THE function of the cutter is to transform the electrical (audio) energy furnished by the recording amplifier into mechanical movement of the stylus. There are 2 common types : the magnetic and the crystal.
DAMPERS POLE PIECES PIVOT STYLUS PERMANENT MAGNET AF. FROM AMPLIFIER MOVEMENT
Fig. 501--Simplified sectional view of magnetic cutting head.
Magnetic Cutters
The magnetic cutter is similar in design to the moving-coil pickup.
But the pickup is used to convert mechanical movement to electrical energy, while the cutter does the opposite. Fig. 501 is a simplified drawing of the working parts of a cutter. A permanent magnet, shaped like an inverted U, supplies a magnetic field which is concentrated by the split pole pieces into a small area. The armature, mounted between sets of dampers of rubber or similar material, pivots between the lower portions of the pole pieces. The upper end of the armature is free to move laterally, and the cutting stylus is fastened to the lower part.
The audio output of the amplifier is fed to the coil, producing a polarity at the ends of the armature. This polarity, of course, changes with each alternation. When the top of the armature has the same polarity as the left pole piece, it will be repelled and will travel to the right. During the next alternation, the polarity will be reversed, and the armature will be attracted to the left pole piece.
Since the armature pivots near its lower end, the stylus which is attached to the lower end will always move in a lateral direction opposite to that taken by the armature's top. If the stylus is in contact with a revolving disc of suitable material, the path of the groove it engraves will vary laterally in accordance with the motion of the stylus.
In playback, the needle of the reproducer, of which Fig. 501 is also a drawing, is moved laterally at an audio rate by the groove undulations. Movement of the needle causes the upper part of the armature to move ; and the moving coil, cutting lines of force from the permanent magnet, has induced in it an audio voltage which can be amplified.
Fig. 502--A magnetic cutter.
Crystal Cutters
The governing principle of the crystal cutter is the piezoelectric effect. A block of crystalline substances-notably quartz and Rochelle salts-will exhibit an electrical potential between its faces when the block is subjected to mechanical pressure or torsion. Conversely, when a voltage is applied to the faces, the crystal will twist or distort. This effect is used in crystal microphones, pickups, and frequency-control instruments as well as in cutters.
The usual crystal cutter is formed of 3 to 5 slabs or plys of Rochelle salt crystals. The stylus is mechanically coupled to the assembly, and an audio voltage is applied to the crystal faces. The mechanical distortion of the crystals produced by the voltage results in movement of the stylus. The direction and amplitude of the movement depend on the polarity and magnitude of the voltage.
The magnetic cutter was developed first, along with the moving-coil pickup, and both units are still used in most of the high-quality re cording and reproducing systems, especially in broadcasting.
To give the best results, the magnetic cutter must be very care fully made. Most of the parts in the assembly are very small, allowing tolerances only in thousandths of an inch. This calls for skilled and expensive workmanship. The cutter may have one or more resonant points caused by dimensions of certain moving parts. In good magnetics, these are almost nonexistent. The rubber used for dampers dries out and must be periodically replaced. Distortion may occur on loud passages because of excessive compression of the dampers at the points of maximum armature travel. Magnetic cutters are available with frequency ranges as high as 10,000 cycles. These are very costly. One is shown in Fig. 502.
Fig. 503--A crystal cutter. Brush Model RC-20 cutter shown.
The ruggedness of the magnetic cutter makes it quite suitable for use by amateurs. It can take considerable overloads and can with stand a fair amount of knocking around without being damaged. It is practically unaffected by temperature.
The crystal, on the other hand, is sensitive to temperature and can easily be cracked by an overload or by being dropped. For low cost high-fidelity results, however, it is often preferred by semiprofessional users. The unit shown in Fig. 503 is flat from 50 to 9,000 cycles and costs about $15. Compare this with the magnetic in Fig. 502, which, with about the same frequency range, costs about $150.
Special circuits are required for connecting a crystal cutter to an amplifier if the usual phonograph record frequency response is desired.
Magnetics are sold with popular impedances--4-8, 15, and 500 ohms--and need only be connected to the amplifier's output transformer. How ever, more flexibility in selection of response curves is possible with the crystal.