Tonearm
The tonearm of a record player is used to guide the pickup needle mounted on it in the groove of a record, which runs as a spiral with an end circle and thus the connection of the mechanical pickup deflections for electronic conversion into sound waves. After placing the record on the - usually - horizontal platter, the arm is lifted off a support and the needle is placed on the record from above . This can be done purely manually with the help of a small part protruding laterally from the tonearm head, which is lifted from below with a finger and placed at the start of the groove, often with the help of a damped automatic lowering system, by using a radius-shaped sheet metal strip with conical countersunk holes on the underside for the Up to 3 plate formats (single to LP), which can then snap into the lowering pin that clicks into place from below and then lower them in a damped manner after actuating a lever or fully automatically, which can also include the automatic detection of the presence of a plate and its diameter.
The conventional and usual design is called the radial tonearm ( commonly just called the tonearm ), as the typically 15-30 cm long arm swivels around its vertical bearing axis at the right rear end of the chassis while the spiral groove is played and, like the circle radius, swivels up to 40 ° Angle sweeps over a large sector of a circle.
The tangential tonearm , which is only a few centimeters short and is only a few centimeters short , is only found from around 1970 and is shifted in parallel by a linear gear, thereby guiding the needle during playback on a radial path in the direction of the turntable in the direction of the turntable rotation axis, while the arm itself continuously moves the perpendicular to it Draws the tangent at the point of contact of the needle. The tracking error (angular deviation) of the sound head in relation to the groove (its local tangent) is avoided in principle, but the construction is significantly more complex.
Both arm types have a horizontal tilting axis at the end that is supported, which is used both for placing and lifting the needle at the beginning and end of the game and for raising and lowering the needle on wavy panels.
The main components of the tonearm are the arm itself, either as a straight arm or, in the case of radial tonearms, also as an S-shaped arm, the fastening mechanism for the cartridge, the counterweight with which the tonearm can be adjusted to different cartridge weights and tracking forces, and for radial tonearms an anti- Skating device. With radial tone arms from the professional sector, such as the electrical measurement technology Wilhelm Franz EMT 929/997, the tracking force was generated by means of spring force. The company Dual also adjusted the tracking force of its tonearms, especially in the 1960s and 1970s, by means of a torsion spring in the horizontal tonearm bearing. Such constructions can be recognized by the fact that the counterweight is mounted so that it can "swing" in a rubber bushing. The record manufacturer Carl Lindström is considered to be the inventor of a forerunner form of the tonearm in gramophones .
In the rare case that a record player can play one or (automatically) several record (s) stacked on top of each other on 3 retaining lugs of a mandrel that protrudes approximately 8 cm from the plate, a second needle pointing upwards is inserted from below onto the underside of the lowest of the stacked plates.
As a shoulder bag portable, hängbare on the wall turntables and some in jukeboxes integrated rotate the record about a horizontal axis, and drop the needle with a tone arm from the side of.
Radial tonearm
The radial tonearm is mounted to rotate around a vertical axis. This changes the toe angle from the outer groove to the inner one. The positioning of a radial tonearm - only finitely long - means that the pickup does not take a constant track angle to the groove of the record as it moves over the record. This results in different degrees of distortion during playback depending on the position of the pickup on the record.
Setting and measuring points
In practice, the tonearm and the pickup are set so that the pickup is exactly tangential to the groove at two positions on the plate (at radius 66.0 and 120.9 mm) to avoid the distortion caused by the tracking error to minimize the entire plate side.
Later findings according to Walter E. Schön have led to the need to redefine the measuring points for the track error angle on the pickup system. His calculations are based on other geometric conditions, such as the average playing time of one side of the record and DIN IEC98. The dimensions are named here as follows: 62.5 and 117.2 mm (distance from the center of the plate axle). If the perpendicularity of the tangent is set exactly at these two measuring points, the dimension for the overhang (the distance from the center of the plate axis to the needle) should also fit exactly. With this version of the calibration, two parameters are precisely defined. ((Source: Operating instructions and explanation of the Schön template type 2 at www.dos-hifi.de))
Compensation tone arm
A variant of the radial tonearms are compensation arms in which the tonearm head and thus the pickup are rotatably mounted about a vertical axis. The head and pickup are turned by a mechanism as they move from the outside to the inside across the plate opposite the arm, thus compensating for the tracking error to a certain extent. The design effort for a backlash and friction-free mounting of the head is enormous.
The best-known example was the Garrard Zero (Great Britain) in the 1970s.
Tangential tonearm
The tangential tonearm is moved on a slide tangential to the turntable axis in a straight (linear) direction. The main advantage of a tangential tone arm is that, ideally, the needle is always exactly tangential to the groove. This means that there are fewer distortions because the needle scans at the same angle as the cutting stylus that forms the groove when the plate is manufactured.
With servo motor
This was achieved in the early days by a servo motor that tracked the tone arm of the record groove. The arm was pivoted on the sledge. An electronic control reacted to the slightest deflection of the arm from the ideal linear alignment by tracking the slide. The disadvantage of this procedure was that a deviation - albeit a very small one - had to occur first, which could then be corrected.
Servo-controlled tangential tonearms allowed technical gimmicks such as title programming, similar to CD players . In the 1980s in particular, some device manufacturers brought out turntables with these functions (e.g. the Technics SL-J3 with features that are otherwise common on a CD, such as programming up to eight tracks, automatic track jump and repeat function, or the HMK PA 1205 from RFT with similar functions ).
With minimized friction
Modern systems are mechanically perfected more and more (high-precision mechanical or air cushion bearings for the smallest friction ), so that the tracking force of the record groove is sufficient so that the tonearm can follow the record groove by itself. The actual tonearm can no longer be turned sideways, but is fixedly mounted on the slide in a tangential orientation. Servo motors and control electronics are completely eliminated here. Even in their simplest form, these tonearms are very complex and can therefore only be found in the high-end segment.
Tangential tonearms have no skating principle and therefore do not require any anti-skating measures.
Skating and anti-skating facility
The anti-skating mechanism of a tonearm compensates for a skating force directed towards the center of the turntable on radial tonearms , which results from the fact that the groove tangent and the imaginary line between the needle support point and the tonearm pivot point are usually not aligned in the same direction . Although the track error angle (angle zw. Pickup and grooves tangent), by a bend ( offset can be reduced (about ± 2 °)) of the tone arm to a relatively small interval, the skating force is, however, not affected.
The real tonearm including the offset can be reduced to an imaginary tonearm (green line) to consider the forces. As a result of the friction between the record groove and the stylus, a force F t tangential to the record groove first acts . The imaginary tonearm can absorb this through two forces that are perpendicular to each other, namely the radial force F r in the direction of the imaginary tonearm and the skating force F s . From the two different positions of the tonearm it can be seen that the skating force would always drive the tonearm towards the center of the record under these conditions, but the force is smaller when running on smaller groove radii. For this reason, anti-skating devices must build up a counterforce that is dependent on the tone arm position. The offset of the tonearm is intentionally exaggerated to show that the resulting skating force is not related to the angles of the real tonearm to the record groove.
The above-mentioned frictional force F t depends, among other things, on the needle shape and the contact force, which is why the anti-skating force is usually set according to a scale of the contact force.
A "static anti-skating mechanism" with a counterweight and a three-stage adjustment option is used in the detailed illustration of the tonearm support. This is achieved dynamically with the help of a spring force arrangement, which is located in the lower part (not visible) of the tonearm. A disadvantage of the dynamic construction is the very high expenditure with possibly more sound-influencing components. The anti-skating force can also be generated and regulated electromagnetically, as was the case in the 1970s, for example. B. von Braun with the then new type of tonearm moved by a moving coil drive.
Mounting standards for pickups
Tonearms can be equipped with different mounting options for the cartridge. Some tonearms are attached directly to a holding device integrated in the tonearm. As a rule, however, the pickup is not mounted directly on the tonearm but on a system carrier ( headshell ).
The system carrier can be attached to the tonearm in different ways. Simple, mostly older devices without high demands on the sound quality often only offer a manufacturer-specific or device-specific attachment of the pickup to the tonearm or head carrier. An exchange of the serial pickup (usually a crystal pickup) for a higher quality one is not intended. In addition, clamping or adhesive connections are also used, while axial screw or bayonet fastenings are common for higher-quality constructions.
There are also other manufacturer-specific quick-change systems for pickups and system carriers, such as B. the exchangeable TK head carrier for half-inch cartridges in older dual turntables, the Japanese T4P standard or the SME bayonet mount.
The electrical connections between the actual pickup and the head support or the inner tonearm wiring are made using color-coded cables. A common color code is: red for "+" and green for "-" of the right audio channel, white for "+" and blue for "-" of the left audio channel.
The half inch standard
In the hi-fi sector, the half-inch screw fastening standardized in DIN IEC 98 is common (see picture). This designation refers to the horizontal distance between the two screws with which the actual pickup is attached to the system carrier. The associated openings in the head carrier are designed as longitudinal slots that allow adjustment of the tracking error angle (minimal deviation from the exact tangent) and precise adjustment of the needle overhang (dimension from the needle to the center of the turntable). The needle overhang is also required for exact scanning, as this in turn depends on the tracking error angle.
The SME attachment
A very common system, which also enables the easy and quick change of the cartridge, is the SME bayonet mounting, which is used on the tonearms of the manufacturer of the same name or on the tonearm of the Technics SL-1200 /1210 turntable.
In the DJ range pickup with integrated are SME -Bajonettanschluss widespread. For example, the tonearm geometry of the Technics SL-1200/1210 and the Ortofon Concorde pickups are structurally matched to one another: Without further adjustment, the needle position is correct for a small tracking error. The dynamically effective mass of the pickup, which is lower than the conventional design, also enables a more flexible mounting of the stylus ( compliance ) and thus a lower tracking force - while maintaining the optimal mechanical resonance frequency of the spring mass consisting of tonearm and pickup -Schwingers .
SME attachment: Technics SL-1200 with an Ortofon Concorde pickup
TK attachment to the tonearm of a dual turntable
Electrical contacts of a TK head carrier with dual M20E pickups
Web links
- Classic tonearms, various manufacturers - technical data ( Memento from February 26, 2018 in the Internet Archive )
- Explanation of the tangential tracking error angle when scanning records (including graphics)
- Relationship between the effective tonearm length, overhang and installation distance (including graphics)
- Brief description of the HMK-PA 1205
literature
- Michael Dickreiter, Volker Dittel, Wolfgang Hoeg, Martin Wöhr: Manual of the recording studio technology. 7th completely revised and expanded edition, published by the ARD.ZDF medienakademie, Nuremberg, 2 volumes, publisher: KG Saur, Munich, 2008 ISBN 978-3-598-11765-7 .