Balanced / Unbalanced

September 28, 2013 in AV Design Tips, The Basics by Sam Davisson

UnbalancedWhen I did the first grounding post people seemed to think I was setting the groundworkk for talking about audio. I was actually setting the foundation for all AV. Although its true that any audio problems are related to grounding issues. Balanced and unbalanced interconnect systems, while mostly related to audio, are prevalent in all AV cabling.

RS-232 is an unbalanced interconnect system. RS-422 or RS-485 are balanced. Analog video and SDI are unbalanced. Analog audio, the pro format is balanced. The consumer format, unbalanced. Digital audio employs both types of signal interconnects as well.

Definition: Impedance –
A measure of the opposition to the flow of alternating current through a circuit. Impedance is measured in ohms. The resistance of a circuit to direct current (also measured in ohms) is generally not the same as its impedance, due to the effects of capacitance and induction in and among the components of the circuit.

Balance is defined in terms of the impedance of the two signal conductors with respect to a reference, which is usually ground. If these impedances are equal and non-zero, the system is balanced. If the impedances are unequal the system is unbalanced. A signal conductor with a grounded return conductor is, therefore, an unbalanced.

A small, common-mode, 60 Hz noise, voltage can exist between the chassis of two AC powered devices regardless of whether they are safety grounded (use a three-wire plug) or not. An isolated ground system is designed to minimize this.

Unbalanced Interconnections

An unbalanced interface uses only two conductors to carry the signal from one device to another, one conductor carries the signal and the other is the grounded return. In consumer audio systems this usually consists of a cable with a center conductor and a shield terminated in an RCA Phono plug.

The “RCA Phono” plug was developed by RCA many years ago to be used for short interconnects between a turntable and amplifier inside a phonograph. This unbalanced
interconnect system is simple and inexpensive, but as with many other connector systems has been adopted for uses other than originally intended. It has become the de-facto connector for consumer audio/video equipment. Note, however, that the design was original intended only for short cable runs within the same piece of equipment. Being an unbalanced system it is susceptible to common-mode noise voltages.

A problem occurs when there is a ground voltage (common-mode voltage) between the two interconnected devices. Because of this voltage, a small current will flow down the cable shield between the devices (sometimes referred to as common-mode current, or most commony as a ground loop current). If the cable shield were ideal (zero impedance) this current would not cause a problem. However, since the shield has a finite resistance, a small noise voltage will appear across the length of the cable shield. The magnitude of this voltage will equal the common-mode current times the shield resistance. This voltage is in series with the signal voltage and will add directly to it at the receiver. In other words, an unbalanced interconnect system has no ability to reject common-mode noise voltages.

This coupling can be referred to as common-impedance coupling, and is the result of the fact
that in an unbalanced two-wire system the shield is performing two functions. It is a shield carrying the common-mode noise current, but it is also one of the signal conductors carrying the return signal current

Consider a typical case of the interface between two grounded (3-prong AC plug) pieces of audio equipment. This example is typical some cases would be better and some worse. Also, note that calculating signal to noise is a future discussion.

The shield resistance of a fifteen-foot cable typically will be 0.25 ohms, depending on cable type used. If the 60-Hertz shield current is 250 uA, the voltage developed across the shield will be 62.5 uV. For consumer audio products the reference signal level is 300 mV (-10 dBV). The signal to noise ratio will therefore be 74 dB. In a high end consumer audio sysem, we would most likely be able to hear some 60-Hertz hum in quiet passages of the program material.

You could conclude, at this point, that ungrounded equipment, those using a 2-prong AC plug, might solve this problem by eliminating the ground connections. This sometimes helps, but it does not necessarily eliminate the problem.

For ungrounded equipment, the common-mode ground current can still flow through the inter-winding capacitance of the power transformer. The impedance of this capacitance will normally reduce the current to 100 uA, or less. However audible noise may still be present.

The impedance of the inter-winding capacitance is frequency dependent, more current will flow at higher frequencies (harmonics of 60 Hz) than at the fundamental frequency (60 Hertz). Therefore, the interference will consist of a high frequency buzz which is more audible by the human ear than the 60 Hz hum.

Despite its shortcomings, this unbalanced system works well. Especially in cases with short cable runs, and the equipment within the same rack so there is very little, or no, 60 Hz voltage between the chassis of the interconnected devices.

Trouble shooting and eliminating audio issues will come in future posts but understanding the issue at hand should help you develop techniques of eliminating problems before they occur making troubleshooting unnecessary.

Advantages of Twisted Pair Wire

Twisted pair wiring, even when unshielded, is very effective in reducing magnetic field coupling to and from the wire pair. There are only two conditions necessary for this to be true. First, the signal must flow equally, and in opposite directions, on the two conductors.

Secondly, the length of the twist must be less than one twentieth of a wavelength at the frequencies of concern. (One twist per inch will be effective up to about 500 MHz).

The above is true whether the terminations are balanced or not. In addition, if the terminations are balanced, twisted pair wiring will also be effective in reducing electric field coupling to and from the wire pair. Even though field coupling is not the primary noise coupling mechanism in audio systems, it is still a good practice to always twist the signal and return conductors in a cable. (Twisting is especially important in the case of microphone cables.)

Balanced Interconnections

For a balanced interconnect system both of the signal conductors have an equal, and non-zero, impedance to ground. Therefore, three conductors are required, signal+, signal-, and ground or shield.

Professional audio installations require long cable runs of 500 feet or more and have to be able to operate at microphone signal levels of 3 mV (-50 dBV) as opposed to a line level of 300 mV for consumer audio. As a result he maority of professional audio equipment is designed using a three-conductor balanced interface (two signal conductors,signal and signal return, and a grounded shield), using XLR connectors, or in some cases 1/4 inch phone plugs.

This three conductor balanced interconnect system avoids the problem of the shield having to serve two purposes. The signal is now carried on the two internal conductors (usually twisted together) and the shield only acts as a shield and not also as a signal return line. The penalty for this improvement in performance is a more complicated and hence more expensive system.

A 60Hz shield current flowing between two interconnected devices will still produces a voltage drop in the shield, but this noise voltage is not in series with the signal. Rather it will be coupled equally (as a common-mode noise voltage) into both signal conductors.

Since the receiver looks at the difference between the two signal conductors (not the voltage between one of them and ground), the common-mode noise voltage cancels out and is not seen by the receiver.

A balanced interface theoretically would be completely immune to noise and interference. But, in the real world, nothing is perfect. Even if you attempt to make the impedance of the two signal conductors to ground the same there will be some difference, if only a fraction of a percent, and this will limit the degree of common-mode voltage rejection and therefore the maximum noise suppression possible.

For this example I’ll assume that the impedance balance is such that the circuit can provide
60 dB of common-mode noise rejection, a very conservative assumption, and that the other
parameters are the same as in the unbalanced system above. A well designed balanced interface can have as much as 80 to 100 dB of common-mode noise rejection.

Conclusions

When using unbalanced interconnects between audio equipment the primary noise-coupling mechanism is due to common-impedance coupling. The cable shield resistance
is the common-impedance, and any small 60 Hz AC potential between the equipment chassis is the noise source producing a common-mode noise current on the cable shield (ground loop
current).

This problem can be minimized, or eliminated, by any one of the following approaches:

  • Minimize the AC voltage between the two pieces of equipment.
  • Minimize the cable shield resistance.
  • Block the common-mode noise current by using a signal isolation transformer.
  • Use a balanced interconnection system.
Print Friendly