REV. A
OP27
–14–
against the RlAA-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 7 can be readily modified for tape use, as shown
by Figure 8.
Ca
Ra
R1
33k
TA P E
HEAD
0.47F
0.01F
R2
5k
100k
15k
T1 = 3180s
T2 = 50s
OP27
+
–
Figure 8.
While the tape-equalization requirement has a flat high-frequency
gain above 3 kHz (T
2
= 50 µs), 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 R1 and R2 to
optimize frequency response for nonideal tapehead performance
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
output offset is just over 500 mV. A single 0.47 µF output capaci-
tor can block this level without affecting the dynamic range.
The tapehead can be coupled directly to the amplifier input,
since the worst-case bias current of 80 nA with a 400 mH, 100
µ inch head (such as the PRB2H7K) will not be troublesome.
One potential tapehead problem is presented by amplifier bias-
current transients which can magnetize a head. The OP27 and
OP37 are free of bias-current transients upon power-up or power-
down. However, it is always advantageous to control the speed
of power supply rise and fall, to eliminate transients.
In addition, the dc resistance of the head should be carefully
controlled, and preferably below 1 kS2. For this configuration,
the bias-current-induced offset voltage can be greater than the
100pV maximum offset if the head resistance is not sufficiently
controlled.
A simple, but effective, fixed-gain transformerless microphone
preamp ( Figure 9) amplifies differential signals from low imped-
ance microphones by 50 dB, and has an input impedance of 2 kΩ.
Because of the high working gain of the circuit, an OP37 helps
to preserve bandwidth, which will be 110 kHz. As the OP37
is a decompensated device (minimum stable gain of 5), a dummy
resistor, Rp, may be necessary, if the microphone is to be
unplugged. Otherwise the 100% feedback from the open input
may cause the amplifier to oscillate.
Common-mode input-noise rejection will depend upon the
match of the bridge-resistor ratios. Either close-tolerance (0.1%)
types should be used, or R4 should be trimmed for best CMRR.
All resistors should be metal film types for best stability and
low noise.
Noise performance of this circuit is limited more by the input
resistors R1 and R2 than by the op amp, as R1 and R2 each gen-
erate a 4 nV/√Hz noise, while the op amp generates a 3.2 nV/√Hz
noise. The rms sum of these predominant noise sources will be
about 6 nV/√Hz, equivalent to 0.9 µV in a 20 kHz noise band-
width, or nearly 61 dB below a 1 mV input signal. Measurements
confirm this predicted performance.
R3
316k
Rp
30k
R1
1k
R4
316k
R2
1k
R7
10k
R6
100
OUTPUT
R3
R1
R4
R2
=
LOW IMPEDANCE
MICROPHONE INPUT
(Z = 50 TO 200 )
C1
5F
OP27/
OP37
+
–
Figure 9.
For applications demanding appreciably lower noise, a high
quality microphone transformer-coupled preamp (Figure 10)
incorporates the internally compensated OP27. T1 is a JE-115K-E
150 Ω/15 kΩ transformer which provides an optimum source
resistance for the OP27 device. The circuit has an overall gain of
40 dB, the product of the transformer’s voltage setup and the op
amp’s voltage gain.
A1
OP27
R3
100
R1
121
R2
1100
C2
1800pF
OUTPUT
150
SOURCE
T1*
T1 – JENSEN JE – 115K – E
JENSEN TRANSFORMERS
10735 BURBANK BLVD.
N. HOLLYWOOD, CA 91601
*
Figure 10.
Gain may be trimmed to other levels, if desired, by adjusting R2
or R1. Because of the low offset voltage of the OP27, the output
offset of this circuit will be very low, 1.7 mV or less, for a 40 dB
gain. The typical output blocking capacitor can be eliminated in
such cases, but is desirable for higher gains to eliminate switch-
ing transients.
OP27
–18V
+18V
Figure 11. Burn-In Circuit
Capacitor C2 and resistor R2 form a 2 µs time constant in this
circuit, as recommended for optimum transient response by the
transformer manufacturer. With C2 in use, A1 must have unity-
gain stability. For situations where the 2 µs time constant is not
necessary, C2 can be deleted, allowing the faster OP37 to be
employed.
