Balanced vs. Unbalanced Cables - page 2
In 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 often require long cable runs of 50 to 100 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 interconnections (a two order of magnitude smaller signal level). As a result most professional audio equipment is interconnected using a three-conductor balanced cable (two signal conductors and a shield) using XLR connectors, or in some cases 1/4-inch phone plugs.1/4" Phone Plug
The 1/4" "phone" plug (not to be confused with the RCA "phono" plug) was developed by AT & T for use in early-day manual telephone switchboards, hence its name "phone plug." It can interconnect three conductors, referred to as tip, ring, and sleeve.
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 conductor. The penalty for this improvement in performance is a more complicated and hence more expensive system.
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 very low-level microphone cables.)
A 60 Hz 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. In practice, however, nothing is perfect. Even if we 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 hence the maximum noise suppression possible.
Example 2: Let's assume that the impedance balance is such that the circuit can provide 60 dB of common-mode noise rejection (an easily obtainable number) and that the other parameters are the same as in the previous example. (Note: a very well designed balanced interface can have as much as 80 to 100 dB of common-mode noise rejection.)
Since the shield resistance of a fifteen-foot cable is about 0.25 ohms and assuming the shield current is 250 uA, the voltage developed across the shield will be 62.5 uV (the same as in Example 1). This noise voltage is, however, not in series with the signal, rather it will couple as a common-mode noise voltage into the balanced circuit. Since (in our example) the balanced circuit has 60 dB of common-mode noise rejection, the noise voltage coupled into the receiver will be reduced by an additional 60 dB. Sixty dB represents a reduction of 1,000 to 1. Therefore the noise voltage coupled into the system will be only 0.063 uV (62.5 uV / 1,000). For the line level signal (300 mV) of Example 1, this represents a S/N ratio of 134 dB. We now have a very good quality audio system with the S/N ratio well above the desired 100 dB. Even for the case of a microphone level signal (3 mV) the S/N ratio is 114 dB, still a quite respectable number.
Note: In order for a balanced system to be effective in reducing common-mode noise not only must the interconnection cable be balanced, but the terminations must also be balanced. Using transformers or balanced amplifiers are two possible approaches to providing balanced terminations. A full discussion of the various ways to balance the terminations (and the advantages and disadvantages of each) is beyond the scope of this discussion.
Notice what has happened. For the case of the 300 mV line level signal, the unbalanced shielded cable of the first example provided a S/N ratio of 74 dB, and the balanced signal conductors of the second example reduced that noise by an additional 60 dB giving us the sum of the two or 134 dB for the overall S/N ratio.
Therefore we can conclude that for the situation described above, regardless of the signal level the balanced interface will have 60 dB (or more, depending upon the degree of balance achieved) less noise than the unbalanced interface.
The Telephone System
An excellent example of the effectiveness of using a balanced system to reduce noise is the telephone system, where signal levels are typically the tens to hundredths of millivolts. Telephone cables often run for many miles parallel to high voltage (4,000 to 14,000 volts) AC power lines and you do not hear any 60 Hertz hum in the telephone. This is because the telephone system is a balanced system. On the rare occasions where you do hear some hum in the telephone it is because something has caused an unbalance to occur in the lines, and it will go away once the balance is restored.
When using unbalanced interconnections 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.
Check Out Our Article on Ground Loops for more information
Electric and magnetic field coupling to the interconnection cables, although possible, is not normally a major noise problem in most audio systems.
Special thanks to Henry Ott Consultants: http://www.hottconsultants.com
For more details on the characteristics of balanced systems see "Noise Reduction Techniques in Electronic Systems," Second Edition, Chapter 4, Balancing and Filtering, by Henry W. Ott, published by Wiley-Interscience, 1988
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