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

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

e

C13

350 mA

e

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

R13 C11 In

t1

051

e

(72 ms)

051

e

141 ms

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

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