Choosing the Correct Cable For Your Application
Let‘s first start with the instrument, the guitar, the bass, and the keyboard.
On stage, the kind of cables that are usually used to connect instruments and amplifiers are single-wire audio cables with shielding. Interference from external sources can cause trouble even if short cables are used. The cable shielding consists of a wire mesh or foil that completely surrounds the actual conductor. The shielding protects the sound-carrying wire against interference caused by electrical and electro-magnetic waves. But the problem for a single-wire shielded cable is that we only have one dedicated positive signal “hot“ wire (+), while the negative “cold“ phase (–) is shared with the outer shielding. This works fine as long as outside interference is low. The shield functions simultaneously as the second conductor that is required to complete the circuit.
For the transmission of audio signals through single-wire shielded cables, interference-free transmission is limited to cables that are only a few meters long.
Unbalanced cables (such as your typical guitar cord) are single-wire audio cables with an inside conductor and outside wire braiding. The plug only requires two contacts (positive “hot“ phase and shielding).
The phase (“hot“) transmits the audio signal from the source to the destination. The shielding serves as protection and as the negative or “cold“ phase.
- DI boxes
- Stage microphones
- Mixing board L/R master outputs
- Mixing board sub groups
- Mixing board matrixes
- Mixing board monitor paths
- Aux paths
- Mixing board XLR inputs
- Mixing board balanced line-inputs
- Mixing board returns
- Microphone pre-amplifiers
- Effect units
Balanced cables even very long ones are immune to electro-magnetic interference and ground loops.
Balanced cables consist of shielding (1) and two conductors (Fig. 3). The audio signal is transmitted independently from the shield and ground. Interferences that are intercepted by the shield and ground wire do not affect the audio signal, thus the term ungrounded. The two conductors are wired phase inverted.
This means that the phase in one conductor is inverted by 180 degrees. Due tothis opposite phasing, the electro-magnetic interference signals that affect the cable from the outside reach both conductors simultaneously and cancel each other out.
For ungrounded balanced transmission to work, it is necessary that the inputs and outputs on the connected devices are both balanced. This can be accomplished electronically, i.e. through the use of balanced input amplifiers with common mode rejection or output amplifiers with optimized unbalanced output voltage ( a-conductor to b-conductor).
Today we can assume that electronically balanced inputs and outputs are superior to most transformer-balanced ones regarding transmission parameters such as frequency response, distortion factor, common mode suppression, and resistance to over-modulation. Nevertheless it is often necessary to use balanced cables without ground.
Genuine ungrounded balanced audio transmission is only possible through the use of audio transformers. These transformers are optimized to handle lowfrequency audio signals. But despite the fact that a vast selection is available on the market many of them can only be used for the ungrounded transmission method.
Those who want genuine transmission quality with the best possible common mode rejection have to depend on toroidal core transformers.
Microphone cables connected to mixing boards carry very low voltage levels (milli-volt range) and are therefore extremely sensitive to interference. This is why it is very important that they always be ungrounded-balanced. But with return cables leading from the mixing board to the amplifier, the risk of interference is not quite as high, because the strength of the signal can reach several volts. Therefore, using ungrounded balanced cable to route the signal to the amp is not absolutely required.
However, the only way to completely eliminate all possibility of interference is by insisting that all cables incorporate an ungrounded balanced design.