Handbook for Sound Engineers

Magnetic Recording and Playback 1077

output to the head for constant current drive must still

be overcome, however, by including either a complex

coupling network or an active feedback network.

In addition to the head driver circuits, which correct

for any response droop due to head inductance, the

record amplifier must provide deliberate frequency

response tailoring to match the desired equalization

standards. The standards usually require an adjustable

boost at 6 dB/octave beginning in the middle of the

audio band, with lower tape speeds generally requiring

greater boosts to overcome the increased tape thickness

and self-erasure losses.

The needed boost is easily implemented by the resis-

tance-capacitance circuit shown in Fig. 28-42A, but the

use of a variable capacitor is inconvenient due to the

limited range of capacitor adjustment and the awkward

size and mounting of the capacitor. Newer designs,

therefore, favor operational amplifier configurations

that control the amount of boost with a potentiometer.

One such circuit, shown in Fig. 28-42B, selectively

adds the output of a differentiator circuit, which rises at

6 dB/octave, to the main signal path.

A secondary benefit of the differentiator circuit is the

phase change introduced by the inverting characteris-

tics of the differentiator amplifier. Unlike most of the

loss-correction circuits of the signal path, which intro-

duce signal delay at high frequencies, the inverted

differentiator advances the high frequencies. The proper

combination of advance and delay can provide less

phase distortion in the signal, yielding improved tran-

sient response with less overshoot. A similar

phase-correcting effect has been implemented in other

designs by providing an all-pass, phase-shifting network

in the reproduce amplifier.

The NAB and compact cassette equalization stan-

dards provide an additional record signal boost at low

frequencies to overcome the hum and noise limitations

of the reproduce heads and amplifiers. Typical circuits

for this purpose are shown in Fig. 28-43. Both cases

achieve a 6 dB/octave rise with decreasing frequency

from 50 Hz or 100 Hz to below 20 Hz.

Abrupt changes in the bias and audio signals on the

record head must be avoided whenever the record mode

is entered or exited. Ramping circuits are employed for

this purpose to control the buildup and decay of these

signals. Typical methods include the use of analog

switching elements such as bipolar-junction or

field-effect transistors. The rate of switching of these

elements is limited to a value that does not create abrupt

transients but, at the same time, is quick enough to avoid

annoying delays, overrecordings, or program holes.

Figure 28-41. Active summer for bias and audio.

Figure 28-42. High-frequency record boost circuits.

Bias

Audio

+ Coupling

network To record

head

Audio

in

Audio in

Output

Equalized

output

A. With variable capacitor.

B. With operational amplifier configuration.

Figure 28-43. Low-frequency record boost circuits.

Boost

circuit

Boost Circuit