Aphex Systems 320A owner manual Appendices

320A 9. Appendices

Compellor

Appendix A: Balanced and Unbalanced Lines and Operating Levels

Interfacing all types of equipment with balanced and unbalanced lines and can sometimes be trouble - some. First you have to somehow connect balanced to unbalanced and then you have to deal with dif - ferent levels. This tutorial will teach you about the principles of balanced and unbalanced lines, wiring standards, and how to e￿ectively interface them.

Standards

Professional audio equipment usually comes equipped with inputs and outputs that are balanced using 3-pin XLR connectors and sometimes 1/4 inch phone jacks as well. This equipment most often is designed to operate at +4dBu, a professional indus - try standard. That translates to a magnitude of 1.23 volts RMS (Root-Mean-Squared).

Consumer gear has unbalanced I/O as standard, usu - ally on RCA jacks. The normal operating signal level follows the IHF (Institute of High Fidelity) standard of -10dBV, or 0.316 volts (316mV) RMS. Converting to dBu dimensions, this works out to be the same as -7.79dBu. There is therefore a di￿erence of 11.79dB between pro and consumer operating levels.

Grounding

There is the notion that some king of earthly “ground” exists out there that sinks all the noise and acts as some kind of a shield. You see wires connected to ground rods and water pipes that are supposed to get a good ground. This is not a correct interpreta - tion of grounding from an audio standpoint. Proper grounding of equipment and wiring is important and you will gain a better understanding of that as you read along.

Balanced -vs- Unbalanced

Every audio signal is connected through a circuit. The circuit must contain two conductors to create a com - plete return path. In other words, a signal voltage is conducted to a piece of equipment by injecting a cur - rent into a wire. That current has ￿ow though to the destination through the wire and return back to the source through another wire. Since audio is an alter - nating voltage, swinging through negative and posi - tive polarity, the current through the two conductors changes direction each alternate half cycle. Which wire is the source and which is the return alternates accordingly. In this regard, balanced and unbalanced

lines are the same. They both need two conductors.

What makes a system unbalanced is when one of the wires is formed into a tube that wraps around the other conductor, without touching it, such that the outer conductor can be said to “shield” the inner conductor. This describes all of the coaxial cable used for video, cable-TV and radio as well as most of the high ￿delity audio cables.

Balancing

If both conductors are identical insulated wires that are twisted together, then they form a balanced line. This describes telephone lines, microphone cables, and most professional audio cables. Typical balanced cables include an additional shield wrap around the twisted pair, but this is not strictly required for bal - anced lines to work properly.

Figure 1 Balanced Line Model

Figure 2 Unbalanced Line Model

Many people, because they have more experience with unbalanced wiring, think that balanced is confusing. Believe it or not, balanced lines are really easier to understand than unbalanced. There is no grounding issue with balanced, and the way it works is perfectly natural and simple. Balancing naturally rejects hum and noise and eliminates all sorts of complications in interfacing.

Balanced transmission works something like this. Your balanced input stage looks at the two wires and detects only the potential (voltage) di￿erence between them. Anything that is the same on the two wires (for all practical purposes as seen measuring from ground) is called a common mode signal and

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