Interconexión De Equipos Balanceados y Desbalanceados

Jensen AN-003

INTERCONNECTION OF BALANCED AND UNBALANCED EQUIPMENT
by Bill Whitlock Techniques for interconnecting balanced and unbalanced equipment in audio systems seems to be a murky topic for many users, technicians, and system designers. It is the subject of numerous calls to our applications engineers. Confusion, and even controversy, is further fueled by some published "guides" whoseengineering basis is dubious, at best. This paper briefly explains the hardware, some underlying engineering theory, and includes wiring diagrams for interfaces that solve many typical system problems. 1 - "UNBALANCED" and "BALANCED" What do these terms really mean? Since I have previously written about the subject of balanced lines in some detail, I will just summarize the most important pointshere. Balance is defined in terms of the impedances of the two signal conductors with respect to a reference, which is usually "ground". An UNBALANCED input or output connects one of its signal conductors to ground and has a non-zero impedance at the other signal conductor. Unbalanced inputs and outputs are very popular in consumer electronics, electronic musical instruments, and low cost (oftencalled "semi-pro") audio equipment. Examples of typical circuits are shown below. A BALANCED input or output uses two signal conductors which have equal impedances to ground. Balanced inputs and outputs are widely used in professional equipment because the input differential amplifier can, in theory at least, totally null its response to ground noise which exists equally on both signal lines.Examples of typical circuits are shown below.

Typical Balanced Input Because of its low cost and small size, this "active balanced" differential input circuit (or some variation of it) is very widely used. The pairs of 10 kS resistors are trimmed to match within 0.01% in order to null response to ground noise. When used in real-world systems, however, a major drawback of this circuit is its exquisitesensitivity to unbalances in the output (source) impedances of the line driver. This seriously compromises ground noise rejection or CMRR (Common-Mode Rejection Ratio) of the system.

Typical Unbalanced Input Usual values for RL are 10 kS to 100 kS, regardless of the equipment type. It is RL that effectively determines the input impedance of the circuit. Typical Balanced Output This outputcircuit provides two equal amplitude but opposite polarity (symmetrical) signals and equal output (source) impedances to the lines. Values for RS and CC are generally in the same range as those for semi-pro equipment and tolerances are typically ±5% for RS and ±20% for CC. Although the balance of these (source) impedances strongly control system CMRR performance, especially when used with activebalanced inputs, few equipment makers seem to recognize its importance. The signal symmetry of this, or any other, balanced line driver has nothing to do with ground noise rejection. 2 - UNBALANCED to BALANCED INTERFACES The interfaces on the following page do NOT provide the 12 dB gain necessary to raise the nominal !10 dBV (316 mV RMS) "consumer" reference level to the nominal +4 dBu (1.23 V RMS)"pro" reference level. If the pro equipment doesn't have enough gain "reach", an active interface may be necessary. A step-up transformer, even an ideal lossless one, is not a viable source of gain in this application. Reflected impedances cause excess level losses and compromise both low frequency response and distortion.

Typical Unbalanced Output Usual values for RS are 330 S to 1 kS and for CCare 4.7 µF to 47 µF in consumer and musical instrument equipment, often specified to drive a 10 kS minimum load. In Semi-pro audio equipment, often specified to drive a 600 S minimum load, RS is usually 47 S to 220 S and CC is 47 µF to 220 µF. The inherent weakness of unbalanced interconnections is that the shield, which is also a signal conductor, is a path for power line related currents that...