Audio Signals - Apogee APL-220 Manual

Audio Signals

(1) Input Impedance and Level
The APL-220, -500, and -800 were designed to receive a balanced-line signal
on their 3-pin female XLR type connector. They are also equipped with a 3-
pin Male XLR output connector, labeled "loop-through," intended for the
purpose of daisy-chaining multiple powered speakers together on a common
signal feed. These input and output connectors are hard-wired in parallel; that
is to say, no active or passive buffer exists between the two connectors, they
are simply wired in parallel. This means that the loop-through connector will
always remain functional, even if AC power is interrupted to a speaker
"upstream" of other speaker(s) on the same feed circuit.
The input impedance is 35k-ohm, active balanced, and is intended to receive
a nominal +3 dBv input level (+3 dBv = full power). Because it's common
practice to loop quite a few powered speakers together on a shared feeder
circuit, care must be taken to insure that the signal source is capable of driv-
ing the group of powered loudspeakers. For example, ten APL-500 speakers
present a 1k-ohm load to the signal source. This is well within the capability
of most line-level output drivers, but depending on the output impedance of
the source, some loss in level may occur. The formula to calculate such
loss is:
R
S
V
V
R
S
Ix
For example, let's say you're driving ten APL-500 speakers with a 100-ohm
source impedance. The formula to calculate loss would then be:
Voltage Loss (dB) = 20 * Log
NOTE: It is important to be aware of the potential losses that may occur
from combining multiple powered speakers on the same feed circuit. This is
particularly important when attempting to adjust audio levels among a large
system that employs multiple feed circuits with different numbers of pow-
ered speakers on each feed.
14
Voltage Loss (dB) = 20 * Log
= Source Equipment Output Impedance
Where: R
S
= Combined Input Impedance of all APL Speakers
R
Ix
I
= Unloaded Source Equipment Signal Voltage
Note: V
S
= Loaded Signal Input Voltage to APL Speakers
V
I
1000
[
(
1000+100
R
[ ]
Ix
( )
R R
+
Ix S
]
= – 0.83 dB
)