By Craig Anderton
In today’s studio, you’re likely to find some gear that has balanced analog inputs and outputs, some with unbalanced inputs and outputs, and some with both. Knowing how to interface them properly can make the difference between a quiet, well-behaved signal path, or one rife with signal loss, mismatches, and general unpleasantness. We’ll give a little theory about the two different types, then some practical interfacing examples.
The typical musical instrument cable is an example of an unbalanced line that has two conductors. One wire is the ground line, which is also referred to as common, earth, shield, or DC return. The other is the signal line, often referred to as the "hot" line. The hot line carries the signal, and the ground acts as a voltage reference (Fig, 1). Unbalanced systems typically use two-wire, 1/4" phone connectors, or RCA jacks and plugs (as used in hi-fi gear).
Fig. 1: The inner, “hot” conductor is surrounded by a grounded shield that fences out hum and interference (although shielding is not 100% effective).
Almost all guitar processors and amps use an unbalanced input to be compatible with conventional guitars; computer audio interfaces often include an unbalanced, guitar-compatible input (Fig. 2) so you can plug your guitar directly into the interface without needing any other preamps or level-matching boxes.
Fig. 3: A balanced line system adds a "cold" line to the "hot" line of unbalanced systems.
This type of signal has to feed a special type of input, found in transformers and certain types of amplifier designs, called a differential or balanced input that responds to the difference between these out-of-phase signals. The differential input is a somewhat unusual beast that rejects in-phase signals that flow into the line, such as hum and noise. This is because the kinds of noise and garbage that get into a balanced line generally spill over both signal lines equally. As the differential input responds only to the differences between the two signal lines, when the same signal is present on both lines, then there isn't any difference, they cancel out, and the differential input ignores them.
The simplest way to explain the concept of a differential input is by analogy (Fig. 4). The balanced signals arriving from the previous stage feed the input, and are “differenced” to produce a final, unbalanced output inside your gear. That's all fine, but the real strength of this approach is that signals that are in phase cancel each other out. Some differential amplifiers can reduce these interfering signals (technically called common-mode signals) by over 90 dB—enough to make the interference fade way into the background.
Fig. 4: Conceptual equivalent of a differential amp.
Balanced lines, which involve more expense than unbalanced lines, are favored for long cable runs involving low-level signals, such as those generated by microphones. Unbalanced lines are usually satisfactory for situations where you have fairly high level signals running a short distance, as is often the case in project studios.
3-pin connectors (also called XLR connectors) are wired so that pin 1 is ground, pin 2 is the “hot” signal, and pin 3 is neutral. Some people will try to tell you that pin 3 is hot; don’t believe them. There is an international standard that specifies pin 2 as hot. Unfortunately, some manufacturers still wire their gear so that pin 3 is hot. There are two ways around this: go inside the unit itself and switch the pin 2 and pin 3 wires, or wire up a special cable that cross-connects these two leads (Fig. 5).
Fig. 5: Adapter cable for converting pin 2 hot to pin 3 hot.
Stereo 1/4” phone connectors are also used for balanced line connections as well. In this case, the tip is the hot lead, the ring the neutral lead, and the sleeve is ground. These are often referred to as TRS, for tip-ring-sleeve, balanced connectors.
Although transformers were used for years to provide balanced inputs and outputs, they are not perfect devices. They can pick up hum and introduce a small amount of distortion and coloration; besides, they’re expensive. These days, it’s more likely that active circuits provide the ins and outs because they cost less and are generally conceded to add less coloration. These often use TRS jacks, which when matched with a properly designed input stage offer the advantage of working as either balanced or unbalanced inputs. If you plug in a stereo cable wired for balanced operation, then the hot goes to the tip, the neutral goes to the ring, and the ground goes to the sleeve. If you plug in a mono cable, the hot goes to the tip, but the ring shorts to the sleeve, as does the ground. The way most inputs are designed, when run in unbalanced mode the input stage has a gain of 6 dB but also includes a 6 dB pad. So the net signal level is the same, although the extra gain can add a tiny bit more noise.
The same principle applies to feeding an XLR balanced input from an unbalanced output. To convert from one to the other, you feed the hot signal out to XLR pin 2. The ground connects to pins 1 and 3 (Fig. 6).
Fig. 6: Driving a balanced input from an unbalanced output.
I’ve made up several cables like this to make it easy to go from unbalanced outs (for example, from guitar effects) to balanced inputs, such as spiffy signal processors or hard disk recording interfaces.
Feeding a balanced output to an unbalanced input generally, but not always, works the same way: XLR pin 2 provides the unbalanced hot line, while XLR pins 1 and 3 are tied together and go to ground. However, there is one caveat. With poorly-designed balanced outputs, tying the neutral output to ground could short the preceeding output stage to ground in such a way as to unbalance the output or worse yet, cause physical damage. This is seldom an issue, but if you’re paranoid or use a lot of older solid state gear with non-transformer-based outputs, consider wiring up a cable as follows (Fig. 7). Here, we’ve added a couple of 100 ohm resistors in series with pin 2 and pin 3. The one in series with pin 3 prevents a direct short circuit to ground. If it gets warm after a few seconds to a minute of operation, then simply omit it and leave pin 3 unconnected. You’ll still be able to get an unbalanced output if you just connect to XLR pins 1 (ground) and 2 (hot).
Fig. 7: Balanced output to unbalanced input for the faint of heart.
As you can see this isn’t really all that difficult. Interconnect your gear in the right way, and you need have no fear about mixing and matching balanced and unbalanced devices.
Mic connectors are almost always balanced, as evidenced by an XLR jack--a common giveaway that the circuitry is balanced (Fig. 8).
However, as mentioned above, 1/4" TRS phone jacks can also be balanced, even though they appear to use standard 1/4" guitar cord-type jacks.
But what if you run 1/4" unbalanced lines, but your gear has 1/4" balanced connectors? No worries. TRS jacks are electronically balanced, so if you plug in a mono plug, they simply turn into an unbalanced output. (The one place where you don't want to plug a mono plug into a stereo connection is the headphone out.)
"Combination" jacks are a welcome addition to the connector scene. These combine an XLR connector and 1/4" TRS connector; the 1/4" connector is located inside the XLR section. This is a truly universal solution, as you can interface XLR balanced, 1/4" balanced, and 1/4" unbalanced lines with one connector.
Fig. 9: Yamaha's GO46 mobile interface has combination front panel input jacks. Note how the pins from an XLR connector can fit in the three small holes bordering the internal, larger hole, which accommodates a 1/4" plug.
In a small studio or onstage setup with relatively short cable runs, unbalanced connections work just fine — although you'll still probably want to use balanced connections for your mics. Most of the time there's no real need to rip your setup apart and rewire it with balanced connections. Where balanced lines shine is in situations with long cable runs, especially ones carrying low signal levels that are subject to noise or interference.
The bottom line is that both technologies have their uses. But balanced lines tend to be more expensive, so if you don't really need to go balanced, you're probably okay sticking with unbalanced lines.
Some gear you already have might work well in a pinch for signal translation. For example, I have an old (okay, ancient, but you get the point) TASCAM DA-30 DAT with both balanced and unbalanced inputs and outputs, as well as both AES/EBU and S/PDIF digital ins and outs. Although I don’t use it any more for recording, it makes a great translator: I can plug in a balanced signal and get unbalanced out, or vice versa, as well as convert AES/EBU in to S/PDIF out or the reverse. It can also serve as an A/D converter for turning an analog signal into a digital one, or a digital input into an analog output. Check out those connectors on your gear; you just might have some “free” translation capabilities lurking around your studio.
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