Balanced

Overview

The term "Balanced" is used in audio to describe a form of signal transmission that has two signal conductors which carry signals which are both: a.) identical except for being of opposite polarity and b.) have a signal amplitude voltage range equally above and below signal ground (equal positive and negative voltage range).

History

In order to minimize noise pick-up by long cables carrying low level signal such as those output by a microphone; a system of transmission was developed using audio "signal" transformers. One of the advantages of an audio transformer is that there is no physical connection between the input circuit ("primary coil") and the output circuit ("secondary").

Before audio electronics were developed that worked on power supplies with both "plus" and "minus" voltages; some method was needed to isolate the DC voltages required for operation of audio circuits operating on "single-ended" DC power from different stages of the circuitry and from devices connected to the input and output. In many cases; "coupling capacitors" were used to allow the AC (audio signal) to pass while "blocking" the DC. Transformers could also serve this function; and provided other useful functions such as impedance and level matching.

A microphone can have a transformer in its output with a "floating" secondary (output). This means simply that neither of the two signal conductors is referenced to ground. At the input of the microphone preamp; there was a second transformer that contains a primary with a "center-tap" which referenced the center of the winding to ground. As a result; the signals in the two signal conductors would be of the "same" except that they were of opposite polarity. The ground reference provided by the center-tap of the micpre input transformer caused the signal voltages to be "centered" on ground (of equal positive and negative voltage range).

The advantage of this approach is that the audio signal was passed from the primary to the secondary for further amplification; but interference signals would appear of equal voltage and the SAME polarity, resulting in them effectively canceling each other in the transformer by generating no signal in the secondary. Thus the low-level audio signal was transmitted and the noise was not.

Basics

Analog

There are basically two types of analog audio interconnects: Balanced and Unbalanced.

The differences are:

1. The Unbalanced connection uses the outer "shield" conductor as the audio signal return conductor. A Balanced connection has a separate shield and two signal conductors.
2. In Unbalanced connections; one of the signal conductors is connected directly to ground. In Balanced connections; neither signal conductor is connected to ground.

Unbalanced connections are simpler and OK for relatively short connections between equipment with relatively high level signals. In the vast majority of cases; internal connections and circuitry of audio equipment is "unbalanced" and the signal is only converted to balanced at the output and converted from balanced at the input.

The advantages of Balanced connections are excellent noise rejection on long cables; even with low level signals, and the possibility of ground isolation between equipment. In most cases; the ground isolation is a matter of degree as there must be some form of ground reference between the connected equipment. In many instances, it is not a matter of the ground connection existing so much as how and where the grounds of the two pieces are connected. The most common way a ground issue manifests itself is as a "hum" in the audio; and this is often due to the presence of a ground loop caused by multiple ground paths that do not take the same physical path between the two pieces of equipment.

The other form of interference is "noise" induced by electromagnetic or electrostatic interference on the audio cable's shield. In the case of an Unbalanced connection; the shield must be connected directly to the audio input of the receiving device to provide the signal return path for the audio signal on the center conductor. Balanced connections allow the shield to be connected to a ground that is isolated from the input audio signal path; or in more extreme cases involving ground loops; to not be connected at all to the input device as the other ground connection provides the necessary reference.

The important aspect of a balanced input is that it is "differential." One input is non-inverting and amplifies the signal with the original polarity. The other input is inverting and amplifies its input signal with the opposite polarity. What makes it work as a system to amplify the desired (audio) signal and cancel the interference (hum and noise) is the fact that the balanced output is configured in the same manner. The output that is inverted feeds the input that is inverted, and the result is the inversion is eliminated and the signal from both inputs are added together "in-phase." This results in an output from the balanced input stage that is the same polarity as the non-inverted output. Noise and interference signals appear with the same polarity on both signal conductors so when the differential input inverts one of these signals and adds it with the non-inverted signal; they cancel each other.

Balanced outputs that employ audio transformers are typically "floating" in that neither of the transformer's output conductors are connected to audio ground. This allows either of the two signal conductors to be connected to ground when feeding an unbalanced input. Because the entire signal voltage appears across the input in either balanced or unbalanced connections; there is also no difference in level if the balanced output is "unbalanced" by the wiring or connection to an unbalanced input. The down-side is that even very expensive high-quality audio transformers are non-linear compared to contemporary audio amplifiers, and are not "DC coupled."

With advances in bipolar audio amplifiers; it became possible to make relatively low-cost electronically balanced outputs. The important difference between electronically balanced outputs and transformer balanced outputs is that electronically balanced output is referenced to ground on both signal outputs. In a practical sense; this means that if one of the two output conductors is connected to ground; an amplifier's output will be "short circuited" unless there is some provision in the design to compensate for this type of connection.

There are amplifier designs that offer the convenience of sensing when one of the two outputs is connected to ground and adjusting the gain so the "shorted" output is no longer outputting a signal and the gain is adjusted on the active output to compensate for the loss of the other output's signal. But this comes at the cost of increased distortion; even when operated as a balanced output. Exactly how one of the two outputs is connected to ground also can have a profound effect on the distortion level of the active output. For example; if the connection is made using twisted-pair cable typical of balanced connections and the "low" signal conductor of the pair is grounded only at the receiving device's end of the cable; the entire circuit made of the cable's shield and one conductor of the pair will be introduced into the output amplifier's "cross-coupling" circuit and could potentially raise the noise and distortion in the active output.

For these reasons; Lavry Engineering's electronically balanced outputs require manual configuration for unbalanced operation. In every case except the LavryBlack DA11, internal jumpers must be set to the proper configuration for unbalanced Pin 2 Hot or unbalanced Pin 3 Hot operation. The DA11 has front panel settings to configure the outputs. In all cases please do not confuse the Balanced/Unbalanced settings with signal polarity settings; as all Lavry DA converters with Polarity settings accomplish this function electronically, before the output stage. This means that the Polarity setting does affect the output regardless of the balanced/unbalanced setting; taking into account the wiring of the output connections and whether Pin 2 or Pin 3 is used as the "+" (non-inverting) connection. The output level will also be 6dB lower in level for the same output gain setting when compared to balanced configuration. This is because the output signal now consists of one-half of the balanced output signal; which is very close to a 6dB difference in level. In the vast majority of cases; the lower level is a benefit as unbalanced inputs typically are designed to accept lower signal levels than balanced inputs.

Digital

There are connections used for digital audio that are similar to analog audio balanced and unbalanced connections; and they share many of the same advantages and disadvantages. But there are important differences due to the dramatically different frequency range of the two types of signal and the fact that most digital circuitry operates on "single-ended" (positive voltage only) power supplies.

With 5 volt DC power; in the absence of a signal transformer, the output voltage can only vary between "0 volts" (or "ground") and 5 volts. In reality; the output voltage cannot even reach 5 volts because there is always going to be some voltage "drop" across the transistor driving the output, and termination divides what is left in half. But the important point is that the signal voltage does not ever enter the negative voltage range and this type of circuitry is therefore referred to as "differential" as versus "balanced." It still employs the basic qualities of having two equal amplitude signals of opposite polarity carried on shielded twisted-pair cables with XLR connectors.

The current AES/EBU standard for differential circuits does include the use of signal transformers at the output and input; so the connection does retain the "floating" quality. This is why Lavry AES/XLR digital input/output ("I/O") can be safely adapted to unbalanced connections for use with most unbalanced RCA/S-PDIF I/O using simple wired adapters or adapter cables. The difference between the 75 Ohm and 110 Ohm impedance of the two formats has a negligible effect with short cable lengths.

In order to minimize interference caused by very high frequency signals "reflecting" at points of impedance miss-match; any signal path of significant length must be "impedance matched." By having the value of the cable's characteristic impedance "terminating" both the transmitting end and the receiving end; reflections are minimized. In the case of digital audio, this value is 75 Ohms for coaxial connections and 110 Ohms for differential connections.

There are two types of coaxial unbalanced digital audio connections: professional and consumer. The professional standard is part of the same AES3 standard as the differential, with the signal format being the same; but the connection is made using 75 Ohm BNC coaxial cable and connectors. The signal voltage is also nominal "5 volt" before termination. This format allows reliable operation with cable lengths up to 100 meters.

The consumer format is the same as the original S-PDIF format; which has been superseded by the IEC 60958 type II standard. Connections are made using 75 Ohm coaxial cable; most frequently with RCA connectors. The signal voltage is nominal 1 Volt before termination. Part of the limitation of the unbalanced consumer format is the relatively low signal voltage of less than one-half volt after termination. The combination of low signal voltage with unbalanced connections places severe length limitations on consumer coaxial digital audio connections. It is recommended that cable lengths be kept below 3 meters whenever possible. With longer cable lengths; it is very important that some form of common ground besides the coaxial connection be present; such as plugging both the sending and receiving device into a common AC power source with 3-prong AC cords.