JBL 7510B User Manual

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Manual 751 OB3.2 Summing Amplifi er and NQM CircuitThe Audio signal from all inputs is combined in an amplifier on the Output Assembly. The gain of thisamplifier is under control of the NOM circuit. The NOM circuit counts the numbw of active inputs, calculatesthe appropriate gain for that number of inputs and increments the gain of the summing ^plifier accordingly toprevent system feedback and give the maximum system gain for Ae actual number of live microphones. Thefollowing table gives the exact gain increments of the NOM circuit.Table 3 NOM GainNumber ofMicrophonesGain Number ofMicrophonesGain Number ofMicrophonesGain1 OdB 9 -9.5 dB 17 -12.3 dB2 -3 dB 10 -10 dB 18 -12.6 dB3 TTSb 11 -10.4 dB 19 -12.8 dB4 -6 dB 12 -10.8 dB 20 -l8 dB5 -7 dB 13 -11.1 dB 21 -llidB6 -7.8 dB 14 -11.5 dB 22 -13.4 dB7 -O^B 15 -11.8 dB 23 -13.6 dB8 -9 dB 16 -12 dB 24 -13.8 dB3 3 Threshold and Background ThresholdThe audio signal on any channel, taken from the wiper of the channel gain control, is convated to a DCvoltage which drives a comparator. If the voltage exceeds the comparator threshold the channel is gated ON.A zero crossing detector circuit restricts the channel from turning ON until the audio signal crosses throughzero volts, thereby eliminating any tum-on pop due to waveform truncation. A threshold trimpot allowsadjustment of the DC voltage level required for threshold, and a release trimpot allows adjustment of the timethat the comparator is held ON. The circuit drives a switch that changes the gain of an operational amplifierin the audio chain. The gain change is 25 dB. The circuit also drives another switch which turns on a frontpanel LED to indicate that the channel is ON and a third switch which provides a contact closure to ground onthe rear panel remote control connector for external switching.The comparator threshold may be either fixed, or may vary with the sum of the audio signal from all channels.In the fixed, preset position of the Background Threshold control a fixed DC voltage is set with trimpot RT-1and feeds establishes the threshold level for all channels. When the control is turned out of its detentedposition, a rectified DC voltage representative of each input signal is summed and establishes the thr^hold forchannel tum-on. The Background Threshold contro changes the gain of an amplifier to adjust the ratiobetween the level of signal provided by an individual channel to that provided by the sum of all channels.3.4 Output AmplifierFollowing the output Gain control, an operational amplifier driving a complementary pair of transistors drivesthe output transfomier which gives the unit a fully floating output at the XL connector. A second operationalamplifier takes signal before the output Gain control and feeds the Auxiliary Output jack.3.5 Power SupplyAC mains power to the mixer is converted to DC in the power supply. The supply is a straightforward linearstyle. Stepdown transformer T1 supplies low voltage AC to a bridge rectifier BRl. The full wave rwbfi^DC is filtered by capacitors C18 and C19 and regulated by Integrated Circuit Voltage Regulators Q1 for +5VDC, Q2 for +8 VDC, Q3 for -8 VDC, Q4 for +15 VDC and Q5 for -15 VDC. The power supply isprotected by an internal 1.5 A, 8 AG fuse (Littlefuse 36101.5) mounted next to the AC Voltage Change switch.Page 18