Handbook for Sound Engineers

1076 Chapter 28

28.4.2 Record Circuits

The primary task of the amplifier that drives the record
head is to convert the input audio signal voltage into a
proportional amount of current flowing in the windings
of the record head. To accomplish this task, the head
driver must overcome the rise in head impedance with
frequency that is due to the inductance of the head. A
common technique to achieve flat current response, as
shown in Fig. 28-39A, is to insert a resistor in series
with the head so that the combined series impedance of
the resistor and the head remain relatively constant
throughout the audio band. If the resistance is chosen to
be two to three times the reactance of the head at the
upper limit of the desired audio band, the desired
constant current characteristics can be closely
approximated.

The primary disadvantage of the series resistor is the
loss of head room due to the extra signal drop across the
resistor. This problem can be overcome with an active
current feedback circuit that senses the current in the
head through a small sampling resistor. Fig. 28-39B
shows a sampling resistor Rs in series with the return leg
of the head. The voltage generated across Rs by the
current flowing in the head is fed back to the inverting
input for comparison with the incoming audio signal.
The high gain of the driver amplifier necessitates only a
very small feedback signal, creating a negligible loss in
head room at high frequencies.

The circuits of Fig. 28-39 oversimplify the task of
driving the record head since no provisions are included
for adding the high-frequency bias signal to the current

in the head. A common method of adding the bias and

audio signals is shown in Fig. 28-40. The audio driver is

isolated from the bias source by a parallel trap tuned to

the bias frequency so that the bias signal does not create

nonlinearities within the audio driver. The high imped-

ance of the trap at the bias frequency also reduces the

loading effect of the audio source on the bias source.

A similar isolation of the bias source is accom-

plished by the capacitor in series with the bias supply.

Since the capacitor looks like a high impedance at audio

frequencies, the loading effect of the bias supply on the

audio source is minimized. At the higher frequencies of

the bias signal, the reactance of the capacitor has

dropped to a relatively low value that provides adequate

coupling of the bias signal into the record head.

An alternate approach that eliminates some of the

previously mentioned isolation requirements is shown

in Fig. 28-41. In this case, the bias and the audio are

added together at the input to a combination bias/audio

head driver amplifier. If the amplifier has sufficient

head room and very low distortion, the two signals can

be amplified simultaneously by the same amplifier

without any interference. The problem of coupling the

Figure 28-37. Unequalized reproduce head output.

Figure 28-38. Simplified playback amplifier.

Attenuation—dB

0

10

20

150 Hz 1.5 kHz 15 kHz

Frequency Hz

Low frequency cut

High frequency

boost

Integrator

Output

Gain

Head damping control

Reproduce

head

Figure 28-39. Constant-current record head drivers.

Figure 28-40. Audio and bias coupling to record head.

R = 3 × Zhead max

Equalized input

Head driver

Record

head

A. Combined series impedance of head

and resistor remains constant.

Head driver

Feedback

resistor

Record head

Sampling

resistor RS

B. Sampling resistor in series with

return leg of the head.

Equalized input

Bias

Audio

Bias adjust

To record head

Bias trap

Cb