Unbalanced Signals

Most unbalanced music instrument cables have two conductor wires. One wire carries the signal and is referred to as the hot line. The other is the ground wire that’s connected to to an insulated wrap and is also called the common ground, earth, or shield. In theory, the ground wire with its insulation shield isolates the hot lead conductor from external interference that can cause hum and other electronic noise. Unbalanced cables usually are terminated with two-wire ¼” phone connectors or the RCA plugs and jacks usually found in consumer audio gear. As we explain below, the ground conductor and insulating shield are only partially successful at eliminating electronic interference.

Signal Conductor Grounded Shield

The signal conductor has a grounded shield that only partially isolates it from electronic noise.

Most instrument amps and guitar effects processors and stompboxes have unbalanced inputs to provide compatibility with standard unbalanced ¼” guitar outputs. This eliminates the need for line-level adapters or preamps with signal-matching capabilities.

Balanced Signals

In balanced signal paths, there is a second signal conductor referred to as the “cold” line. The hot and cold lines transmit identical signals that are 180 degrees out of phase with each other. As the signal voltage in the hot line increases, the cold line’s signal voltage decreases, precisely mirroring the hot signal in reverse. The combined hot and cold signal is fed into a circuit containing what’s known as a differential input that detects any difference between the signals. Because hum and other noise are present in both conductors more or less equally, they appear to the input as in-phase signals.

Balanced Circuit Phase Mirroring

A balanced circuit uses out of phase mirroring to filter out noise.

The differential circuit ignores such in-phase signals, hence canceling out the noise. Amplifiers with differential circuits can cancel out most interference—up to 90 dB or more—essentially reducing noise to inaudible levels.

Differential Amp Noise Cancellation

A differential amp “sees” noise as an in-phase signal thus canceling it out of the transmitted sound.

Balanced lines are ideal in situations involving lengthy cables and low signal levels where noise can be a problem. Mic cables are a good example of such situations. On the other hand, short cable runs carrying higher signal levels are less prone to audible noise.

Balanced Line Connectors

Balanced XLR connectors have 3 pins. An international standard dictates that pin 1 is the ground, pin 2 carries the “hot” signal, and pin 3 is neutral. A few manufacturers unfortunately wire their I/O so that pin 3 carries the hot signal. This can be addressed by reversing the pin 2 and 3 conductors inside the gear or by using an adapter cable that does the same thing.

A cross-connect adapter cable reverses the pin 2 and pin 3 signals.

A differential amp “sees” noise as an in-phase signal thus canceling it out of the transmitted sound.

In addition to XLR plugs and jacks, stereo ¼” TRS phone connectors are also wired for balanced operation. The jack tip (T) is hot, the ring (R) is the neutral lead, and the sleeve (S) carries the ground lead.

Auto Balancing Input Circuits

In the past transformers were used to convert unbalanced signals to balanced operation. They were imperfect, introducing distortion and other artifacts that affected the purity of the signal.

Focusrite Scarlett 2i2 USB Audio Interface

The Focusrite Scarlett 2i2 USB Audio Interface has combination line/mic inputs on its front panel that accept ¼” TRS or XLR connectors.

Today’s gear typically has active circuits that produce less coloration while automatically detecting the signal type based on the type of jack inserted, routing the signal accordingly. Gear with auto-switching I/O often provides a spec referring to the function as “mic/line level I/O”. Such gear usually has level adjustment capabilities to handle varying input voltages. They also often contain 6 dB pads that equalize balanced/unbalanced gain.

Similarly, an XLR balanced input can be fed from an unbalanced output. In this scenario, the hot signal is output the the XLR pin 2 while pins 1 and 3 carry the ground.

XLR connector configured for an unbalanced output

An XLR connector configured for an unbalanced output.

Having cables with this kind of output makes it easy to directly connect unbalanced sources such as guitar effects directly into recording gear, DAWs, and signal processors with balanced inputs.

In many cases you can reverse the process when feeding a balanced output signal to an unbalanced input. However, some older gear with poorly designed balanced outputs could potentially short the output to the ground, producing a noisy, unbalanced signal, or worse, damage the input stage it’s connected to. This is a pretty unlikely scenario, but if you have older gear that has outputs that do not use transformers, and are handy with a soldering iron, you can wire an adapter cable as illustrated below. For safety’s sake we’ve added a couple of 100 ohm resistors in series with pins 2 and 3. The resistor wired to pin 3 prevents short circuiting with the ground. If the resistor becomes warm within a minute of operation, remove it and leave pin 3 unconnected. By connecting pin 1 to the ground and pin 2 to the hot conductor you’ll still get an unbalanced input signal.

XLR balanced to unbalanced connector

Resistors add insurance to this XLR balanced to unbalanced connector.

Do I Really Need Balanced I/O?

Unbalanced connections are usually adequate for home studio and stage rigs where there are no long cable runs. But if if you need lengthy cables in your setup or are working with very low signal levels such as recording a softly played acoustic guitar with a dynamic mic, balanced connections will help keep your high-gain signal noise-free.