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Audio ICs
Dual-channel, auto-reverse tape preamplifier
BA3402
The BA3402 is a dual preamplifier developed for auto-reverse car tape decks. An electronic switch on the chip switches between the forward and reverse heads, and this has the benefits of greater reliability, easier wiring, lower space requirements, and improved assembly efficiency when compared with conventional mechanical switches. The circuit is designed in such a way that coupling capacitors are not required. In addition to reducing the number of external components required, this ensures that the head is not magnetized when the power is applied.
FApplications Car stereos and play-only auto-reverse cassette decks. FFeatures 1) Low noise (VNIN = 1.2Vrms). 2) High open loop gain (GVO = 85dB). 3) Reliable electronic switch on the chip. Head switching for both left and right channels is possible with the addition of one external switch circuit. 4) Input-side virtual-ground design means that input coupling capacitors are not required. FBlock diagram
5) Built-in bias circuit reduces attached component requirements. 6) Low "pop" noise during head switching. 7) Emitter-follower output circuit with large drive capacity. 8) Pin layout allows easy PCB design. 9) Compact package allows high PCB wiring density.
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Audio ICs
FInternal circuit configuration
BA3402
FAbsolute maximum ratings (Ta = 25_C)
FElectrical characteristics (unless otherwise noted, Ta = 25_C and VCC = 8V)
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Audio ICs
FMeasurement circuit
BA3402
FApplication example
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Audio ICs
FElectrical characteristics curves
BA3402
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Audio ICs
FElectrical characteristics curves
BA3402
FOperation notes (1) Input-side virtual ground If coupling capacitors are used to connect the head with the preamplifier input, the capacitor charging current flows in the head and can magnetize it. To prevent this, a virtual ground is used on the input side, and coupling capacitors are not required, even with a single-rail supply. This is the bias pin input and the virtual ground point for the negative feedback, and the bias for the input transistor is taken from this terminal. The only DC current that flows in the magnetic head is the base bias current for the input transistor, which is not enough to magnetize the head. If a floating input is not possible due to the construction of the magnetic head, or wiring considerations, connect the input via a coupling capacitor and bias the transistor using a resistor as shown in Fig.15.
This resistor, the capacitance of the capacitor connected between the control pin and GND, and the 20k pulldown resistor on the IC determine the switching speed. Switching speed increases as the value of the capacitor is reduced. Set the switching speed so that it matches the timing of the reversing mechanism. Note, that if the switching speed is too fast, a "pop" noise will be generated. The circuit in Fig.16 illustrates how to perform channel switching by connecting the control pin to GND. Note, that the switching speed from channel 1 to channel 2 and from channel 2 to channel 1 is different.
(2) Channel switching control When the control pin is connected to VCC via a resistor, channels A-1 and B-1 operate, and when it is open circuit, channels A-2 and B-2 operate. The recommended value for the resistor is 15k.
(3) Output coupling capacitors In car stereos, the preamplifier output passes through the tone control circuits before going to the power amplifier, so it is possible to eliminate the output coupling capacitors. Coupling capacitors are required when the preamplifier output is used directly as the deck output with no tone control, or when it is connected directly to power amplifier.
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Audio ICs
(4) Capacitor for prevention of oscillation and RF mixing With the BA3402, a capacitor must be connected between each input pin and the bias pins to prevent oscillation and RF mixing. The recommended value for this capacitor is 1000pF to 2000pF. If it is too small, oscillation and RF noise can adversely influence circuit performance. Conversely, if the value of the capacitor is too large, it will adversely influence the audio frequency characteristics. These capacitors can also be used for treble compensation. Determine the value based on the relationship with the magnetic head impedance. (5) Bypass capacitor The capacitor connected in series with the gain-setting resistor between the NF pin and the bias pin is the bypass capacitor. The value of this capacitor determines the bass characteristics. The variation in bass characteristics for different values of this capacitor is shown in Fig. 5.
BA3402
(6) Playback equalizer The playback equalizer curve is determined by the RC circuit connected between the output pin and the NF pin. For the circuit shown in Fig. 17, with a closed-loop gain of GVC (dB) at a frequency of 1kHz, the value of the circuit components is given by the following equations: R2 = 15.6 R3 10Gvc / 20 () R1 = 2 53 R2 ()
C=
3180 10*6 R2
(F)
The recommended value for R3 is about 100. If R3 is too large, the voltage gain will not match the calculated value.
FExternal dimensions (Units: mm)
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