REV. A

OP37

–14–

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 k

Ω. For this configuration,

the bias-current induced offset voltage can be greater than the

170 pV maximum offset if the head resistance is not sufficiently

controlled.

A simple, but effective, fixed-gain transformerless microphone

preamp (Figure 10) 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

unplugged. Otherwise the 100% feedback from the open input

may cause the amplifier to oscillate.

OP37

+

–

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

Figure 10. TBD

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

generate 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 bandwidth,

or nearly 61 dB below a l mV input signal. Measurements confirm

this predicted performance.

For applications demanding appreciably lower noise, a high quality

microphone-transformer-coupled preamp (Figure 11) incorporates

the internally compensated. 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.

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 switching

transients.

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 11. TBD

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 con-

stant is not necessary, C2 can be deleted, allowing the faster

OP37 to be employed.

Some comment on noise is appropriate to understand the

capability of this circuit. A 150

Ω resistor and R1 and R2 gain

resistors connected to a noiseless amplifier will generate 220 nV

of noise in a 20 kHz bandwidth, or 73 dB below a 1 mV reference

level. Any practical amplifier can only approach this noise level;

it can never exceed it. With the OP27 and T1 specified, the

additional noise degradation will be close to 3.6 dB (or –69.5

referenced to 1 mV).

References

1. Lipshitz, S.P, “On RIAA Equalization Networks,” JAES, Vol. 27, June 1979,

p. 458-4S1.

2. Jung, W.G., IC

Op Amp Cookbook, 2nd Ed., H.W. Sams and Company,

1980.

3. Jung, W.G., Audio /C

Op Amp Applications, 2nd Ed., H.W. Sams and Com-

pany, 1978.

4. Jung, W.G., and Marsh, R.M., “Picking Capacitors.” Audio, February &

March, 1980.

5. Otala, M., “Feedback-Generated Phase Nonlinearity in Audio Amplifiers,”

London AES Convention, March 1980, preprint 197B.

6. Stout, D.F., and Kaufman, M.,

Handbook of Operational Amplifier Circuit

Design, New York, McGraw Hill, 1976.