Design tips for balanced audio connections
Balanced connections does not automatically guarantee hum free connections, but well designed balanced connection avoid problems in very many cases. Infortunately there are many equipments where the balanced connections are not designed well, so they do not perform as well as you woudl expect them to do.
Grounding of inputs and outputs
The grounding of audio cables have
Grounding in balanced audio connections
It is absurd that you cannot go out and buy pro audio equipment from several different manufacturers, buy standard off-the-shelf cable assemblies, come home, hook it all up and have it work hum and noise free. Use only balanced lines because they were develloped to provide good isolation against external interfences. Tie the shield to the chassis, at the point it enters the chassis, and at both ends of the cable. Standard XLR cables come with their shields tied to pin 1 at each end (the shells are not tied, nor need be), this means equipment using 3-pin, XLR-type connectors must tie pin 1 to the chassis (usually called chassis ground) not the audio signal ground as is most common.
If the grounding od XLR pin 1 is done correctly in the equipments the currents running in the cable shield easily cause humming inside the equipment electronics. This has lead to a situation that not using signal ground is a common practice in pro-audio practice. This is not a good solution to the problem.
Design rules used in professional audio equipment design
Audio-frequency groundloop problems are typically in the low millivolt range. As a general design rule, true professional equipment designed for the real world of Pro Audio should not have its mains/chassis ground and electronics ground permanently and directly connected. It should either be totally isolated / switchable or have the chassis and electronic ground connected via a soft coupling resistive/capacitive network.
Good balancing of the signal is absolutely necessary for the balanced inputs to work as espected. Balancing means that the inputs and outpus use different signals, but that's not all. Balancing also means that the cables are impedance balanced: both wires in the pair see the same impedance. The impedance balancing is absolutely necessary for reliable operation in real world situations. If the impedances are not properly balanced the equipment might give good CMRR figures on typical test bench measuring setup but perform quite poorly on real-world situations.
In the sending end the ba÷ancing is easy to use. Just match the output impedances of the output amplifier stages by using same size output resistor for both inverted and non-inverted outputs.
For ba÷anced inputs you must select a differential input circuit which really shows the same impedance to both inverting and non-inverting inputs. Quite many basic differential amplifier schematics show different imepdance values to those input. So be careful in the design process and analyze the input impedances carefully.
Floating transformer inputs and outputs are also a good way to do the impedance balancing. Floating input and output will be automatically balanced if a good audio transformer is used.
Tomi Engdahl <Tomi.Engdahl@iki.fi>