OPA1641 Datasheet, PDF (25/31 Pages) Texas Instruments – High-Performance, JFET-Input AUDIO OPERATIONAL AMPLIFIERS

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Typical Application (continued)
OPA1641, OPA1642, OPA1644
SBOS484C – DECEMBER 2009 – REVISED DECEMBER 2015
20
15
Playback Curve
10
5
0
Combined Response
-5
-10
Recording Curve
-15
-20
10
100
1000
Frequency (Hz)
10000
C009
Figure 39. RIAA Recording and Playback Curves Normalized at 1 kHz
The basic RIAA playback curve implements three time constants: 75-μs, 380-μs, and 3180-μs. An IEC
amendment was later added to the playback curve and implemented a pole in the curve at 20 Hz with the intent
of protecting loudspeakers from excessive low frequency content. Rather than strictly adhering to the IEC
amendment, this design moves this pole to a lower frequency to improve low frequency response while still
providing protection for loudspeakers.
Resistor R1 and Capacitor C1 are selected to provide the proper input impedance for the moving magnet
cartridge. The cartridge loading is specified by the manufacturer in the cartridge datasheet and is absolutely
crucial for proper response at high frequency. 47 kΩ is a common value for the input resistor, and the capacitive
loading is usually specified to 200 pF to 300 pF per channel. This capacitive loading specification includes the
capacitance of the cable connecting the turntable to the preamplifier, as well as any additional parasitic
capacitances at the preamplifier input. Therefore, the value of C1 needs to be less than the loading specification
to account for these additional capacitances.
The output network consisting of R5, R6, and C5 serves to AC couple the preamplifier circuit to any subsequent
electronics in the signal path. 100-Ω resistor R5 limits in-rush current into coupling capacitor C5 and prevents
parasitic capacitance from cabling from causing instability. R6 prevents charge accumulation on C5. Capacitor
C5 was chosen to be the same value as C4; for simplicity however, its value must be large enough to avoid
attenuating low frequency information.
The feedback resistor elements must be selected to provide the correct response within the audio bandwidth. In
order to achieve the correct frequency response, the passive components in the above circuit must satisfy the
equations:
R₂ u C₂ 3180Ps
(1)
R₃ u C₃ 75Ps
(2)
R₂ || R₃ u C₂ C₃ 318Ps
(3)
R2, R3, and R4 must also be selected to meet the design requirements for gain. The gain at 1 kHz is determined
by subtracting 20 dB from gain of the circuit at very low frequency (near dc):
A_1kHz A_LF 20dB
(4)
Therefore the low frequency gain of the circuit must be 60 dB to meet the goal of 40 dB at 1 kHz.
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