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FM-55 / Aug 2014
G E N E R A L I N F O R M A T I O N
If RF interference is an issue, the far (floating
and ungrounded) end of an audio cable
shield can be experimentally bypassed to
RF ground via a 0.01µF, 250V AC rated
capacitor. Suitable capacitors are:
TDK
CS17-F2GA103MYGS
Murata DE2F3KH103MA3B
Panasonic ECK-ATS103MF
AVX
65N103MBLCP
Analog Audio Output Connections
Balanced audio loads are recommended and should be connected to the AM-55’s
outputs using standard two-conductor shielded audio cable. Unbalanced audio loads
should be avoided, but if they can’t they should be connected using shielded two conduc-
tor cable such as Belden 8451 or 9451 (as if connecting a balanced source).
The AM-55 is equipped with active balanced output stage that behaves like a transformer.
Because of this behavior the correct wiring method may be different than expected.
The unbalanced load’s “Hi” lead should be
connected to XLR connector Pin 2 (“Hi”).
The unbalanced load’s shield should be
connected to the AM-55’s output XLR Pin 3
(“Lo”). Then, and to enable the AM-55’s
balanced output amplifier to operate correctly when driving the unbalanced load, it is
recommended that the AM-55’s XLR output Pin 1 (ground/shield) also be connected to
Pin 3 (“Lo”), noting that this MUST BE DONE at the AM-55’s output connector (see
the diagram above).
Note that it is not advisable to connect Pin 1 and Pin 3 conductors together at the far
end
of the cable. Doing so can induce external noise and crosstalk on the output amplifier’s
“load sense” lead which is XLR Pin 3 when Pin 2 is being used as the “Hot.”
As in the input case, if RF interference is
an issue the far (floating) end of the cable
shield can be experimentally bypassed to
RF ground through a 0.01µF 250V AC
rated capacitor to see if it helps. Please
refer to the listing above for capacitors
known to be suitable for this task.
Digital Audio Connections
For digital audio connections always use a good quality digital audio cable (or twisted
pair Category 5E/6 Network cable) having a characteristic impedance of 110 ohms. This
cable should be shielded where possible, and in the case of multi-pair cable, each pair
should be individually shielded. Foil shielding is recommended for permanent installations,
and a cable with foil shield plus an overall braid should be used in applications where
frequent flexing of cables might occur.
