3-2
Rev 2.2, 10/31/94
of rating. If so, then the input impedance of the succeeding input must be equal to or greater
than the output impedance of the driving device.
Symetrix equipment inputs are designed to bridge (be greater than 10 times the actual source
impedance) the output of whatever device drives the input. Symetrix equipment outputs are
designed to drive 600-ohm or higher loads (600-ohm loads are an archaic practice that won't go
away). You don't need to terminate the output with a 600-ohm resistor if you aren't driving a
600-ohm load. If you don't understand the concept of termination, you probably don't need to
anyway.
The two facts that you need to derive from this discussion are:
1.
Match signal levels for best headroom and signal-to-noise ratio.
2.
For audio, impedance matching is only needed for antique equipment and power amplifier
outputs. In all other cases, ensure that your inputs bridge (are in the range of 2 to 200
times the output source impedance) your outputs.
3.2 Signal Levels
The 601 is designed around studio/professional line levels: +4 dBu or 1.23 volts. The unit is
quiet enough to operate at lower signal levels such as those found in semi-pro or
musical-instrument (MI) equipment (-10 dBu or 300 millivolts).
The microphone input is designed around the output levels delivered by typical professional
microphones (-60 dBm to -20 dBm, -45 dBm to -5 dBm with the pad engaged). +48V phantom
powering is available for condenser microphones. Be sure to depress the rear panel switch to
turn phantom powering on. Additional information on phantom powering can be found at the
end of this chapter and in the references.
3.3 I/O Impedances
The 601 is designed to interface into almost any recording studio or sound reinforcement
application. This includes:
❒
600-ohm systems where input and output impedances are matched.
❒
Unbalanced semi-professional equipment applications.
❒
Modern bridging systems where inputs bridge and outputs are low source impedances
(voltage transmission systems).
The 601's input impedance is 12.5 k
Ω
balanced, and 9.4 k
Ω
unbalanced. The inputs may be
driven from any source (balanced or unbalanced) capable of delivering at least -10 dBu into the
aforementioned impedances.
The 601's output impedance is 300-ohms balanced, 150-ohms unbalanced. The output line
driver de21.5 dBm into 600-ohm balanced loads or +15.5 dBm into 600-ohm
unbalanced loads.
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