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BA3423S

Double cassette tape recorder system preamplifier

ROHM Semiconductor 

Audio ICs

BA3423S

FCircuit operation

(1) Playback equalizer amplifier

The standard input level is *76dBm at 315Hz. The volt-

age gain at 315Hz is 53dB, so the standard output level

for the playback equalizer amplifier is *23dBm. The

playback head is connected between the bias pin (pin 28)

and the input pins without coupling capacitors, and the

amplifier input stage is biased through the head.

When the A / B switch pin (pin 20) is “H”, the input goes

to B, and when it is open the input goes to A. The play-

back equalizer NAB characteristics are determined as

follows:

External CR timing circuit: 270kΩ, 4.7kΩ, 0.012µF

On-chip resistors: 5.3kΩ, 500Ω

Bass time constant:

= 270kΩ 0.012µF 3180µs

Treble time constant:

Normal-speed mode:

= (4.7kΩ + 5.3kΩ) 0.012µF = 120µs

High-speed mode:

= (4.7kΩ + 500Ω) 0.012µF = 62.4µs

Switching between normal-speed and high-speed

modes is controlled by pin 21 (Nor / HS). When pin 21 is

HIGH, high-speed mode is selected, and when open,

normal-speed mode is selected.

(2) Aux input amplifier

The standard input level is *23dBm, and the input resis-

tance is 47kΩ.

Input the CD or radio signal (or other) via a coupling ca-

pacitor. Switch the Tape / Aux input (pin 18) “H” for Aux in-

put mode, and leave it open for tape input mode.

(3) Microphone amplifier

The standard microphone input level is *50dBm when

the NF resistor connected to the Mic NF pin (pin 22) is

820Ω. The microphone amplifier gain at this time is 27dB,

and the microphone amplifier output is *23dB. The input

resistance of the Mic In pin (pin 23) is 30kΩ. Connect the

input to pin 23 via a coupling capacitor. The frequency

characteristics of the microphone amplifier are deter-

mined by the bass cutoff frequency of the CR circuit con-

nected to the Mic NF pin. For treble cutoff, connect a low-

pass filter circuit to the input pin.

(4) Function switching (Mic / mixing circuit)

The signal switched by the Tape / Aux switching pin (pin

18) and the microphone amplifier output are mixed by a

6.8kΩ mixing resistor. However, when the Nor / HS pin is

set to high-speed mode (“H”), the microphone amplifier

output is muted. When in internal microphone recording

mode (only when: pin 18 is open (tape mode), pin 19 is

“H” (recording mode), pin 20 is “H” (Tape B mode), and

pin 21 is open (normal-speed mode)), the tape and Aux

inputs are muted, and only the Mic output is output. If pin

21 is driven “H” (high-speed mode) in this state, all inputs

are muted. Pins 16 and 17 are for the low-pass filter.

These pins cut bias oscillation leakage during dubbing.

The 6.8kΩ mixing resistor and an external capacitor set

the treble cutoff frequency according to the following for-

mula

fHC = 1 / (πCR)

Treble cutoff frequency: fHC (Hz)

Mixing resistor: R = 6.8k (Ω)

External capacitor: C (F)

(5) Line amplifier

The standard level of the signal mixed by the mixing re-

sistor is *30dBm. This level is input as is to the line am-

plifier. The gain of the line amplifier is 11dB, so the stan-

dard output is *19dBm, and this is output to the Line Out

pins (pins 14 and 15). The line amplifier output has a

mute switch, but it is only muted when in internal micro-

phone recording mode (described in the previous item).

The line mute operates to suppress the switching “pop”

sound that is generated when switching between modes

(transient muting). The transient muting time is set by the

time constants of the external circuits connected the con-

trol pins (pins 18 to 21). However, in one case only, when

pin 18 is “H” (Aux input mode), line output muting does

not occur when the other control pins are switched. This

is to disable transient line out muting when recording is

started while listening to the radio (for example).

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