Audio ICs BBS (bass boost system) for radio cassette players BA3870 The BA3870 is a bass boost audio IC for radio cassette players. BBS adjusts the base boost to a level appropriate for the volume, achieving a rich sound without distortion. Frequency characteristics can be set externally, enabling system design to meet the needs of the application. Applications Radio cassette players Features 1) Adjustment of the base boost to a level appropriate for the volume through BBS. 2) Maximum bass boost level can be set externally. (20dB recommended) Absolute maximum ratings (Ta = 25C) Recommended operating conditions (Ta = 25C) 420 3) Treble boost circuit (constant at 4dB) included onchip to preserve overall balance. 4) BBS on / off “pop” sound almost nonexistent. Audio ICs BA3870 Block diagram 421 Audio ICs Pin descriptions 422 BA3870 Audio ICs BA3870 Input / output circuits Electrical characteristics (unless otherwise noted, Ta = 25C, VCC = 5.0V and f = 1kHz) 423 Audio ICs Measurement circuit 424 BA3870 Audio ICs BA3870 Circuit Operation (1) Overview of BBS BBS is a system which boosts the low end of the audible frequency range and at the same time linearly controls the amount of boost according to the volume to produce a rich sound without distortion through small diameter speakers. It also adjusts the gain of the high-frequency range to attain a balanced sound with natural volume. (2) Operation of BBS (1) BBS on The signal system when BBS is on is shown in Fig. 2. 425 Audio ICs BA3870 1. Low range signal system The transmission of low range signals in Fig. 2 is as shown below. The frequency characteristics of each block are as shown in Fig. 3, and the gain and cutoff frequency of each are given by the following equations. Cutoff frequency of LPF1 : fCL1 = 1 2π (R4//R5) C2 Attenuation of LPF1 : GLP1 = 20log Gain of Amp A : GA = 0 Attenuation of ATT1 Gain of Amp B R5 R4R5 (Hz) (dB) (dB) EVR R6EV R7RR6 : GB = 20log R7 : GATT1 = 20log (dB) (dB) Cutoff frequency of Amp B: fCL3 = 1 2π R7 C3 (Hz) Cutoff frequency of LPF2: fCL2 = 1 2π R9 C4 (Hz) Gain of Amp C : GC = 0 Gain of Amp D : GDL = 20log 426 (dB) R11 R10 (dB) Here, R4 = R5 = 2R9 and C2 = C4, thus fCL1 = fCL2 and the total frequency shows a 12dB / OCT bass boost as indicated in Fig. 4. Also, as R11 = 2R10, GLP1 + GDL = 6 + 6 GLP1 + GDL = 0 (dB) and thus the total gain during EVR Max. is GL Max. = GLP1 + GA + GB + GC + GDL GL Max. = GB (dB) Audio ICs BA3870 2. High-range signal system The transmission of high frequency signals in Fig. 2 is as shown below. The frequency characteristics of each block are shown in Fig. 5, and the gain and cutoff frequency of each are given by the following equations. Cutoff frequency 1 of HPF: fCH1 = 1 2π (R1)R2) C1 (Hz) (R2)R3) Cutoff frequency 2 of HPF: fCH2 = 2π (R1R2)R2R3 )R1R3) C1 (Hz) Attenuation of HPF in frequency range f t fCH1 : GHP1 = 20log Attenuation of HPF in frequency range fCH2 t f : GHP2 = 20log Gain of Amp D : GDH = 20log R3 R2)R3 (dB) R3 R1//R2)R3 (dB) R10)R11 R10 (dB) The total frequency characteristics show a +6dB / OCT treble boost as shown in Fig. 6. Also, R11 = 2R10, thus GDH = +9.5 (dB) and the total gain GH1 in the f t fCH1 frequency range is GH1 = GHP1 + GDH GH1 = GHP1 + 9.5 (dB) The total gain GH2 in the fCH2 t f frequency range is GH2 = GHP2 + GDH GH2 = GHP2 + 9.5 (dB) 3. Combination of frequency characteristics By combining the low-range characteristics (Fig. 4) and the high-range characteristics (Fig. 6), ideal BBS characteristics can be obtained as shown in Fig. 7. 427 Audio ICs 428 BA3870 Audio ICs BA3870 4. Bass boost control system Bass boost control is as follows in Fig. 8. The comparator level and gain of each block are given by the following equations. Attenuation of ATT2: R13//R14 (dB) : GATT2 = 20log R12R13//R14 Gain of Amp E: R15R16 (dB) : GE = 20log R16 Comparator level of COMP: VB R17 ) (dBV) : VCOM = 20log ( 2 R17R18 VB: DC voltage at BIAS OUT pin If the volume output level at the beginning of control of the low range signal system ATT1 is VBBS (dBV), VBBS + GATT2 + GE = VCOM In this way, bass boost control is carried out based on the volume as shown in Fig. 9. 429 Audio ICs 2) BBS off When BBS is off, the signal system is as shown in Fig. 10 due to the switches in the IC, and signal flow is as follows. The frequency characteristics of each block are shown in Fig. 11, and the gains are given by the following equations: R20 (dB) Attenuation of ATT3: GATT3 = 20log R19R20 R21R22 (dB) Gain of Amp D’: GD’ = 20log R21 As R19 = R20 and R21 = R22, the total gain GOFF when BBS is off is GOFF = GATT3GD’ GOFF = 0 (dB) and the characteristics are flat as shown in Fig. 12 430 BA3870 Audio ICs BA3870 Application examples 431 Audio ICs Operation notes (1) External value at CONTτ pin The BBS on / off “pop” sound can be reduced by increasing the value of the capacitor connected to the CONTτ pin, however, on / off switching time will increase. Select a value appropriate for the application. (2) External value at DET NF pin The gain GE can be decreased by connecting a series RC circuit to the DET NF pin, however, feedback will increase causing greater susceptibility to oscillation. We recommend a gain GE of 10dB or more. (3) External value at BBS LEVEL pin The comparator level can be increased by reducing the ratio of the resistors connected to the BBS LEVEL pin, however, taking into consideration the power supply voltage characteristics and transient characteristics, we recommend the following: VBuVBBS LEVELyVB*500mV : DC voltage at BIAS OUT pin VB ( = 1 / 2 VCC) VBBS LEVEL : DC voltage at BBS LEVEL pin 432 BA3870 (4) External value at LP NF pin The low frequency gain GB can be decreased by increasing the value of the resistor in the series RC circuit connected to the LP NF pin, however, feedback will increase causing greater susceptibility to oscillation. We recommend a gain GB of 12dB or more. (5) Coupling capacitors Decide the polarity of the input and output coupling capacitors based on the electrical potentials of the devices to which they are being connected. Audio ICs BA3870 Electrical characteristic curves External dimensions (Units: mm) 433