Handbook for Sound Engineers

Magnetic Recording and Playback 1075

higher, more usable level by the first stage of the play-
back amplifier. Special low-noise amplifier circuits
developed for this purpose provide at least 20 dB of
gain so that subsequent amplifier stages will not be
required to operate near their noise limits.
Since the reproduce head produces an output voltage
that is related to the rate of change of the flux on the
tape, dI/dt, the output voltage will rise at a rate of
6 dB/octave. A compensating circuit with a falling
6 dB/octave response, known as an integrator, is used in
the playback amplifier to correct for this rise and give a
voltage that is proportional to the value of flux sensed
by the head.
When the effects of playback head resonance
peaking and gap length, spacing, eddy current, and
thickness losses are included, the output from the
low-noise amplifier and integrator would follow the
falling curve in Fig. 28-37 for 15 in/s (38 cm/s) opera-
tion, see Fig. 28-12 for details. This curve must be
reshaped by the combined effect of the record and play-
back equalizers to yield a flat response. The method of
partitioning this correction between the record and play-
back circuits is dictated by the equalization standard
chosen by the operator. Since all users of a given equal-
ization standard will be using the same partitioning, the
recorded tapes will all be interchangeable.

Fig. 28-38 shows a simplified schematic of a typical

operational amplifier type of a playback amplifier

capable of the necessary playback corrections. The

low-frequency-cut circuit is utilized in some NAB and

cassette standards to achieve a decrease in

low-frequency playback noise below 100 Hz at the

expense of low-frequency headroom. A typical design

would include additional ancillary components for

amplifier biasing and stabilization.

With one common exception, the same type of

circuitry is utilized in the sync/overdub mode to condi-

tion and amplify the playback signal from the record

head. The exception is in the form of an added

voltage-boosting transformer that is commonly neces-

sary to get the signal above the noise floor of the

low-noise input section. This problem arises from the

low inductance and few turns of wire that are typically

found in a record head. The record head must pass the

audio plus the high-frequency bias signal; therefore, the

inductance must be kept low enough to avoid self-reso-

nance with the head cables at the bias frequency. When

fewer turns are used to reduce the inductance, the output

voltage goes down proportionately. In essence, these

turns are restored in the transformer by a step-up turns

ratio ranging from 3:1 to as high as 10:1.

Figure 28-36. Tape recorder signal block diagram.

Tape direction

Erase

Bias/erase

source

Bias

Bias

trap

Record

amplifier

Record level

Line in

Record

EQ

Sync switching

Record Play

High-frequency

equalization

Low-frequency

equalization

Playback

preamplifier

Playback

level

Line amplifier

Monitor selector

source tape

Record cal

Use step-up transformer at

Point A for low-impedance record & high-impedance playback heads

Point B for low-impedance record & playback heads

Line meter

Line out