R a d i o S y s t e m s , I n c .
D A - 2 / 4 x 4 b M a n u a l
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P a g e 1 2
Using Actve Balanced Circuitry
Balanced lines have been used for many years and are in continuing 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 advantage 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
hook-ups. For the 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 though 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 sources 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
Summary of Contents for DA 2x4b
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