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" (a positive and negative voltage).

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

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 converter 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 is 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 connections are connected to audio ground. This provides flexibility in connection as this allows either of the two signal connections to be connected to ground for connection to 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 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 conductors. 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. I every case except the LavryBlack DA11, internal jumpers must be set to the proper setting 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 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.