JBL EON15-G2 Technical & Service Manual page 12

Signal Input Circuitry
Voltage measurements are at frequency of 100 Hz with –20 dB on input at XLR (J2).
Balanced input is connected to J2 and travels to impedance selector switch SW2. This switch inserts
R33, R52 and R45 for line level inputs and removes same for microphone level inputs. It also shorts
the "mic" indicator LED (allowing the illumination of the "signal" LED) and changes the gain of
summing amplifier IC U5B. D1 and D2 add input circuitry protection if the input signal is in excess
of ±15Vdc. The signal travels to pins 2 and 3 of buffer amplifier U5A where the Common mode
rejection cleans up the extraneous noise from being amplified.
The signal is output on pin 1 of U5 to the control PCB via connectors RC1/PA1 so the user can adjust
to the desired level using volume control VR1. The signal is returned to the input PCB through
RC1/PA1 and is input to pins 5 and 6 of U5B. This output on pin 7 of .4Vpp, branches into the
mic/signal mode LED indication circuitry (comparator U6), and the equalization circuitry on pin 1 of
RC2.