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LM1818 Scheda tecnica(PDF) 9 Page - National Semiconductor (TI) |
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LM1818 Scheda tecnica(HTML) 9 Page - National Semiconductor (TI) |
9 / 14 page Application Hints (Continued) Anti-Pop Circuitry (Figure 8) The capacitor on pin 3 is used in a time delay system in conjunction with C13 the ALC capacitor to suppress pops when switching between record and playback Figure 8 illus- trates how this is done The output amplifier either record or playback is shut off prior to switching and carefully rebiased after switching takes place It is therefore required that a proper ratio is selected between the ALC capacitor and the logic input RC time constant The ALC capacitor must be discharged to 07V within the time it takes the logic input capacitor to 1) charge from VCC 2 to 07 VCC when switch- ing from record to playback or 2) discharge from VCC 2 to 03 VCC when switching from playback to record These times would normally be similar however the ALC capacitor can be charged to a different initial value depending upon the input to the ALC circuit The maximum value to which the ALC memory capacitor will normally charge is 32V therefore the maximum time allowed for discharging C13 is given by t1 e (C13 c DV) I1 e C13 (32V b 07V) 350 mA e C13 c 72 c 104 If C13 e 10 mF t1 e 72 ms It is now necessary to determine the minimum value for the RP logic capacitor This is done by computing the time between the 2 voltage switching points using the exponen- tial equations for a single RC network t2 e R13 C11 In VCC 03 VCC ( b R13 C11 In VCC 05 VCC ( e051R13C11 To be sure that C13 is completely discharged let t2 l t1 R13 C11 l t1 051 e (72 ms) 051 e 141 ms If C11 e 10 mF R13 e 15 kX R13 should be kept to a value less than 50 kX to insure that bias current existing from pin 3 does not cause an offset voltage above 200 mV Typically this bias current is less than 3 mA Record Playback Switch When the voltage on pin 3 of the IC is greater than 05 VCC the internal record-playback switch switches into the play- back mode During playback the record preamplifier re- mains partially biased but the input signal to this preamp does not appear at the preamplifier output In addition dur- ing the playback mode the record monitor output (pin 9) is disabled and the ALC circuit operates to minimize the signal into the record preamp input The meter circuit is operation- al in the playback as well as the record mode Similarly during the record mode the playback preamp input is ig- nored and the playback monitor output is disabled In addi- tion a pin is available to hold one side of the record head at ground potential while sinking up to 500 mA of AC bias and record current TLH7894 – 10 FIGURE 8A Anti-Pop Circuit 9 |
Codice articolo simile - LM1818 |
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Descrizione simile - LM1818 |
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