OP37
Table I.
by only 0.7 dB. With a 1 kW source, the circuit noise measures
63 dB below a 1 mV reference level, unweighted, in a 20 kHz
noise bandwidth.
Source
Impedance Comments
Device
Gain (G) of the circuit at 1 kHz can be calculated by the expression:
Straln Gauge
<500 W
Typically used in low-frequency
applications.
Ê
Ë
R1 ˆ
G = 0.101 1+
Á
˜
Magnetic
Tapehead
<1500 W
Low IB very important to reduce
set-magnetization problems when
direct coupling is used. OP37
IB can be neglected.
Similar need for low IB in direct
coupled applications. OP37 will not
introduce any self-magnetization
problem.
R3
¯
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.
Magnetic
Phonograph
Cartridges
<1500 W
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.
Linear Variable <1500 W
Differential
Transformer
Used in rugged servo-feedback
applications. Bandwidth of interest
is 400 Hz to 5 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.
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.
C4 (2)
R5
220ꢁF
100kꢄ
+
+
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.
MOVING MAGNET
CARTRIDGE INPUT
LF ROLLOFF
OUT
C3
0.47ꢁF
IN
A1
Ca
150pF
OP27
Ra
R4
47.5kꢄ
OUTPUT
75kꢄ
R1
C1
97.6kꢄ
0.03ꢁF
–
0.47ꢁF
R2
C2
OP37
7.87kꢄ
TAPE
0.01ꢁF
Ra
Ca
HEAD
+
15kꢄ
R3
R1
100ꢄ
33kꢄ
R2
G = 1kHz GAIN
0.01ꢁF
5kꢄ
R1
R3
1 +
= 0.101 (
)
= 98.677 (39.9dB) AS SHOWN
100kꢄ
T1 = 3180ꢁs
T2 = 50ꢁs
Figure 8. Phono Pre-Amplifier Circuit
Figure 9. Tape-Head Preamplifier
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 ms, 318 ms, and 75 ms.1
While the tape-equalization requirement has a flat high frequency
gain above 3 kHz (t2 = 50 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
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.4 (High-K ceramic
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 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.
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
One potential tape-head problem is presented by amplifier bias-
current transients which can magnetize a head. The OP27 and
REV. B
–13–
ADI [ ADI ]
