JBUUREI MODEL 7110
2.6 Using Floating or Balanced Cables for Longer Puns
Shielded cables are required for all input and output signals. If the output cable lengths are
greater than 4.5 mto 6m(15 to 20 ft), consider installing isolation transformers (600 ohm:600
ohm) at each end of the cable. This allows dual-conductor shielded cable to be used in a
floating mode, greatly reducing the possibility of radio frequency Interference (RFI) or hum. If
the equipment to which the 7110 output is connected has a balanced or floating input, only
one transformer need be installed .An accessory output transformer is available for the 7110
which wires directly to the circuit board inside the unit. See Section 2.10 for details of
installation and wiring.
Similarly, longer input cables should be balanced or floating to reduce susceptibility to RFI
and hum. If the output of the device feeding the 7110 Is balanced or floating, no transformer
is required at all; simply use adual-conductor shielded cable. If the input source is
unbalanced, an isolation transformer should be used at the source device's output.
With shorter cables, particularly where interconnected equipment is mounted in asingle rack
or is powered by the same AC receptacle, there is less need for balanced or floating input
and output cables.
If the unit is modified to incorporate the optional output transformer, it is important to
remember that connection must be made to both “+” and terminals of the output. The
output will drive any load of 600 ohms or greater and does not require aresistive termination.
If the 7110 is driving along line (in excess of 30 meters [100 feet]) a620 ohm termination
resistor at the other end of the line from the 71 10, in combination with the use of balanced
lines, will reduce the possibility of noise pickup in those lines.
2.7 Impedance and Termination for 600 Ohm Lines
In the USA the early history of the audio industry is very closely tied to the history and
technology of the telephone industry. Much early equipment used for public address systems,
recording, broadcast and reproduction of sound was either designed by or heavily influenced
by the scientists and engineers at Bell Telephone Laboratories. The technology that they and
others developed has had alasting influence on the design and specification of all audio
products. Among their contributions was the 600 ohm transmission line.
The 600 ohm line was developed because of aneed for astandardized impedance for long
distance transmission lines. Transmission and reception equipment using vacuum tubes,
transformers, and passive equalization and mixing networks require known source and load
impedances to achieve predictable results. Because much of the early professional audio
equipment was designed by telephone company people or used similar types of equipment,
it is not surprising that the 600 ohm line became astandard in the professional audio
industry.
Correct use of the 600 ohm transmission line requires asignal source with an exact 600 ohm
source impedance, and areceiving device which also has a600 ohm input impedance. If a
device does not have the correct impedance, it must be modified until it does. In the case of a
device with alower than required source impedance this would require addition of aseries
buildout resistor (or two in the case of abalanced line). In the case of ahigher impedance
input aresistor across the input will suffice. Other variations require either aresistive network
or transformer to match impedances.
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