- 3 - - 24 -
2. Volume adjustment, sound field processing and EQ adjustment
circuits.
All channel signalsare sent toN402 inside whichthe independent volumeadjustment,
EQ adjustment andall sound fieldmodes process areperformed.
The sound fieldprocessing and EQadjusting circuit ismainly processing theL&R
channel signals.According to theschematic diagram, theL&R channel signalsare
added simultaneously tothe pins 15,17, 13 and16 of N402.When the unitmode is in
the Hi-Fi mode,the internal circuitof the pins17&16 is connectedand the otherinput
signals are cutoff.At this time,only the L&Rchannel volume canbe adjusted andonly
the pins 31&32send out signals.Therefore, theunit is inthe 2CH outputmode. The
unit mode isnot in theHi-Fi mode, otherinput signals areconnected but thepins
17&16 signals arecutoff.At this time,all channel volumescan be adjusted
independently and thesound field processingor EQ adjustingof the L&Rchannels
can be performed.Finally,all channel signalspass out fromthe pins 31,32, 33, 34,
35&36. TheSW channel signalfrom the pin36 reaches theamplified speakers tobe
amplified through theSW output terminals.Other channel signalsreach to thepower
st
amplifying circuit tobe amplified.The L&R channelsignals will gothrough 1 grade
LPF and MIXamplification (Karaoke signalsare overlapped intoL& R channels).
3. Input signals detect, search and frequency spectrum sampling cir
cuits
3.1 Input signalsdetect and searchcircuit: Thesix channel signallines of theinput IC
N402 are connectedwith 100K samplingresistors R533, R534,R657, R676 andR678
respectively.The signalsare mixed bythese resistors andadded to theopposite-phase
input terminal tobe amplified. VD431and C481 connectedto N403B's outputend
constitute half-wave rectifyingfilter circuit.Then the signalsreach the voltagecomparer
composed of N403A.The outputend of N403A(SEARCH)is connected tothe pin 28of
CPU. Thiscontrol signal isthe search anddetect signal: whenit is lowlevel, it entersthe
search mode; whenit is highlevel, it stopssearching. Its worksas follows:
3.1.1 When thisunit is gettingstarted, theA&B control signalsfrom the pins38&39 in
the domination ofthe CPU's interprogram are addedto the inputselect circuit to
search circularly once.When there areno signals inthese four inputconnectors, the
VCD mode stopsautomatically.When there aresignals in oneof the fourconnectors,
AC signals willappear in allchannels of theinput N402.These ACsignals are
amplified by N403Band rectified andfiltered by VD434and C481 tobecome DC
signals. Atthis time, theopposite-phase voltage ofN403A is0.01V.When this DC
voltage surpasses 0.01V, the outputend of N403Bsends out ahigh level (SEARCH)
close to positivepower supply voltage(A+6V) which reachesthe pin 28of CPU. CPU
keeps searching inthe connector inwhich there areinput signals andthe unit willplay
normally.
1.2.2 When pressthe SEARCH onthe front panel,CPU sends outA&B control
signals again tostart searching. Meanwhile,the pin 27(EX) sends outa high level
which makes V446inductive. Theemitter of V446sends out ahigh level whichpasses
through R498 whichmakes the opposite-phasevoltage of N403Ato be 0.4V. Thatis
to say, if youwant to stopsearching of CPU,the gained voltageafter the inputsignals
are rectified andfiltered must exceed0.4V.This voltageis higher than0.01V when
this unit isgetting started inorder to avoidthat the CPUreceives signals mistakenly
and stops searchingdue to thelarge external interferencesignals. If theinput signals'
amplitude is nothigh enough, CPUwill continue searching.When the amplitudeis
high enough, N403Asends out highlevel to thepin 28 tostop searching.The pin 27
(EX) will becomelow level againand the opposite-phasevoltage of N403Awill also
returns back to0.01V.The wholesearching process isfinished.
