Playback head

The playback head , also known as the audio head (HK), is a magnetic head and a component of a tape recorder . It serves as an electromagnetic transducer for scanning the signal stored in the magnetic layer of a tape .

The signals stored as magnetic flux in the magnetic layer are converted into electrical signals by the playback head. The signal voltage emitted by the playback head is in the millivolt range and is therefore so small that it has to be electronically amplified before it can, for. B. can be fed to a loudspeaker and emitted as an acoustic signal.

The playback head is designed as a ring system and consists of a ferromagnetic core with a working gap ('head gap'), filled with non-ferromagnetic material, and a coil with a high number of turns.

The core consists of high-quality, soft magnetic metal (mumetal, ferrite) with special requirements in terms of high initial permeability , low coercive field strength and high specific resistance . To reduce eddy current losses , the magnetic core is laminated or sintered ferrite is used. The magnetic resistance should be small, which amounts to a large iron cross section and a small iron path length.

The head gap is chosen to be so narrow that the highest possible cut-off frequency of the tape recorder is achieved, but the head is still adequately flooded by the magnetized layer of the tape.

The function of the playback head is based on the law of induction . The recorded magnetic tape is moved past the working gap and induces a voltage in the head coil according to the relationship:

{\ displaystyle U_ {i} = K \ cdot {\ frac {d \ Phi} {dt}} \,}

.

A recording leaves larger areas of identically ordered dipoles in the magnetic layer. When playing back a sinusoidal recording, assuming that the gap width is small compared to the smallest wavelength and the pole length large compared to the largest recorded wavelength , a flux amplitude Φ ₀ proportional voltage amplitude results as follows: ${\ displaystyle \ lambda = {\ frac {v} {f}} \,}$

{\ displaystyle U_ {0} = w \ cdot \ omega \ cdot \ Phi _ {0} \,}

The following mean:
U ₀ = voltage
amplitude w = number of coil turns
Φ₀ = magnetic flux (magnetic flux, flux amplitude)
ω = angular frequency = 2 ${\ displaystyle \ pi \ cdot f \,}$

The induced voltage therefore increases proportionally with the frequency at 6 dB / octave (omega gear). In practice, however, wavelength-dependent losses increasingly occur at higher frequencies, of which the gap width attenuation and the distance attenuation are the two most important.

Playback head

See also