Circuits - A Balanced Opinion
Equipment - The Myths What
is a Balanced Circuit? CMRR
Conversion Conclusion Sources for this article Comments?
This article was prompted by a discussion at one of the London
Live DIY HiFi Circle pub meetings.
As is often the case no one appeared to definitively answer the
questions being raised about balanced circuits but the discussion
did at least start me thinking. I can't claim to know a lot about
the subject and I can't claim that what follows has been extensively
researched but it hopefully it will give you some "pub level"
answers to "pub level" questions.
Most of you will have heard about balanced equipment, balanced
inputs and balanced outputs. You may have the idea that "balanced"
is better, that it is somehow connected with high end or more
expensive equipment but don't know why. I hope by the end of
this article you will understand the issues involved and be able
to make your own judgement.
Balanced Equipment - The Myths
A balanced circuit is different from an unbalanced circuit
(see the next section for an explanation) but is not necessarily,
especially in a HiFi environment, any better. A balanced circuit
has nothing to do with the quality of the circuit or of its components.
It is perfectly possible to design and construct a balanced circuit
that is in audio terms inferior to an unbalanced one. In audio
terms neither is superior, just different. However designing
a balanced circuit does require more care than the design of
an unbalanced circuit and in general requires more components
and so is more expensive. The effect of both these influences
is that balanced circuitry tends to appear more often in high
What is a Balanced Circuit?
A balanced circuit is one where the +ve and -ve signal paths
are identical. That's a short statement that requires a bit more
explanation to expose its full meaning. Any analogue system (I
don't think anyone applies the term "balanced" to digital
systems but who knows it could become next years marketing scam!)
consists of a series of signal sources (outputs) connected to
signal sinks (inputs). Every signal source has two connections,
a +ve and a -ve, a stereo connection consisting of two signals
therefore has four connections. "Hold on a bit," you
may be thinking here if you have ever wired up your own interconnect,
"my stereo interconnect consists of three wires so how come
you are saying there should be four?" Well your cable must
have been an unbalanced interconnect and this illustrates the
difference between a balanced and an unbalanced circuit. In an
unbalanced circuit the -ve left and right connections share a
single wire typically called signal ground, in a balanced circuit
the -ve connections are kept separate from each other and from
ground. Although the number of wires is the most obvious visible
difference between a balanced and unbalanced circuit it is really
only a by-product and is not the real purpose of balanced circuit
which is to minimise the amount of noise picked up by the circuit.
It does by having the +ve and -ve signal paths that are as near
as possible to being identical. This means that for every resistor
in the +ve signal path there is also a resistor in the -ve signal
path, for every transistor amplifying the +ve signal a corresponding
transistor amplifies the -ve signal, and so on. Why this should
reduce noise is explained later in the section on cmrr.
Only an ideal circuit is ever balanced because no two resistors
are identical, no two transistors exactly alike, circuit layouts
don't normally have signal paths that are mirror images of each
other. No actual circuit can achieve the goal of being perfectly
balanced; each circuit is only balanced to a certain degree.
However the difference between a balanced and unbalanced circuit
is that, as designed, the unbalanced circuit can never be balanced.
On the other hand a circuit designed as balanced can be built
and with careful component selection and layout the builder can
improve the balanced nature of the circuit.
Common mode rejection ratio (CMRR) is used as a measure of
the quality of a balanced circuit. It was said earlier that the
aim of a balanced circuit was noise reduction, cmrr is a measure
of the susceptibility of the circuit to external noise. Making
a circuit more balanced improves its cmrr, meaning it is less
susceptible to external noise sources. In any circuit an external
noise source will disturb both the +ve and -ve signals. In an
unbalanced circuit the -ve signal path is signal ground and has
no circuit elements in it. The +ve signal path however has components
in it, most importantly transistors which amplify the noise.
The result is that unbalanced circuits have a poor performance
when subject to external noise sources because once in the circuit
the noise gets amplified. In a balanced circuit any noise passes
through similar circuit elements in both the +ve and -ve signal
paths, when the signals are amplified so is the noise. However
at the balanced circuit's outputs the signal appears as the difference
between the outputs, while the noise has caused the same
change in each of the outputs and so does not change the difference
between the outputs. Considering a more specific example, say
there is a 2mV signal at the input of a balanced circuit meaning
the +ve signal path has +1mV and the -ve signal path -1mV. If
there is also 20mV of external noise picked up by both signal
paths the +ve signal path has +21mV and the -ve +19mV. If each
path is amplified equally by a gain of 10 the output signals
are +210mV and +190mV. The signal at the output of the balanced
circuit is the difference between the +ve and -ve outputs, so
the output is (+210mV) - (+190mV) = 20mV, the signal has been
amplified by 10 but the noise has gone.
As an aside a twin pair, where the wires run parallel in a
flat cable can be considered an unbalanced circuit whereas a
twisted pair is a balanced one. Why is this? It's because in
the twisted pair each wire curves to take the place of the other,
if there is an electromagnetic field causing noise in the cable
each wire moves towards and away from the field as they twist
so equalising the effect of the noise on each wire.
This section covers ways of connecting unbalanced signals
to balanced inputs and vice versa. As a reminder, an unbalanced
line consists of two wires, a signal and a ground while for stereo
there are three wires, a left and right signal and a ground.
The balanced equivalents have 3 wires for mono, signal+, signal-
and ground, while stereo has 5 wires. Unbalanced and balanced
lines are illustrated in the first diagram.
How can you connect an unbalanced signal to a balanced input?
One way is to connect the signal to one of the balanced inputs
and the ground to the other balanced input. The line will still
be balanced provided the unbalanced output driving the line does
not have an output resistor in series at its output. If it does
the line will be unbalanced and the advantages of a balanced
line described above will be lost. An unbalanced line can also
be converted into a balanaced line using a circuit like that
shown in the second diagram.
So how can a balanced line be connected to an unbalanced input?
If the line is "floating" such as the output of a transformer
(or a cartridge provided none of the arm leads are grounded)
then one end can be grounded and the other connected to the unbalanced
input. If the balanced input does have some reference to ground,
such as the output of a transistor amplifier, then half the output
can be used by connecting one of the balanced signals to the
unbalanced input, connecting the grounds together and leaving
the second balanced signal unconnected. Unfortunately this connection
means a 6db loss in signal level and the line is no longer balanced.
This is because the second signal is unused so it can't be used
to cancel any noise picked up by the cable. An alternative is
to use a circuit as shown in the third diagram.
An alternative to all the conversion circuits described above
is to use a signal transformer. However for the same quality
this is a more expensive option as quality transformers dont
You should now see that balanced circuits are an advantage
in noisy environments or in situations where the pickup of noise
is more likely. Do either of these apply to your domestic environment?
They may, but generally the domestic environment is not noisy
and there should not be a problem with noise unless you are running
very long interconnects, for instance to power amps located next
to the speakers. High end equipment and a balanced configuration
are not synonymous, the single ended amplifier design is not
balanced but is considered suitable for high end equipment. If
your system comprises unbalanced components I hope you can now
be happy with it, balanced equipment is not necessarily better.
Sources for this article
- The Pro Audio FAQ of the Internet Usenet group rec.audio.pro.
- The articles "A Balance Opinion" and "A
Balanced View" in the April and May 1997 issues of "Electronics
World incorporating Wireless World".
- The 1993 Borbely Audio catalogue.
Send mail to Neil Mackie