Talk About Balanced vs Unbalanced Systems

Talk About Balanced vs Unbalanced Systems
 E
talk about
BALANCED
By Jack Sondermeyer
The idea of a balanced system is one that has confused even
the most experienced audiophile. The actual application of
balanced systems are further confused by relating them to the
characteristics of low vs. high impedance. A balanced system is
unique in that it possesses an electrical characteristic called
common mode rejection. If one can grasp the idea of common
mode rejection, then a balanced system can be easily understood.
The fact that a system is balanced or unbalanced has nothing
whatever to do with its impedance level, although usually balanced
systems are low impedance, but do not necessarily have to be. To
begin with however, let's start with an unbalanced system, which is
so called because it is simply not balanced. Whenever separate
chassis are used for different parts of an audio chaim-such as a.
separate mixer and power amplifier, the signal must be paiched
through some connector and cable system. The most common
connector used is the telephone plug and jack developed by Ma
Bell many years ago. These are simply called a “phone” plug and
jack, but other type connectors will work equally well. The cable
used must be some sort of shielded type, required to reduce stray
pickup of extraneous hum and noise and R.F. signal always
present around us. An unshielded cable should never be used in
this application. Such a system is shown in figure 1.
FIGURE 1
MIXER POWER AMP
ee es pe aa a
tm dd mp ey. fm dep UNBALANCED SYSTEM
Da
O
AC MAINS AC MAINS
Notice the ends of the shield are grounded at each chassis and
the signal is present on the single conductor with reference to the
chassis grounds. Usually the mixer output will have a low
impedance source which will help to minimize external
interference and fairly short cable runs using this configuration are
generally satisfactory. Where the problems usually begin is when
the cable length gets excessive (20 Ft. or more), and the separate
chassis are connected to different AC mains sources. Such is the
. condition in a typical sound system with the mixer in the audience
and the power amplifiers on stage near the speaker system.
The first problem encountered is an increase in hum and noise
due to the failure of the shielded cable to provide adequate
shielding in the longer cable run: Any such interference that gets
into the shielded conductor simply adds to the signal and ¢annot
be eliminated. If a powerful radio station or a boosted CB radio is
located nearby, it too can play havoc with this system.
A similar problem exists whenever unbalanced microphones
and other sources are used with a mixer over long cable runs and
usually is worse because of the lower signal levels involved and the
fact that often those microphones are high impedance.
The second and more critical problem, however, is the
different AC mains sources which cause a completely different set
of problems. Now mixers and power amplifiers employ a three wire
fine cord with the usual grounding type plug with the large pin that
is supposed to be left alone (not broken off as is normally done).
This, of course, is provided to minimize the risk of shock hazard
dictated by our various testing agencies. The ground pin is
supposed to be grounded through a separate third wire in the
typical AC mains recepticle. This ground pin is always connected
internally to chassis ground of the associated equipment. Thus the
individual chassis are supposed to be grounded to good old
Mother Earth. Unfortunately, what is said to ground, really is not.
Often ground systems are inadvertently. miswired. Other
equipment tied to the same ground introduces voltage transients
and spikes which cannot be eliminated. Three phase AC voltage
systems introduce additional hum voltages into the grounds. If the
AC distribution system transformers are located significant
distances from the equipment, the long ground wires can actually
pick up local radio and TV stations’ signals. Lighting dimmers also
introduce spikes into the grounds. So, our separate chassis are
actually connected to different interference sources rather than
common ground as we thought.
The obvious question is, why is this difference in chassis
voltage so bad?
Reviewing figure 1, we see the shielded cable connects the two
chassis together via the shield itself. Now, that shield might seem
an effective connection between those chassis, but it actualiy
ismE...not even close. The different ground signals that appear at
each AC mains recepticle will drive the individual chassis much
more so than that small gauge shield wire can short out. In reality,
the shield does little for the grounds except shield the signal wire
itself. The power amplifier is “looking” for a signal from the mixer to
amplify and actually “sees” the mixer signal plus the difference in
ground signals that appears between chassis. So, that different
signal, which is interference, is actually amplified along with the
desired audio signal... The results which are less than desirable.
it might seem that an obvious solution might be to break the
ground pin on the units...Heaven forbid....And interestingly
enough, sometimes that really works. Another solution is to
connect the mixer through'a long extension cord to the same AC
mains as the power amplifiers, thereby assuring that the grounds
are the same, À third alternative is to tie the two chassis together
with a very heavy gauge external grounding wire. .
All these “fixes” are not totally effective, however, and the only
real solution lies in the use of a balanced system, to be discussed
next. ! Ш
Consider the transformer diagram shown in figure 2.
FIGURE 2
yor
я Iy = к
| — q > “|
dd. -
в P q D
Primary Secondary
MIT . AE]
Turm Turns
пр р 3
L
Transformers are relatively simple devices consisting of
separate turns of wire on a magnetic core. In this case we have
construcied a transformer using a certain number of turns on the
primary (terminals A 8 B), and a certain number of turns on the .
secondary {terms C & D ), again both wound on the same magnetic
core. These turns are specified as N {1) & N (2) respectively. The
actual number of turns and wire size is of no particular importance
at this point.
H a signal voltage is applied across terminals A & B, a signal
current will flow in the primary windings proportional to that signal
voltage. This current will in turn cause a magnetic field to circulate
' in the core which will be proportional to the signal current and the
number of turns used in the primary winding. Continuing, this
circulating magnetic field will induce a current into the secondary
windings proportional to the magnetic field and the number of
secondary turns. This, in turn, will produce a voltage across the
secondary terminals C & D which is proportional to that secondary
current. |
We have just described typical transformer action. Notice
there is an energy transformation from electrical to magnetic and
back to electrical. Also notice that if both primary and secondary
turns are the same, then the output voltage is the same as the input
voltage. The actual voltage transformation is determined by the
turns ratio as indicated in the following formula:
N (2)
V (2) = V(1) —
N (1)
Thus a transformer can “transform” voltages up or down
depending upon the turns ratio involved. The actual number of
turns are not important....just the ratio....in this formula. There are
other considerations such as voltage and power levels which
determine the actual turns required in a typical transformer but
these will not be treated here.
The important thing to notice about the transformér is that
signal will only get through itifasignal current is established in the
primary. This current is only possible if a signal voltage 15
impressed across it. In other words, there must be a difference in
potential between terminals A & B in order for current flow. Such a
condition would occur if, for example, a microphone were
connected across the primary terminals. The microphone signal
itself would be this difference in potential. tf both primary terminals
were connected together and a signal connected to that common
point with reference to ground, then both primary terminals would
have signal on them but both would be at the same potential and
thus no primary current could be established and consequently
this signal would not get through the transformer. This is referred
to as common mode signal..the same signal on both
terminals...and the transformer is said to have “common mode
rejection.” In other words, common signals don't get through the
transformer...and uncommon’ or difference signals do get through
it.
FIGURE 3
2 Cord
Fig- X Sneigod Cats Primary Secondary
Referring to figure 3, we have connected a two conductor
shielded cable to a typical input transformer. Notice the two
conductors are wired to the transformer primary and the shield is
grounded to the chassis. In our previous unbalanced situation, we
mentioned the failure of the shield itself to adequately prevent
outside interference from getting into the single conductor and
thus this interference would add to the audio signal. In this case,
we have two conductors, both being the same length and both
reasonably close to one another. Therefore, it's safe to assume that
any outside interference that does get through the shield will be
picked up by both conductors equally. Thus this “common mode”
signal will be rejected by the input transformer and will not be a
problem. Any signal that is applied across these two wires, suchas
a balanced microphone or mixer output will not be a “common
mode signal”, but rather a difference signal and this will cause
transformer action to take place and this signal will get through
and be amplified by the system free of outside interference. This
then is why balanced systems should always be used for long cable
runs....Because they have the inherent capability to reject
interference.
We will also show that a balanced system will effectively solve
the differences in chassis potentials that exist in separately
grounded systems. First, notice that a balanced system always
needs two conductors plus ground to work. A single conductor
shielded cable can never be balanced. This is why all baianced
system connectors have at least three pins. The most common
connector used is the three pin XLR type which has become a
standard on most contemporary mixing consoles and equipment.
Most balanced microphones are also fitted with it. The three pins
are always conveniently numbered: 1, 2, and 3, with pin 1 always
the ground pin. Pins 2 and 3 are the respective conductors in the
cable, and become the differential input for the system. A polarity
standard exists for most audio equipment using this connector.
Simply described...."A positive voltage potential applied to pin 3 of
an input connector will produce an associated positive output
voltage potential of the equipment.” In other words, pin 3 is defined
as the positive input to the mixer or power amplifier. Unfortunately,
many times this standard is ignored. A similar standard exists for
balanced microphones in that a positive or inward audio pressure
wave will produce a positive output on piri 3 of the associated XLR
connector. Notice that for transformer balanced systems, this
standard is adhered to by simply connecting the primary winding
up the correct way depending on how the transformer is
constructed. A phase reversal can be accomplished by simply
flipping it. over. This feature is often included in many mixing
consoles....especially studio equipment. A mis-wired XLR
connector can do the same thing. In any sound system it is
important to maintain the same phase relationship in ali the
microphones. An out-of-phase microphone will cause endless
problems for an unknowing audio technician. A point to
remember...the fact that a system uses the XLR connector does not
necessarily make it-a balanced system: Many times, especially on
inexpensive equipment, the XLR connector is wired with pin 3 as
the input to an unbalanced system. Pin 2 simply goes to ground
and, of course, so does pin 1. Occasionally pin 2 might be the input
instead. Now two ‘conductor shielded systems such as
microphones and mixer outputs will effectively drive such an
arrangement, but obviously it is not balanced, and naturally will be
susceptible to interference such as light dimmers and radio station
pickup.
. In order for the connector to be a balanced input, both pins 2
and 3 must connect to some “common mode rejection system”
such as a transformer primary. Consider the wiring arrangement
shown in figure 4.
FIGURE 4
POWER AMP
AS MAINS AC MAINE
Notice that the input to the power amplifier is wired in the
conventiona! balanced configuration with the two-conductor
cable wired through an XLR connector to the transformer primary
and the shield tied to chassis ground. At the mixer end, however,
we are driving.the cable from an unbalanced output (referred to as
single ended...one conductor output plus chassis ground). Such
an output, which is usually asimple phone jack, is only available on
many high quality mixers.
Again notice the wiring arrangement. One of the two wires in
the two-conductor shielded cable (usually the wire associated with
pin 3 on the XLR connector to maintain a proper phase relationship
discussed) connects to the actual mixer output signal (the tip of the
phone jack). The remaining wire, together with the cable shield,
connects to the chassis ground of the mixer (the sleeve of the
phone jack). This system is usually referred to as an “unbalanced
to balanced” configuration.
Now, referring back to our previous discussion of differences
in chassis potentials, notice that one of our differential inputs is
connected (by a separate wire) to the mixer chassis. The other
input is connected to the actual mixer output. In other words, our
input transformer in the power amplifier “thinks” {of course, it's
wired to it) it's actually at the mixer chassis and thus it simply
“ignores” any differences in chassis potential interferences that
exist. Put another way, because the power amplifier input has
“common mode rejection” and the differencesin chassis potential
andas E a
Sala EEE TI =
are common mode signal, they don't get through the transformer.
The mixer output is applied to the transformer as a differential
signal. Thus it gets through and gets amplified.
Various adaptors are available to convert a standard three-
wire XLR connector to a two-wire phone plug to make a convenient
conversion. Of course, for permanent installations, the XLR
connector can be removed and a simple phone plug substituted
and correctly wired. Remember, the two-conductor shielded cable
must be maintained the entire cable length to préserve the
balanced system.
This unbalanced to balanced conftguration performance is
usually quite acceptable for most applications. Occasionally,
however, levels in interference or professional requirements
simply demand a completely balanced distribution system, but
first we must discuss typical applications and types of audio
transformers.
Audio transformers are generally designed for two basic levels
of application, line levels and miggophone levels. Line level types
are designed to work at relatively high: voltage leveis of 1V RMS or
more and are usually found in balanced: outputs of mixers and
balanced inputs of power amplifiers. These are almost always one
to one turns ratio types, providing no voltage gain {or impedance
transformation), but rather are employed to provide the balanced
condition. Occasionally the 1:1 ratio might be adjusted slightly to
provide additional output voltage capability.
Microphone level types are usually designed tówork at lower
primary levels and the turns ratio is adjusted to provide a-voltage
step-up to raise the microphone signal to so called line levels,
Typically a 1 to 10 turns ratio might be used to raise the
microphone signal by a factor of 10. This then would effectively
convert the typical low impedance microphone to high impedance
and raise the signal level to adequately drive standard high
impedance inputs on sound equipment which does riot have low Z
inputs. The cable/transformer combination su pplied with Peavey's
PBH™ microphone has such an arrangement. The cable usedis the
two-conductor shielded type previously described ahd fitted with a
standard male XLR connector wired as outlined. It is designed to
work with a standard low impedance microphone and as such, the
system is balanced due to the transformer. Cable lengths are of no
important consequence due to the characteristics of low
impedance systems, and impedance conversion occurs at the
mixer end of the cable. Again, this transformer is referred to as a
mic to line level type. Notice that this plug-in transformer itself
effectively converts high impedance unbalanced inputs to low
impedance balanced types providing a way to use low Z balanced
microphones with all the obvious advantages of such. Separate
transformer packages are available from many manufacturers for
this purpose with a wide variety of connector options. Both mic-to-
line and line-to-line types are available, the latter being used to
balance graphic equalizer and power amplifier inputs. In this case,
an impedance conversion is not required and signal conditions are
usually at line levels. These types can also be used to provide a
balanced output for mixers and equipment which only have
unbalanced outputs. These applications will be discussed later.
Obviously, if the equipment has low Z transformer balanced in's
and out's with the proper XLR connectors, the transformers are
provided internally.
Many microphones are available with the conversion
transformer in the mic itself. Unfortunately, the space available will
only allow a very small transformer to be used and often both the
low and high end response is degraded. These microphones are
usually fitted with a standard phone plug for use with conventional
high impedance inputs. Remember, in this case the microphone is
no longer balanced (and cannot be used in balanced inputs) and
since it is now a high impedance device, excessive cable lengths
cannot be used.
The truly optimum distribution system is the so called
“balanced to hglanced” configuration. Such a system is shown in
figure 5. :
FIGURE 5
MIXER POWER AMP
BALANCED TO BALANCED SYSTEM DU mm m
A am dak nm he sh ls lis MU ar
a
i
AC MAINS
AC MAINS
‚ 15 a "true differential output”...
In this case, the mixer output uses a line-to-line transformer
and is connected through an XLR connector to the two-conductor
shielded cable. The power amplifier arrangement is similar to that
of figure 4, again using a line-to-line transformer connected
through an XLR connector. The output connector on the mixer is
usually a male type and the power amp input is usually a female
type, as are most microphone inputs. This, too, is sort of a
standard, compatible with most “snakes” on the market.
The system is completely balanced since the mixer signal
comes from a transformer winding and the power amplifier input is
supplied through a transformer winding. Again, of course, our two-
conductor shielded cable is used.
Our input has a "common mode rejection” due to the “true
differential input”... These points have been discussed. Our output
.That is, the output is completely
floating....there is no ground reference. This output provides a
perfect differential signal which will drive the input transformer
and be amplified. Notice the current induced in the secondary of
the mixer output transformer is the same current which will cause
transformer action in the amplifier input transformer. Outside
interference is minimized by the common mode rejection of the
input arrangement and differences in chassis potential are not part
of the circuit because of the differential output. This is truly the
perfect, professional system....one that should always be used if
possible.
If a mixer does not have a balanced output, it can be done
easily with a plug-in transformer package. Similar possibilities
exist for unbalanced power amp inputs. (Be sure to use a line-to-
line type.) This distribution system is so effective in eliminating
outside interference that it is generally used by radio and TY
stations to set up a simple remote broadcast using a telephone
distribution line for the audio feed. In this case, the balanced-to-
balanced system effectively eliminates crosstalk commonly
encountered with the telephone company, and minimizes hum and
noise pickup, even though telephone lines are usually not
shielded.
One more distribution combination exists. This
balanced-to-unbalanced system as shown in figure 6.
FIGURE 6
is the
MIXER POWER AMP
AC MAINE
AC MATHS
In this case, the mixer has a transformer balanced output with
the usual XLR connector, but the power amplifier is unbalanced,
using a standard phone plug. Often, for economic or simplistic
reasons, this is all you have to work with,. Also, as previously
mentioned, the power amplifier orin-line effects device might have
an XLR input connector, butitis not balanced. A typical example of
this is on the Peavey CS-400™ and CS-800™ power amplifiers,
where the balanced transformer module, PL-2™, is not used.
Instead, a PL-1"* blank plug is used, rendering the XLR connector
unbalanced.
In the diagram of figure 6, our two-conductor shielded cable is
converted to a single-ended configuration at the power amplifier
by wiring a phone plug in the same manner as our example of
figure 4. As before, adaptors are also available for this. Obviously,
with the input unbalanced, it no longer has common mode
rejection capability, and any interference that gets through the
shielding of the cable will not be rejected. The signal from the
mixer, however, is transformer balanced, with the secondary
winding connected to the two available conductors. This true
differential output will effectively isolate the chassis differences in
potential problems caused by the different grounding locations of
the equipment. This condition is usually more troublesome than
cable pickup so this arrangement, although not as good as the
previous two examples, will at Jeast solve the worst problem. Again,
it is a compromise... The best situation is still balanced-to-
balanced.
it should now be obvious that whenever parts of the audio
chain are separated by any distance and powered from different
AC mains, the distribution cable or cables must be balanced. In a
typical sound system there might be one main output and two
PANA iad
REIRME UL IRM III
4
monitor outputs and in this case, all three signal cables must be
balanced for optimum performance. However, once these signals
reach their destination, any further distribution of them does not
necessarily have to be balanced, provided that portion of the
distribution system is accomplished on equipment ali powered
from the same AC mains source. Such a system might be several
on-stage power amps connected to a loudspeaker array to
produce the desired sound pressure levels with the same signal
“driving all amplifier channels. In this case, one of these amplifier
Channels wouid require a balanced input transformer to maintaina
balanced condition in the distribution line from the mixer. The
remaining power amplifier inputs can be connected together using
single-conductor shieided cables plugged into the unbalanced
phone jack inputs, provided there is access to the audio signal after
the balanced input transformer. This access point exists on the
Peavey CS-400™/C5-800™ power amps and the dual (bridging)
phone jack inputs will allow “daisy-chaining” between various
channels.
Always remember the concept of “balancing a line”, not
“balancing a channel” of a power amplifier, if the same signal is to
drive each of those channels. Obviously a “stereo” system requires
two “lines.”
A new technique is beginning to become more popular on
contemporary equipment. This is the so called “electronic
balanced” inputs and outputs. Here the input transformer is
replaced by an electronic device called an operational amplifier,
This integrated circuit hastwo input ports, as does the transformer,
and electronically duplicates the common mode rejection
characteristic required to create a balanced input. The balanced
outputs are usually arranged using two “op amps” to generate two
out-of-phase signals necessary to create a true differential output.
Now, it is desirable to design mixers, and especially powered
mixers (mixers with internal power amplifiers), with the power
transformer as an integral part of the chassis. The power
transformer is the device that converts normal 120 VA® line voltage
to levels which can be used by the various electronic circuits. As
such, it generates high levels of external magnetic fields {@ 60 Hz,
the power line frequency....Hum), which are extremely difficult to
eliminate. This external hum field will interact with the audio
transformer and cause an interference signal to be generated
within the transformer itself due to the external magnetic coupling.
Although costly shielding techniques, including MU metal cans
and covers, are usually provided on most high quality audio
transformers, this injected hum can not be completely eliminated.
Consequently, some manufacturers remove the power
transformer from the mixer itself and offer it as an add-on in a
separate box using cables between the box and the mixer. This, of
course, is extremely undesirable...especially in. portable
equipment. For powered mixers, it's impossible.
The alternatives are to either use electronically balanced
inputs and outputs, which then eliminate the audio transformers
themselves, or use special power transformers which have very low
external hum fields. The latter is usually possible in non-powered
mixers (Peavey does this in their Mark MI" mixers), but is totally
impractical in powered mixers due to the size and physical location
requirements. (Again Peavey uses electronically balanced
microphone inputs on all their powered mixers.) Given a choice,
the use of a transformer is always the better approach due to the
superior common mode rejection ratio performance. Also,
because of the “gain function" characteristics of transformers,
input noise levels are usually lower with input transformers. In the
electronically balanced case, all the mic gain will have to come
from the “op amp” stage itself. However, with the new operational
amplifiers available in today's marketplace, good levels of
performance can be achieved economically in both these critical
areas.
Unless very carefully designed, the electronic approach will be
somewhat more susceptible to radio frequency and light dimmer
interference, but in this case, it's a matter of severity.
Often, electronically balanced outputs are simply two out-of-
phase line signals which are not “floating” and thus will not isolate
differences in chassis grounding problems. However, more
elaborate designs will electronically create this important feature.
The best, but unfortunately the most expensive approach, is still
the transformer...which will always be available.
IAE EL Aa e CU
Ta TFT Rima
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement