Radio Systems DA-16, Руководство пользователя

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10
USING ACTIVE BALANCED CIRCUITRY
Balanced lines have been used for many years and are in continued use today because of their immunity to
stray pickup. Induced signals appear on both sides of the balanced line. The receiving end of the balanced line
responds only to the difference voltage between the lines which is the desired signal. Induced signals are common to
both and are balanced out.
Transformers have been the mainstay of balanced circuitry for decades. Unfortunately, transformers cause
distortion and ringing, and are susceptible to magnetic flux pickup. Further, good quality audio transformers are very
expensive.
The use of op-amp balanced circuitry has the advantages of transformers without the disadvantages. The only
caveat is that careful wiring practices are more important with active balanced than with transformers.
Active balanced outputs and inputs use three wires: +,-, and ground. The + and - terminals are both driven
and neither should ever be connected to ground. For best performance, a three-conductor shielded wire should be
used. The third wire completes the ground circuit. The shield should be connected to the ground at one end of the
wire only. If a two-wire shielded cable is used, it is important that a ground connection be made between the sending
and receiving units. A ground circuit through equipment chassis or through three-prong AC cord ground is also
acceptable.
Single-ended audio interconnections lack the interference immunity of balanced hookups. For that reason,
keep unbalanced connections short, direct, and well separated from AC power wires. To drive a single-ended load from
an active balanced source, use coaxial wire: + to center conductor and ground to shield, leaving the - output
unconnected. To feed an active balanced input from a single-ended source, use coaxial wire, connecting the hot center
conductor to +. Connect the shield to ground and put a jumper from ground to -.
When driving an active balanced input from a transformer balanced floating source, use two conductor
shielded wire. Ground the shield at the source end. Establish good ground between the chassis either directly or
through AC plug ground prongs. At the load, connect the + lead to the + input and the - lead to the - input. Put two
300 ohm resistors in series between the + input and the - input and connect their mid-point to the load ground. This
correctly terminates the source output transformer for optimum frequency and transient response (freedom from
ringing) and provides a low impedance return path for leakage and induced hum. If more than one active balanced load
is to be placed across a floating balanced transformer source, install this resistive termination once only. From that
location to the active balanced loads, run three-conductor shielded wire, shield continued from the source chassis, +
from +, - from -, and ground from the mid-point of the terminating resistors.
To drive a balanced floating transformer load from an active balanced source, use shielded wire. Connect the
shield to source ground and leave the shield open at the load end. C to + and - to -, and establish a good
source ground to load chassis connection, either through a third wire in the interconnect cable or through chassis
contact or AC cord third wire ground.
Interconnections between pieces of stereo equipment require doubling the connections described above without
duplicating the ground connection. Between pieces of active balanced stereo equipment, then, 5 shielded conductors
should be run.
When testing active balanced equipment with single ended test equipment, do not connect the - to test
equipment ground. Most modern test equipment provides balanced inputs. In many dual-trace oscilloscopes, balanced
signals may be displayed by running the two inputs in the “add” mode with one input switched to invert. To perform a
test with single-ended equipment, + and - outputs must be tested independently and their results added. Testing only
a single output results in a 6 db loss in output level.
The active balanced equipment interconnection format makes possible state of the art fidelity. Careful
attention to detail and conservative practice will be rewarded with outstanding flat frequency response, low distortion,
and wide dynamic range.