21
Headphone Monitor
The headphone monitor output appears on a 6.3 mm (1/4 in.) three-conductor stereo phone jack on the front panel. The minimum
recommended impedance of the headphones is 200 Ohms per side.
IMPEDANCE AND TERMINATION
Modern audio systems utilise amplifiers and other active devices which have very low output impedances and high (10k to 50k) input
impedances. These products may thus be operated in series, or many inputs may be connected to a single output of a preceding device,
without regard to impedance “matching”. “Floating” (ungrounded) transformer outputs minimize ground loop problems, and differential
transformer-less circuitry (or input transformers) minimize common mode noise or interference which may be induced into the
interconnecting wires or cables.
Where audio must be transmitted through cables or wire pairs of more than 100 metres or so (several hundred feet) in length,
transmission line termination practices should still be observed.
The Model 1620LE’s microphone input is electronically balanced and symmetrical. It is designed for use with a 150 Ohm microphone.
The line and effects loop inputs are unbalanced (one side grounded) and have a 10 k Ohm impedance. This makes these inputs
suitable for use with any nominal source impedance, low or high. A source termination resistor will only be required when the 1620LE
line inputs are connected to a source which requires a low impedance termination.
