OP176 datasheet(14/21 Pages) AD

14 / 21 page

OP176

REV. 0

–14–

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

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.

Figure 43. An RIAA Phono Preamplifier Circuit

Ω100kΩ

ΩR7

100k

Ω

150pF

100

µF/25V

MOVING

MAGNET

PICKUP

OP176

+VS

–VS

Rt = R6| |R7

~ 50k

Ω

–

0.03µF

0.01µF

Ω200Ω (34dB)

Ω100Ω (40dB)

1000µF/16V

100

µF/25V

ΩR5

499

Ω

ΩR4

100k

Ω

3nF

VOUT

0.1µF

+VS

–VS

100µF

+18V

–18V

100k

Ω

[97.6k

Ω

]

8.25k

Ω

[7.87k

Ω]

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