OP176 Datasheet, PDF (14/21 Pages) Analog Devices – Bipolar/JFET, Audio Operational Amplifier

OP176
An RIAA Phono Preamp
Figure 43 illustrates a simple phono preamplifier using RIAA
equalization. The OP176 is used here to provide gain and is
chosen for its low input voltage noise and high speed perfor-
mance. The feedback equalization network (R1, R2, C1, and
C2) forms a three time constant network, providing reasonably
accurate equalization with standard component values. The
input components terminate a moving magnet phono cartridge
as recommended by the manufacturer, the element values
shown being typical. When this ac coupled circuit is built with a
low noise bipolar input device such as the OP176, amplifier bias
current makes direct cartridge coupling difficult. This circuit
uses input and output capacitor coupling to minimize biasing
interactions.
Input ac coupling to the amplifier is provided via C5, and the
low frequency termination resistance, R_T, is the parallel equiva-
lent of R6 and R7. R3 of the feedback network is ac grounded
via C4, a large value electrolytic. Additionally, this resistor is
set to a low value to minimize circuit noise from nonamplifier
sources. These design measures reduce the dc offset at the
output of the OP176 to a few millivolts. The output coupling
network of C3 and R4 is shown as suitable for wide band
response, but it can be set to a 7950 µs time constant for use as
a 20 Hz rumble filter.
The 1 kHz gain (“G”) of this circuit, controlled by R3, is
calculated as:
G (@ 1 kHz) = 0.101 × 1 + ^R1
R3
For an R3 of 200 Ω, the circuit gain is just under 50 × (≈ 34 dB),
and higher gains are possible by decreasing R3. For any value
of R3, the R5-C6 time constant should be equal to R3 and the
series equivalent of C1 and C2.
Using readily available standard values for network elements
(R1, R2, C1, and C2) makes the design easily reproducible and
inexpensive. These components are ideally high quality
precision types, for low equalization errors and minimum
parasitics. One percent metal-film resistors and two percent
film capacitors of polystyrene or polypropylene are recom-
mended. Using the suggested values, the frequency response
relative to the ideal RIAA characteristic is within ± 0.2 dB over
20 Hz–20 kHz. Even tighter response can be achieved by using
the alternate values, shown in brackets “[ ],” with the trade-off
of a non off-the-shelf part.
As previously mentioned, the OP176 was chosen for three
reasons: (1) For optimal circuit noise performance, the
amplifier used should exhibit voltage and current noise densities
of 5 nV/√Hz and 1 pA/√Hz, respectively. (2) For high gain
accuracy, especially at high stage gains, the amplifier should
exhibit a gain bandwidth product in excess of 5 MHz. (3)
Equally important because of the 100% feedback through the
network at high frequencies, the amplifier must be unity gain
stable. With the OP176, the circuit exhibits low distortion over
the entire range, generally well below 0.01% at outputs levels of
5 V rms using ± 18 V supplies. To achieve maximum perfor-
mance from this high gain, low level circuit, power supplies
should be well regulated and noise free, and care should be
taken with shielding and conductor layout.
Active Filter Circuits Using the OP176
A general active filter topology that lends itself to both high-pass
(HP) and low-pass (LP) filters is the well known Sallen-Key
(SK) VCVS (Voltage-Controlled, Voltage Source) architecture.
This filter type uses the op amp as a fixed gain voltage follower
at either unity or a higher gain. Discussed here are simplified 2-
pole, unity gain forms of these filters, which are attractive for
several reasons: One, at audio frequencies, using an amplifier
with a 10 MHz bandwidth such as the OP176, these filters
exhibit reasonably low sensitivities for unity gain and high
damping (low Q). Second, as voltage followers, they are also
inherently gain accurate within their pass band; hence, no gain
resistor scaling errors are generated. Third, they can also be
made “dc accurate,” with output dc errors of only a few
millivolts. The specific filter response in terms of HP, LP and
damping is determined by the RC network around the op amp,
as shown in Figure 44a.
MOVING
MAGNET
PICKUP
R6
Ω100kΩ
+V_S
+18V
0.1µF
100µF
0.1µF
100µF
C5
100µF/25V
+V_S
–V_S
3
76
U1
Ct
150pF
ΩR7 ₂ OP176
100kΩ
4
Rt = R6| |R7
–~ ^50kΩ
–V_S
R1
100kΩ
1%
[97.6kΩ
]
R2
8.25kΩ
1%
[7.87kΩ]
–18V
C3
100µF/25V
C1
0.03µF
2%
C2
0.01µF
2%
ΩR5
499Ω
Ω200Ω (34dB)
Ω100Ω (40dB)
R3
ΩR4
100kΩ
C4
1000µF/16V
V_OUT
C6
3nF
Figure 43. An RIAA Phono Preamplifier Circuit
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