Back in my college days when PortaPaks were all the rage, one of the most helpful and practical resources I relied on was the "The Spaghetti City Video Manual: A Guide to Use, Repair and Maintenance," by the collective Videofreex. This book clearly explained not only the gear but all the connections, so that very quickly I was able to identify and wire all the various audio connectors--RCA, mini plug, 1/4-inch tip-sleeve or TRS (tip-ring-sleeve) plug, and the XLR connector (then often referred to as the Canon connector).
What about today's young video/audio folks? A chief engineer at a post house in New York recently shared this observation with me. The computer generation, he said, tended not to know about audio connections or connections in general, and was often mystified on how to deal with different audio connectors as well as balanced and unbalanced I/Os (inputs and outputs).
As previously discussed in this column, audio is not necessarily plug-and-play. It's not as simple as connecting a keyboard, monitor and mouse to a computer with pre-wired supplied cables.
So here's a brief overview of the basics of balanced and unbalanced audio I/O.
BASICS OF BALANCE
Balanced audio is carried on a pair of wires, neither of which is grounded; the lines being driven with equal voltage, but each with opposite polarity. It's important that the two lines have the same source impedance, but this impedance can be anything. In the days of power-matched lines, this impedance was often 600 ohms or 150 ohms, and is now 110 ohms for balanced digital audio (AES-3).
In today's analog audio world, where bridging circuits are the norm, typical source impedances (outputs) are around 50 to 60 ohms. (These sources are designed to operate into a higher impedance, up to 10,000 ohms.)
Balance by itself doesn't infer any particular type of cable topology, but in practice, to reduce susceptibility to magnetic interference, balanced audio cable is comprised of a twisted-pair for the signal.
Common terminology for those two wires is: high/low; plus/minus; noninverting/inverting.
Wire color is not always a good indicator of which is which, but for single cables having red and black wires, generally the red wire is the high side, while the black is the low.
Common connectors used for balanced audio include the XLR, 1/4-inch TRS phone plug and jack, terminal strips, and various multiconductor connectors like D-25, AMP Champ 50-pin, and Elco (or Edac).
There has been some attempt at standardizing wiring for multiconductor connectors, but there are differences between manufacturers. It's always best to check the equipment manual for wiring information.
(A reminder: Especially when using gear with multiconductor connectors--check for the pin-1 problem.)
Here are some typical balanced audio connections:
Another key property of balanced lines is that the shield is not part of the signal path. The shield is just that, provided it's properly connected at the chassis of the gear it's interconnecting.
Balanced circuits offer good rejection of noise induced equally on both wires (i.e., they offer high common mode rejection), and allow long cable runs.
Balanced circuitry used to be primarily transformer-derived, but now differential active circuitry is used in most applications, although even now sometimes only a transformer will do. Good circuit design with true balanced inputs is crucial in obtaining the benefits balance has to offer.
An unbalanced line uses one wire for the signal and another for the ground. Unlike balanced lines, unbalanced lines use the ground for signal return. In typical unbalanced cables, the ground is the cable shield.
Unbalanced lines are more susceptible to noise, and cable runs should be short. Common connectors used for unbalanced lines include RCA, 1/4-inch tip-sleeve and 1/8-inch mini (commonly found on computer sound cards). One-quarter-inch TRS plugs and jacks also can be used for unbalanced two-channel circuits.
For example, for unbalanced stereo, the tip-sleeve part of the connector is for the left channel while the ring-sleeve part is for the right. The sleeves are used to connect shields from both left and right.
The source impedance for unbalanced audio circuits tends to be higher than unbalanced, around 1,000 to 10,000 ohms, for operating into a receive impedance of typically 50,000 ohms or higher.
Typical unbalanced wiring for an RCA or 1/4-inch or mini phone TS connector:
Because of the advantages of balanced circuits, these tend to be found in most professional audio gear. But look carefully at monitor outputs or insert I/Os on audio consoles. Especially on low- and mid-priced consoles, the inserts may actually be unbalanced, even though the channel inputs and main outputs are balanced.
Some typical connections for inserts in consoles in this price range include TS phone jacks (separate jacks for insert sends and returns); or to save space, TRS jacks that connect the tip and ring together until a plug is inserted. The tip, as an example, could be the send, while the ring could be the return.
Unbalanced circuits are typically found in consumer gear and often in so-called prosumer or semi-pro gear. But there's still much of this gear used in broadcast and post facilities--just look at any dub rack of DVDs and VHS VCRs.
Impedance isn't the only difference between the way balanced and unbalanced audio circuits are implemented. There's a significant level difference as well. (Note that level and impedance do not define whether a circuit is unbalanced or balanced. But engineering practices have developed for each.)
For a professional audio console, a typical balanced line level output is +4 dBv (referenced to 0.775 volts) or about 1.2 volts. A typical consumer unbalanced audio device would have an output around -10 dBV (referenced to 1 volt) or 316 mV.
A consumer device feeding into this pro console would probably not have sufficient level to drive the console's input to normal operating level. The input gain on the channel strip could be raised to maximum, but that would raise the noise as well.
Looking at this from another perspective, this pro console would easily overdrive the input of a consumer device, like a DVD recorder. Consumer devices tend not to have input attenuation to deal with this.
So connecting balanced and unbalanced circuits is not as simple as making up adapter cables, although in a pinch, this can be the solution of choice.
Common practice today is to use an active device for balanced-to-unbalanced (and vice versa) conversion. There are many brands of these interface boxes available. These interfaces take care of both the impedance and level differences between the balanced and unbalanced lines, and many have level controls for fine tweaking. Because unbalanced lines need to be kept short, install the interface box close to the equipment with the unbalanced I/Os. Keep unbalanced lines away from power cables and supplies. Especially avoid running unbalanced lines parallel or close to power lines.
Sometimes even a good interface box can't provide sufficient isolation. In that case, a good transformer will be needed. This will take care of the balanced/unbalanced dynamic and impedance conversion, but usually not the level. But when noise is an issue, a level difference could be tolerated.
As an example, the wiring for a transformer for a unbalanced-to-balanced connection:
On the primary side of the transformer (high impedance), wire the tip of an RCA or TS connector to one side of the transformer and the sleeve (shield) to the other terminal. On the secondary side (low impedance), wire one side to pin 2 of an XLR (or tip of a TRS connector) and the other side to pin 3 of an XLR (or the ring). Pin 1 of the XLR (or sleeve) should be connected to the chassis housing the transformer.
A last resort for interconnecting balanced and unbalanced equipment is the use of adapter cables. The problem with adapters is that these provide neither level nor impedance transformation.
Pre-wired adapters or cables can be purchased. Check the wiring, especially the shield. The shield can be connected in a few ways--the sleeve of the unbalanced connector to pin 1 of an XLR (or the sleeve of a TRS connector); the sleeve to pin 3 of the XLR (or ring of the TRS); the sleeve to both pins 1 and 3 of an XLR (or ring and sleeve of a TRS connector). On some equipment, one type may work better than another.
Here is another way to wire an unbalanced-to-balanced cable, using a twisted pair audio cable with red and black wires. This configuration is often preferable to those listed previously.
I hope that this overview has provided some appreciation of the importance of balanced audio circuits and how to interface balanced with the more problematic unbalanced gear.
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