With rare exception, when it comes to audio system performance, the whole is much less than the sum of its parts. When audio components are interconnected to form a system, it usually has far more noise and distortion than would be predicted from test bench measurements on each component.
BALANCED AUDIO INTERFACESThe goal of modern audio interfaces is to transfer maximum signal voltage while, of course, rejecting ground voltage differences and interference from external electrostatic and magnetic fields. To accomplish this requires a balanced interface with low differential output impedance at the driver and high differential input impedance at the receiver. This method, sometimes called “voltage matching”, should not be confused with impedance matching where output and input impedances are made equal, wasting half the driver voltage in order to transfer maximum power to the receiver. Line drivers with low differential output impedance generally have inherently low common-mode impedances (usually half the differential or “output” impedance). Because of normal tolerances in the resistors and capacitors which usually determine a driver’s output impedance, imbalances up to about 20 S should be routinely expected. This defines a “real-world” source.
In a previous paper, this author has examined balanced audio interfaces in some detail, including performance comparisons of various receiver types.[2] It was concluded that, regardless of their circuit topology, popular active receivers have very poor CMRR when driven from real-world sources. This poor performance is a direct result of their low common-mode input impedances.
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A BALANCED SYSTEM IS A BRIDGE
In the basic balanced interface of Figure 1, the output impedances of the driver and the input impedances of the receiver effectively form the Wheatstone bridge shown in Figure 2. If the bridge is not “balanced” or “nulled”, some of the ground noise Vcm will be “converted” to a differential signal on the line. Rejection or nulling of the common-mode voltage is critically dependent on the ratio matching of the driver/receiver common-mode impedances in the two circuit sides or branches.
The bridge is most sensitive to small fractional impedance changes in one of its arms when all arms have the same impedance. It is least sensitive when upper and lower arms have widely differing impedances, for example when upper arms are very low and lower arms are very high. Impedances must be changed in pairs because the impedance ratios of the two sides must match in order to null the bridge. Therefore, we can minimize the sensitivity of a balanced system (bridge) to impedance imbalances by making common-mode impedances very low at one end of the line and very high at the other. This condition is consistent with the previously mentioned requirements for “voltage matching”.
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