REV. B

OP37

–13–

Table I.

Source

Device

Impedance

Comments

Straln Gauge

<500

W

Typically used in low-frequency

applications.

Magnetic

<1500

W

Tapehead

set-magnetization problems when

direct coupling is used. OP37

Magnetic

<1500

W

Phonograph

coupled applications. OP37 will not

Cartridges

introduce any self-magnetization

problem.

Linear Variable <1500

W

Used in rugged servo-feedback

Differential

applications. Bandwidth of interest

Transformer

is 400 Hz to 5 kHz.

Audio Applications

The following applications information has been abstracted from

a PMI article in the 12/20/80 issue of Electronic Design magazine

and updated.

Ca

150pF

A1

OP27

Ra

47.5k

R1

97.6k

MOVING MAGNET

CARTRIDGE INPUT

R2

7.87k

R3

100

C1

0.03 F

C2

0.01 F

C3

0.47 F

R4

75k

++

C4 (2)

220 F

LF ROLLOFF

OUT

IN

OUTPUT

R5

100k

G = 1kHz GAIN

= 0.101 (

)

R1

R3

1 +

= 98.677 (39.9dB) AS SHOWN

Figure 8. Phono Pre-Amplifier Circuit

Figure 8 is an example of a phono pre-amplifier circuit using the

OP27 for A1; R1-R2-C1-C2 form a very accurate RIAA net-

work with standard component values. The popular method to

accomplish RIAA phono equalization is to employ frequency-

dependent feedback around a high-quality gain block. Properly

chosen, an RC network can provide the three necessary time

constants of 3180

For initial equalization accuracy and stability, precision metal-

film resistors and film capacitors of polystyrene or polypropylene

are recommended since they have low voltage coefficients,

dissipation factors, and dielectric absorption.

capacitors should be avoided here, though low-K ceramics—

such as NPO types, which have excellent dissipation factors,

and somewhat lower dielectric absorption—can be considered

for small values or where space is at a premium.)

The OP37 brings a 3.2 nV/

÷Hz voltage noise and 0.45 pA/÷Hz

current noise to this circuit. To minimize noise from other sources,

R3 is set to a value of 100

W, which generates a voltage noise of

1.3 nV/

÷Hz. The noise increases the 3.2 nV/÷Hz of the amplifier

by only 0.7 dB. With a 1 k

W source, the circuit noise measures

63 dB below a 1 mV reference level, unweighted, in a 20 kHz

noise bandwidth.

Gain (G) of the circuit at 1 kHz can be calculated by the expression:

G

R

R

=+

Ê

ËÁ

ˆ

¯˜

0 101 1

1

3

.

For the values shown, the gain is just under 100 (or 40 dB).

Lower gains can be accommodated by increasing R3, but gains

higher than 40 dB will show more equalization errors because of

the 8 MHz gain bandwidth of the OP27.

This circuit is capable of very low distortion over its entire range,

generally below 0.01% at levels up to 7 V rms. At 3 V output

levels, it will produce less than 0.03% total harmonic distortion

at frequencies up to 20 kHz.

Capacitor C3 and resistor R4 form a simple –6 dB per octave

rumble filter, with a corner at 22 Hz. As an option, the switch

selected shunt capacitor C4, a nonpolarized electrolytic, bypasses

the low-frequency rolloff. Placing the rumble filter’s high-pass

action after the preamp has the desirable result of discriminating

against the RIAA amplified low frequency noise components

and pickup-produced low-frequency disturbances.

A preamplifier for NAB tape playback is similar to an RIAA

phono preamp, though more gain is typically demanded, along

with equalization requiring a heavy low-frequency boost. The

circuit In Figure 8 can be readily modified for tape use, as

shown by Figure 9.

Ca

Ra

R1

33k

TAPE

HEAD

0.47 F

0.01 F

R2

5k

100k

15k

T1 = 3180 s

T2 = 50 s

OP37

+

–

Figure 9. Tape-Head Preamplifier

While the tape-equalization requirement has a flat high frequency

ms), the amplifier need not be stabilized

for unity gain. The decompensated OP37 provides a greater

bandwidth and slew rate. For many applications, the idealized

time constants shown may require trimming of Ra and R2 to

optimize frequency response for non ideal tape head perfor-

mance and other factors.

5

The network values of the configuration yield a 50 dB gain at 1 kHz,

and the dc gain is greater than 70 dB. Thus, the worst-case out-

put offset is just over 500 mV. A single 0.47

mF output capacitor

can block this level without affecting the dynamic range.

The tape head can be coupled directly to the amplifier input,

since the worst-case bias current of 85 nA with a 400 mH, 100

min.

head (such as the PRB2H7K) will not be troublesome.

One potential tape-head problem is presented by amplifier bias-

current transients which can magnetize a head. The OP27 and