High signal-to-noise ratio is an important goal for most audio systems. However, ac power connections unavoidably create ground voltage differences, magnetic fields, and electric fields. Balanced interfaces, in theory, are totally immune to such interfere
TESTING BALANCED LINE RECEIVERS
Noise rejection in a real-world balanced interface is often far less than that touted for the receiving input. That's because the performance of balanced inputs have traditionally been measured in ways that ignore the effects of line driver and cable impedance imbalances.
For example, the old IEC method essentially "tweaked" the driving source impedance until it had zero imbalance. Another method, which simply ties the two inputs together and is still used by many engineers, is equally unrealistic and its results essentially meaningless. This author is pleased to have convinced the IEC, with the help of John Woodgate, to adopt a new CMRR test that inserts realistic impedance imbalances in the driving source. The new test is part of the third edition of IEC Standard 60268-3, Sound System Equipment - Part 3: Amplifiers
, issued in August 2000. A schematic of the old and new test methods is shown on the next page. It's very important to understand that noise rejection in a balanced interface isn't just a function of the receiver — actual performance in a real system depends on how the driver, cable, and receiver interact.
A NEW LINE RECEIVER CIRCUIT
The new circuit uses a technique known as "bootstrapping" to raise the ac common-mode input impedance of the receiver to over 10 MΩ at audio frequencies. The schematic
shows the basic technique. By driving the lower end of R2 to nearly same ac voltage as the upper end, current flow through R2 is greatly reduced, effectively increasing its value. At dc, of course, Z is simply R1 + R2. If gain G is unity, for frequencies within the passband of the high-pass filter formed by C and R1, the effective value of R2 is increased and will approach infinity at sufficiently high frequencies. For example, if R1 and R2 are 10 kΩ each, the input impedance at dc is 20 kΩ. This resistance provides a dc path for amplifier bias current as well as leakage current that might flow from a signal source. At higher frequencies, the bootstrap greatly increases the input impedance, limited ultimately by the gain and bandwidth of amplifier G. Impedances greater than 10 MΩ across the audio spectrum can be achieved.