MONITOR AND RECORD AMPLIFIERS

(Figure 6)

The monitor and record amplifiers share common input and

feedback connections but have separate outputs During

playback the input signal is amplified and appears only at

the playback monitor output Because the outputs are sepa-

rate different feedback components can be used and as a

result totally different responses can be set The amplifiers

are stable for all closed loop gains above 3 and have an

open loop gain of typically 80 dB The outputs are capable

of supplying a minimum of 400 mA into a load and swing

is needed to drive a load an external pull-up resistor on the

output of these amplifiers can increase the load driving ca-

pability

AUTOMATIC LEVEL CONTROLALC

(Figure 7)

The automatic level control provides a constant output level

for a wide range of record source input levels The ALC

works on the varying impedance characteristic of a saturat-

ed transistor The impedance of the saturated transistor

forms a voltage divider with the source impedance of a se-

ries resistor (R1 in

Figure 9 ) The input signal is decreased

as the ALC transistor is increasingly forward biased The

ALC transistor will be forward biased when the preampli-

fiers’s AC output (pin 14) coupled to the combination ALC-

meter drive input (pin 4) reaches 40 mV peak (25 mVrms)

The gain of the ALC loop is such that a preamp input signal

increase of 10 dB will result ina2dB increase on the AC

output of the preamplifier If greater than 25 mVrms is de-

sired at the output of the preamp a series resistor can be

added between the preamp output coupling capacitor and

the ALC input (pin 4) The input impedance of the ALC cir-

cuit is 2 kX therefore ifa2kX series resistor is added

ALC action will begin at 50 mVrms

The ALC memory capacitor connected to pin 6 has the addi-

tional function of amplifier anti-pop control for this reason it

is necessary that a capacitor be connected to pin 6 even if

ALC is not used

R15

R14

R16

R14

TLH7894 – 8

FIGURE 6 Monitor Amplifier

7