BA3423S 查看數據表（PDF） - ROHM Semiconductor

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BA3423S

Double cassette tape recorder system preamplifier

ROHM Semiconductor 

Audio ICs

BA3423S

(6) ALC amplifier

The *30dBm standard level signal mixed by the mixing

resistor enters a *17dB attenuator before the ALC am-

plifier. This attenuator resistance and an electronic vol-

ume are used by the ALC. The ALC amplifier standard

input level is *47dBm, and its gain is 40dB, giving a

standard output level of *7dBm. The standard output

level of the ALC amplifier is compared to the ALC level

(*3.7dBm), and if signals above this level are input the

ALC operates. The attack and recovery times are deter-

mined by the CR circuit connected to the T pin (pin 9).

(7) Recording equalizer amplifier

The standard signal level is *7dBm (from the ALC am-

plifier output), and has treble peaking characteristic at-

tributed to it by the recording equalizer amplifier. The ex-

ternal capacitors connected between pins 5, 6 and 7, and

pins 10, 11 and 12 form an equivalent inductor circuit, and

the series impedance of this LCR circuit connected in

parallel with the on-chip resister form the input imped-

ance of the inverting amplifier and give it its peaking char-

acteristic. The equivalent circuit is shown in Fig. 1. If the

Nor / HS pin (pin 21) is driven HIGH, the values of the two

resisters change, to give the peaking characteristic for

high-speed dubbing. In the past, the recording current

peaking characteristic was formed by switching the cir-

cuit constant values of a fixed-current CR circuit. With the

BA3423AS, this is incorporated on the chip. The gain of

this stage is 0dB at 1kHz, and the recording output stan-

dard level is *7dBm. As the maximum output voltage of

the recording output pin (Tape B pins 3 and 4) is 4.5dBm

(at VCC = 5.5V), for sudden, large inputs, even during the

attack time period until the ALC starts operating, the dy-

namic range is 4.5dBm - the recording output standard

level (*7dBm), i.e. –11.5dBm. This allows low-distortion

recording. The frequency response of the recording cur-

rent flowing through the head is shown in Figs. 17 to 19.

This is the response characteristic for fixed input voltage

to the Aux In pins (pins 24 and 25). The factors that deter-

mine the recording current characteristics are the peak-

ing characteristics of the peaking circuit, and the external

fixed-current circuit. The value of the fixed-current circuit

resistor (Ro) is calculated using the following formula.

The recording output standard level is *7dBm, f = 1kHz,

and the standard recording current required is 37µA

(rms).

*7dBm / (RH + RO) = 37µArms

RH: DC resistance at the head

If RH is 500Ω, Ro is approximately 8.2kΩ. The basic val-

ues for the capacitors for the external constant-current

circuit and recording equalizer amplifier are:

C0 = 820pF

CL1 (between pins 6 and 7, and pins 11

and 12) = 2700pF

CL2 (between pins 5 and 6, and pins 10

and 11) = 470pF

Characteristic curves for different values of C0, CL1 and

CL2 are given in Figs. 17, 18 and 19 respectively. Select

the values for these components after considering the

characteristics of the head to be used, and the overall re-

cording and playback frequency characteristics.

(8) Recording output switch

The Tape B pins (pins 3 and 4) are used as both the B

mechanism input pins, and recording output pins, and

the recording output switch is used to switch between

them. When the input to the Rec / PB pin (pin 19) is “H”,

recording output is selected, and when it is open, Tape

B input is selected. This reduces the complexity of the re-

cording head switch circuitry, and requires just two cir-

cuits. Transient muting is used to suppress the “pop”

sound that accompanies switching.

(9) Control pins

The control pin inputs and the corresponding states of

the various inputs and outputs are summarized in the in-

put / output pin status table that follows.

261

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