOhm resistor
pair. Now you can safely connect e.g. a tuner to either extra hole,
without shifting the stereo image (this would happen if a low im-
pedance tuner input was connected directly across one side of
the monitor output). Now, whenever you monitor an instrument's
input level with the PFL/SOLO function, you can check its tuning
also. That should impress the customers. Especially those using
old, unstable, but very desirable analog synths.
Fig. 11.1: Resistor-buffered parallel wiring for
bay 8 (see section 8"The patchfield")
Balanced inputs and outputs are offered on most audio connec-
tions on the EURODESK SX4882 (inserts and direct outs being
the major exceptions).
Why? Though all audio cables (except speaker cables) have
earthed screens, the shielding they afford from the electromagnetic
garbage that permeates the atmosphere is never perfect. The
balanced line is a simple but effective mechanism to overcome
this problem. Instead of one insulated audio conductor, two, usu-
ally twisted together, are contained within a single screen. One
conductor, wired to pin 2 of an XLR-type connector by international
convention (after decades of total confusion!) carries a signal vari-
ously referred to as "hot" or "positive". Pin 3 is wired to the "cold"
or "negative" conductor.
What does this mean? Consider an unbalanced line. Now, that's
much easier to understand. You have one "hot" or "positive" core,
and an earthed screen. The "hot" wire's waveform, if looked at on
an oscilloscope, would be directly correlated to the audio signal
waveform. If you looked closely at the trace, you'd see random
noise along the X axis. What you probably wouldn't see, however,
is any superposition of 50, 100 Hz, etc. corresponding to mains
hum interference, since these frequencies would be tangled up
in the audio signal (to spot them visually you'd need to perform a
FOURIER TRANSFORM). AC mains frequency and its overtones
are picked up by any wire, and some will always leak through a
cable screen. The question is, when does it become audible?
Well, all other things being equal, the amount of mains hum picked
up by a cable is independent of the signal level. Speaker lines run
50 or more volts, enough to diminish the effect of mains radiation
to vanishingly small even with no screen. (In fact, at these voltages
another effect comes into play: capacitive resistance. It is positively
undesirable to use screened cable to wire an amp to a speaker.
Speaker leads should be as thick and short as possible, with XLR
or wound post terminals.) Line-level signals can usually be run
unbalanced over short or moderate distances (rack to desk etc.),
but NOT from the back of the hall to the stage, always provided
that there are no earth loops (see section 8.3 "Looming problems"
A loop acts as an aerial, positively inviting electromagnetic radia-
tion to flow around the system). Microphone lines, however, are
another story altogether.
Most microphones generate not volts, but millivolts. Protecting
such a low level signal requires a more sophisticated solution.
Hence, all mic networks run along balanced lines. It works like
this. The mic diaphragm moves forwards and backwards accord-
ing to the air pressure increases and decreases that constitute
sound waves. Diaphragm movement generates a corresponding
electrical signal, which is either positive or negative depending on
the direction of travel. The +ve and -ve signals are mirror images
of each other: if you shorted + and - you'd end up with nothing:
one would cancel out the other. In fact this cancelling effect is
what makes the balanced line work. Instead of simply shorting
the negative line to earth, as would be the case in an unbalanced
system (losing half the signal, or 6 dB, in the process), the two
lines are kept apart until they reach an electronic (or transformer)
balanced input.
Here something exquisitely simple happens:
You may not know this, but whenever a signal is amplified, its
polarity is reversed. By inverting the negative side and adding it
1:1 to an unchanged positive, a balanced input wastes none of
the available signal energy. In doing so, it also subtracts all the
radiation picked up along the line. Random noise is unaffected,
but you'll hear no hum, and much reduced thyristor noise (from
poorly-screened lighting dimmers). Live, you could not run a rig
without balanced mic lines, and although in the studio cable runs
are shorter, the recorded medium's demand on signal to noise
is far greater.
When patching a balanced input/output to an unbalanced one,
simply short the -ve and screen together at the unbalanced input
or output.

(Un)balanced lines

EURODESK SX4882
12. (Un)balanced lines
17