London Live DIY HiFi Circle

Circuits - A Balanced Opinion

by Neil J Mackie

  • Introduction
  • Balanced Equipment - The Myths
  • What is a Balanced Circuit?
  • CMRR
  • Conversion
  • Conclusion
  • Sources for this article
  • Comments?
  • Introduction

    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 end equipment. 

    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 come cheap. 


    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
    • 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.


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