Bedienungsanleitung Midas VeniceU U24 (Seite 112 von 122) (Englisch)

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102 Appendix E: Best Grounding Practice

VeniceU

Operator Manual

Balanced transformers

Active electronic circuits achieve good results, but for ultimate noise rejection,

transformers are needed. Transformers colour the sound especially at low and high

frequencies due to core saturation and impedance rises. They are also expensive, so

they are typically only used as external problem solvers when all else fails.

Screen termination

Sometimes ground loop currents are so strong that even balanced systems can pick up

some noise. This is because the currents manage to find a way into a piece of

equipment’s internal unbalanced circuits. This is easily cured by breaking the screen

ground connection, so that it is only connected at one end of the cable. Opinions as to

which end to break differ within the audio industry but:

• Induced noises in electronically balanced systems are typically best rejected by input

stages, so breaking the ground there will generally give the best audio performance.

This also allows any common mode AC currents from the output driver to return via

the screen ground to their source rather than finding a longer path through the

power cables, which may give rise to noise or crosstalk elsewhere.

• Alternately, breaking the screen connection at the send end provides a much more

practical grounding scheme if cables are to be hot plugged. Then any cables that are

plugged first (that is, only) into an input will have a screen and thus will not be a

source of severe electromagnetic interference (which would be the case without the

screen connection).

• If a transformer is utilised within the balanced connection the best CMRR

performance will be obtained by breaking the screen ground at the transformer end

of the cable.

With long cable runs, sometimes the impedance of the screen conductor is high enough

that disconnecting one end allows some electromagnetic interference through the

screen onto the signal conductors. This is typically audible as stray local radio

transitions, typically at very low levels. If this is a problem, it can be cured by bridging

the break in the screen connection with a capacitor. This improves the screen at high

frequencies, but maintains the isolation that breaks ground loops at lower frequencies.

10nF to 50nF are good values to use and are available in ceramic packages that are

small enough to fit inside connector shells. Some experimentation may be necessary,

especially if multiple cables are bridging two units, because in this case the capacitors

are effectively connected in parallel and their combined value will grow (for example,

32 channels at 50nF will equate to 1,600nF). It is probably better to use 10nF for this

kind of multiple connection, so that the overall value only grows to 160nF.

Screen termination diagram showing screen broken at destination

Chassis

Screen

Source

Destination

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