this type 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:
r
Match signal levels for best headroom and signal-to-noise ratio.
r
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 425 is designed around studio/professional line levels: +4 dBu or 1.23 volts RMS. 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).
3.3. I/O Impedances
The 425 is designed to interface into almost any recording studio or sound reinforcement
application. This includes:
r
600 ohm systems where input and output impedances are matched.
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Unbalanced semi-professional equipment applications.
r
Modern bridging systems where inputs bridge and outputs are low source impedances
(voltage transmission systems).
The 425's input impedance is greater than 30-kilohms balanced or unbalanced. The inputs
may be driven from any source (balanced or unbalanced) capable of delivering at least -10 dBu
into the aforementioned impedances.
The 425's output impedance is 300 ohms balanced, 150 ohms unbalanced. The output line
driver delivers +23 dBm into 600 ohm balanced loads or +18 dBm into 600 ohm unbalanced
loads.
3.4. Polarity Convention
The 425 uses the international standard polarity convention of pin 2 hot. Therefore:
If your system uses balanced inputs and outputs,
and uses the 425 this way, then the polarity
convention is unimportant. If your system is both
balanced and unbalanced, then you must pay
attention to this, especially when going in and
coming out through different connector types
(like input on an XLR, output on a phone jack).
XLR Tip-Ring-
Sleeve
Signal
1 Sleeve Ground
2 Tip High
3 Ring Low
Rev 2.0, 10/29/93
3-2
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