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參數(shù)資料
| 型號： | LA3161 |
| 元件分類： | 音頻／視頻放大 |
| 英文描述： | 2 CHANNEL, AUDIO PREAMPLIFIER, PSIP8 |
| 封裝： | SIP-8 |
| 文件頁數(shù)： | 4/7頁 |
| 文件大?。?/td> | 154K |
| 代理商： | LA3161 |
LA3161
No.573–4/7
Function of External Parts
C2, C4 are input coupling capacitors. In NAB equalizer amplifier, the gain at low frequencies is high and 1/f noise
inside the IC is emphasized as output noise. Therefore, if the reactance of capacitor at low frequencies is increased,
the dependence of 1/f noise on the signal source resistance causes the output noise voltage to deteriorate, and the
value of reactance must be made small enough as compared with the signal source resistance. C2, C4 also influence
the operation start time and the adequate value of these capacitors is 10
μF. (Since C2, C4 of less than 4.7μF make the
operation start time longer, use C2, C4, of 4.7μF or more).
C5, C11 are NF capacitors. The lower cut-off frequency depends on the value of these capacitors.
If the lower cut-off frequency is taken as fL :
C5 (C11) = 1/2
π fL R2 (R7)
If the value of this capacitor is made larger, the operation start time of amplifier is more delayed. The adequate value
of capacitor is 47
μF.
The frequency characteristic of the equalizer amplifier depends on C6 and R4, R3 (C10 and R5, R6).
The time constants to obtain the standard NAB characteristic are as shown below.
Tape speed
9.5cm/s
4.75cm/s
C6 (R3
+ R4)
3180
μs
1590
μs
R3 C6
90
μs
120
μs
C8 is bias capacitor for the power line. C8 of 47μF is inserted at a point as close to the power supply pin (pin 4) as
possible.
C1, C3 are for preventing radio interference in the strong electric field, interference attributable to engine noise, and
blocking oscillation at the time of large amplitude operation. The adequate value of C1, C3 is approximately 1000pF.
C7, C9 are output coupling capacitors. The adequate value of C7, C9 is 10μF.
NAB element and determination of gain
Since the DC feedback is provided by R1, R2 of NAB element, which brings about DC output potential at pins 3, 6, it
is impossible to change the value of R1, R2 of NAB element greatly. Therefore, when determining the gain, change
RNF with R1, R2, C1 (NAB element) kept constant.
(1) How to obtain RNF
Impedance Z of NAB element is
Z =
1
1/R1 + j
ωC1
+ R2
= (R1 + R2)
1 + j
ωC1{R1 R2/(R1 + R2)}
1 + j
ωC1R1
For a general negative feedback amplifier circuit, A = Ao/ (1 + Ao
β) applies, and Z = A RNF is obtained under
conditions of Ao>>A, A>>1 (
β = RNF/ (RNF + Z), Ao = open-loop gain, A = feedback gain).
Therefore, we can use an approximation of RNF = Z/A.
A = (VG for 1kHz) times, (Set R1, R2 at approximately 100kΩ)
Each time constant of NAB characteristic.
Tape speed
9.5cm/s
4.75cm/s
T1 C1, R1
3180
μs
1590
μs
T2 C1 (R1//R2)
90
μs
120
μs
(2) Examples of NAB Constants
(a) Tape speed : 9.5cm/s. (8 tracks)
VG : RNF (VG/f = 1kHz)
VG
30
35
40
dB
RNF
180
100
56
Ω
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