390

Audio ICs

BA3528AFP / BA3529AFP

FCircuit operation

(1)

Preamplifier

In the preamplifier input stage the pin 22 bias is the input

and the negative feedback virtual earth, and the bias for

the input stage transistor is taken from pin 22 via the tape

head to allow direct coupling. Connect a 1000pF capaci-

tor in parallel with the tape head to prevent high-frequen-

cy interference (see Fig. 2).

(2)

Mute amplifier

Preamplifier output muting can be switched on and off.

The mute is off when the mute switch input (pin 1) is low

or open, and on when the mute switch input is high (tied

(3)

Equalizer

The preamplifier is based on an NAB120

µs NF-type

equalizer. It is possible to add a switching function for the

equalizer using the mute amplifier. Switching of the

equalizer constant is controlled by the voltage on pin 1

(low or high). Note, however, when this is done, preampli-

fier muting no longer operates (see Fig. 4).

(4)

Power amplifier

The power amplifier employs an NF circuit with fixed

(BA3529AFP).

For the input stage, the pin 22 bias point is the input and

the negative feedback virtual earth point, and the first

amplifier uses the same bias point as its input, and its out-

put voltage is about the same as DC output voltage from

the power amplifier. This becomes the virtual earth for the

headphones (see Fig. 5).