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

TAPE

HEAD

0.47 F

0.01 F

R2

5k

100k

15k

T1 = 3180 s

T2 = 50 s

OP27

+

–

Figure 8.

While the tape-equalization requirement has a flat high-frequency

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 k

Ω.

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

5 F

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.