Last Updated: 20060803, mei
M3 - BALANCED INTERFACES
Presenter:
Bill Whitlock, Jensen Transformers, Inc. - Chatsworth, CA, USA
Abstract:
One goal in the design of audio equipment is to maintain a high signal-to-noise ratio. But audio equipment most often operates on utility AC power, which, even under ideal conditions, normally creates ground voltage differences, magnetic fields, and electric fields. RF energy is increasingly omnipresent, too. Balanced interfaces are capable of conveying wide dynamic range analog audio signals while giving them unrivaled immunity to interference. Realizing this full capability in real-world, mass-produced equipment is not necessarily costly but requires some understanding of several common mistakes made by equipment designers. The telephone company pioneered the widespread use of balanced lines and for 50 years virtually all audio equipment used transformers at its balanced inputs and outputs—their high noise rejection was taken for granted.
When solid-state differential amplifiers began replacing transformers, most designers failed to recognize the importance of common-mode impedances—which are solely responsible for noise rejection. Instead, most believed that “balance” meant equal and opposite signal swings—which is a myth. As a result, most modern audio equipment has poor noise rejection when operating in real-world systems, even though it may have impressive rejection in a laboratory test. The IEC recognized this dichotomy when they revised their CMRR test standards in 2000 (at the urging of this author). A new IC uses bootstrap techniques to raise its common-mode impedances, and real-world noise rejection, to levels comparable to the finest transformers.
The three basic types of balanced output circuits, each with a peculiar set of tradeoffs, must be accommodated by balanced input circuits. Further, certain cable constructions and shield connections can degrade noise rejection of an otherwise perfect interface. A very common equipment design error, the “pin 1 problem,” causes shield connections to behave as a low-impedance audio inputs, allowing power-line noise and RF interference to enter the signal path.
