Audio ICs Double cassette tape recorder system preamplifier BA3423S The BA3423S is a record / playback system preamplifier for radio cassette decks. With four control pins it allows switching between I / O for the A (play only) and B (record and play) mechanisms, input for tape, radio, and microphone, on and off switching for line out and recording output, and equalizer switching for normal and high-speed dubbing. It requires about half the number of external components of its predecessors which means simplified assembly and overall cost savings. Applications Dual-cassette radio cassette players. Features 1) Built-in switch for playback equalizer amplifier for the A and B mechanisms. 2) Built-in normal / high-speed equalizer switch for the playback equalizer amplifier. 3) The B mechanism input is also used as the recording output, so a switch is not required. 4) Built-in tape / radio input switch, and microphone mixing. 5) The microphone amplifier is used as the mixing amplifier, and the line can be muted to allow the built-in microphone to be used for recording. 6) Built-in ALC circuit. 7) Built-in recording equalizer circuit, and internal switching of the normal / high-speed equalizer is possible. 8) All stages are directly connected, so coupling capacitors are not required. 9) Muting is applied as required during switching to reduce switching noise (transition muting). 10) Pin assignments has been designed to facilitate placement of attached components and PCB design. Absolute maximum ratings (Ta = 25C) Recommended operating conditions (Ta = 25C) 251 Audio ICs Block diagram Pin assignments 252 BA3423S Audio ICs BA3423S Pin descriptions 253 Audio ICs Input / output circuits 254 BA3423S Audio ICs BA3423S 255 Audio ICs 256 BA3423S Audio ICs BA3423S Electrical characteristics (unless otherwise noted, Ta = 25C and VCC = 5.5V, f = 1kHz, RG = 680Ω, Tape input = 66dB, Mic. input = 50dB, and AUX input = 23dB) 257 Audio ICs Explanation of electrical characteristics 258 BA3423S Audio ICs BA3423S Measurement circuit 259 Audio ICs Circuit operation (1) Playback equalizer amplifier The standard input level is *76dBm at 315Hz. The voltage 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 playback 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 resistance is 47kΩ. Input the CD or radio signal (or other) via a coupling capacitor. Switch the Tape / Aux input (pin 18) “H” for Aux input 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 determined by the bass cutoff frequency of the CR circuit connected to the Mic NF pin. For treble cutoff, connect a lowpass filter circuit to the input pin. 260 BA3423S (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 formula 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 resistor is *30dBm. This level is input as is to the line amplifier. The gain of the line amplifier is 11dB, so the standard 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 microphone 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 control 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). Audio ICs (6) ALC amplifier The 30dBm standard level signal mixed by the mixing resistor enters a 17dB attenuator before the ALC amplifier. This attenuator resistance and an electronic volume 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 determined by the CR circuit connected to the T pin (pin 9). (7) Recording equalizer amplifier The standard signal level is 7dBm (from the ALC amplifier output), and has treble peaking characteristic attributed to it by the recording equalizer amplifier. The external 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 impedance of the inverting amplifier and give it its peaking characteristic. 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 circuit 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 standard 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 dynamic 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 current 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 determine the recording current characteristics are the peaking 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). BA3423S 7dBm / (RH + RO) = 37µArms RH: DC resistance at the head If RH is 500Ω, Ro is approximately 8.2kΩ. The basic values 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 recording 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 recording head switch circuitry, and requires just two circuits. 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 input / output pin status table that follows. 261 Audio ICs Input / output pin status 262 BA3423S Audio ICs BA3423S Application example 1 263 Audio ICs Application example 2 264 BA3423S Audio ICs BA3423S Application example 3 Operation notes (1) Amplifier oscillation As the BA3423S incorporates dual-cassette dubbing functions on a single IC, it has extremely high input / output gain. In particular, in normal-speed mode, the gain at around the recording equalizer peaking characteristic f0 frequency is about 70dB. The phases of the input and output are reversed to reduce the chance of oscillation due to influence of the PCB pattern, but due consideration must be given to the PCB pattern design to prevent oscillation. In particular, the PCB tracks to the Tape A pins (pins 31 and 32) and Tape B pins (pins 3 and 4) should be sufficiently far apart that there is no coupling capacitance between them, or they should be shielded by having a GND or bias track between them. (2) Strong RF signals To prevent signal mixing due to strong electric fields, connect a capacitor (of a few hundred pF) to each input pin. These must be connected close to the pins of the IC to have any effect. Design the PCB track layout so that the capacitors can be connected as close to the base of the pins as possible. (3) External time constants for the control pins By keeping the external time constants of the circuits connected to the control pins small, the switching time can be kept low, but as the switching time is reduced, the switching noise increases. Choose the time constants to match the set design. We recommend that you use 33kΩ pullup resistors (these will not effect the maximum output voltage of the recording output). 265 Audio ICs Electrical characteristic curves 266 BA3423S Audio ICs BA3423S 267 Audio ICs External dimensions (Units: mm) 268 BA3423S