Z4II & Z8II: Installation and User Guide 15
26 Solving problems
26.1 Ground loops (aka Earth loops)
Despite your best efforts, if the completed sound system ‘hums’ you probably have a ‘ground
loop’; the offending signal source can be found by setting the volume control to minimum
then disconnecting the input leads (both left & right channels) on each line input until the
‘hum’ disappears. This problem is often caused by terminating a screened input cable into a
signal source positioned a significant distance from the mixer. A good way of avoiding this
potential problem is to use signal sources (CD players and the like) that are double insulated
with no connection to the mains supply earth. If a signal feed were derived from a second
mixer (a club or microphone mixer for example) it would be perfectly normal to expect this to
be earthed; we suggest that a transformer be used to isolate the signal and prevent a noisy
loop (see section 25.2).
26.2 Connecting balanced signals to the unbalanced line inputs.
We recommend the use of a transformer to convert a balanced signal to an unbalanced
signal suitable for direct connection to the mixer line inputs. The transformer should be
mounted close to the Zone Mixer and the unbalanced output lead should be kept as short as
possible. Where both the source unit and destination mixer are earthed, it is important to
isolate the primary and secondary windings to avoid a potential ground loop; if there is any
doubt about this, we suggest that the balanced cable screen is not connected at the
transformer end. Suitable transformers for this application are the RS Components 210-6447
(primary & secondary series connected) and we recommend the fitting of the screening can
(part number 210-6469); Canford Audio supply a similar transformer OEP Z1604 but we
suggest that these should be fitted in a screened housing. All transformers should be wired
to give a ratio of 1:1.
30/05/03 V8
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