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AM-10HD / Aug 2008
G E N E R A L I N F O R M A T I O N
The AM-10HD may be mounted between other devices in the equipment rack and,
in accordance with good engineering practice, should not be mounted directly above de-
vices that generate significant amounts of heat. If such a location is unavoidable then it
is advisable to utilize an extra 1RU blank rack panel between the AM-10HD and devices
immediately above and/or below it.
WARNING! Under no circumstances should the AM-10HD unit be opened! The
unit contains high voltage circuits that are hazardous and potentially harmful. The
unit has no user-serviceable parts inside! If you have a problem the unit must be
returned to Wheatstone Corporation for repair.
Installation Tips:
• Place surge protection circuits as close as possible to the AM-10HD or other device
being protected. This is especially important at AM transmitting plants which often
have less than stellar AC power quality.
• Establish a low impedance common ground in your facility and try to route all grounds
to that point. Transmitter cabinets, phasor cabinets, equipment racks, and, where per-
mitted by electrical codes, the AC power line neutral, should be securely bonded to
the station’s AM ground radial system by low impedance means.
• Choose the best power conditioning / UPS units that you can afford and suitable for
your equipment — focus on the features and options you need. The better UPS prod-
ucts can prevent thousands of dollars in equipment damage — some even come with
an external equipment damage warranty.
• Unbalanced audio connections to the AM-10HD are not recommended in a high-RF
environment such as an AM transmission site. If such interfacing is required it should
be made with shielded two conductor cable such as Belden 8451 or 9451 as if connect-
ing a balanced source. At the unbalanced source’s output connect the + Output to the
HI input wire and connect the source GND wire to the LO wire. Connect the shield
at the AM-10HD end only, noting that in some instances this connection may need to
be made through a 0.01uF capacitor in order to “break” a potential 60Hz ground loop.
• For digital audio connections always use a good quality digital audio cable with a
characteristic impedance of 110 ohms.
The AES/EBU specification, with its broad impedance tolerance, allows for cables with imped-
ances from 88 ohms to 132 ohms - 110 ohms is ideal. Twisted pair cable should be shielded, and in the
case of multi-pair cable, each pair should be individually shielded. Foil shielding is recommended for
permanent installations and foil shield plus overall braid should be used in applications where frequent
flexing of the cable will occur. One cable pair is capable of carrying two channels of digital audio.
Generic “audio” cable such as Belden 8451 may be used for interconnecting AES3
digital audio devices but only for distances of less than about 25 feet. The actual cable
length that will work satisfactorily in an installation is primarily determined by the
error correction and jitter tolerance of the AES3 receiver device and the cable used.
The impedance of most “analog” cables ranges from 40 ohms to 70 ohms and represents a large
impedance mismatch from the nominal 110 ohms required in the AES3 standard. Such mismatch will
result in signal reflections, causing bit errors at the AES3 receiver. The higher capacitance of generic
analog cables also slows down the rise time of the digital data signals, impairing the ability of the AES3
receiver to accurately detect digital signal transitions. This may result in increased jitter.
