Balance: Benefit or Bluff? Page 2

In balanced working, the signal's positive-polarity connection (+, or hot) and negative-polarity connection (-, or cold) are segregated so that the - is no longer combined with the earth, shield, or ground line. Instead, a three-wire connection gives the ground line its own separate identity. The opposite-polarity + and - signal lines may then be said to be balanced, or set equal with respect to the ground (figs.3 & 4). This balance is engineered, not for aesthetic or philosophical reasons, but so that the next input in the chain can be arranged just to look at the difference between the two audio lines. Every proper balanced input operates as a differential circuit, and any unwanted noise, hum, or interference common to both lines is therefore canceled (fig.4). The recovered signal level is the difference between the hot and cold signals, and is therefore twice the level of either on its own, which is why substituting a true balanced connection boosts the level by 6dB compared with an unbalanced one, all things being equal.

Fig.3 A balanced signal connection.

Fig.4 A typical all-balanced preamplifier line stage. The hot signal (H) consists of the wanted signal (A) plus noise (N); ie, H=A+N. The wanted cold signal is A in antiphase, -A, and again the same noise picked up by the cable, N; ie, C=-A+N. Because the noise is the same in both hot and cold lines, it is termed "common-mode" noise. Provided the amplifier's differential input has total common-mode rejection, it subtracts the cold signal from the hot, with an output D = (A+N)-(-A+N) = A+A+N-N = 2A. Note complexity compared with fig.1 circuit. In addition, the resistors marked "R" must be closely matched between hot and cold signal polarities to maintain common-mode rejection and minimize distortion.

This is a powerful and valuable technique when you need to use it. When a newscaster needs an audio connection from her mike to the producer's console three rooms away, the cable carrying the low-level signal may traverse power and control ducting for the building—typically a run of 100' or more. An effective means of noise-rejecting transmission is vital in this situation. Take a large orchestra at a recording session: Local spotlight mikes may need 300' of cable to get to a remote mixing desk. Likewise, a crossed pair of mikes hung from the auditorium ceiling for ambience pickup will need a long run of cable.

If the high rejection levels possible with balanced mode are not required, however, then the technique's drawbacks come into focus—it adds complication, complexity, and cost to audio components. Disciples of short-path design know only too well how each unnecessary stage can potentially be a step down the fidelity ladder.

Consider the arguments for and against balanced design for high-quality consumer audio components:

For:
1) High electrical noise immunity to both induced signals (Radio Frequency and Electromagnetic Interference, RFI and EMI) and chassis ground-hum loops.

2) Essentially noiseless, hands-on cable connection; with the ground made first, contact transients are suppressed.

3) Standardized XLR connectors are mechanically self-locking; make good, gas-tight contact to a specified close tolerance; and generally have good cable strain-relief fittings.

4) With balanced cable, the + and - signal paths are equal; ie, they have the same conductor, which improves the potential for good cable sound.

5) Effective over very long runs—greater than 60'—and/or for very-low-level signals of very wide dynamic range, such as those from microphones.

6) Professional and broadcast studio use implies quality by association.

7) Safety requirements for effective chassis grounding can be easily met.

8) Encourages good practice for low electromagnetic radiation and good immunity.

9) For the digital interface, it provides a beneficially higher operating level.

Against:
1) Greater cost.

2) Greater length and complexity of audio circuits.

3) Often results in matching difficulties with existing normal equipment.

4) Restricted choice of cables.

5) Restricted choice of connector grades; the best XLR types are technologically inferior to the best, albeit costly, RCA plug types.

6) Partial disagreement between Europe and the US over pin connections.

7) Poorer sound?

Skirmishes
Although my primary reference system is not balanced, until recently I've gotten by without serious trouble. I've had many skirmishes with balanced equipment, but I've been able to supplement my few balanced reference audio components with validated, current-review loan stock as and when required. However, when I was asked to review the Audio Research LS5 line preamplifier and its matching PH2 phono preamp (footnote 1), I was brought face to face with the full implications of true balanced operation.



Footnote 1: Hi-Fi News & Record Review, February 1994, Vol.39 No.2, pp.30-35.
Article Contents
Share | |
Site Map / Direct Links