lauantai 16. helmikuuta 2013

Beogram 3000 and Beogram 4000 record turntable

Beogram 4000
 Michael Kenward describes the Danish Bang & Olufsen Beogram 4000 record turntable which is likely to lead to a new generation of audio equipment

Manufacturers of high-fidelity audio equipment are constantly seeking technical innovations to meet the enthusiast’s quest for perfection. More often than not “improved” designs are little more than tarted-up versions of what went before a slight refinement here. a stylistic alteration there. Bang & Olufsen, however, has introduced a truly revolutionary record turntable in the Beogram 4000 designed by Jakob Jensen and the engineering team of Egon Bjerre, Anders Adelstrop and Eigil Thomson. B&O has done away with the pivoted pick-up arm that describes an arc as the record playing stylus progresses across the record. Instead, B&O has designed a servo-controlled pick-up arm which tracks the record tangentially. As the record is played the whole arm moves across the record, a servo-motor drives what is the pivot of a conventional arm along a rail.

Beogram 3000  and  Beogram 4000  (1972)
 During record manufacture a tangential tracking system is used to transfer the recorded sound from tape to disc. To adapt the tangential arm for the hi-fi market proved to be too expensive until now. Various arrangements of weights, pulleys and levers have been attached to conventional pick-up arms to reduce the sideways forces present when a long arm describes an arc during the progression of the stylus from the edge of a record inward. One sideways force is that needed to ‘drag’ the pick-up stylus inwards. Another problem with elliptical styli - favoured for their superior ability to reproduce the signal impressed in a record groove - is that the angle of the diamond stylus in the groove changes with the angle of the arm.

The Beogram 4000 actually has two arms. They are side by side, about a centimetre apart. The inner arm is a control device. It carries a photocell which recognises the presence of a record on the turntable. The photocell senses light reflected from the surface of the turntable. Radial ribs on the turntable mean that the reflected light is pulsed at a rate depending upon the speed of the turntable. The Beogram’s electronic circuitry recognises these pulses. This arrangement can locate the edge of a record and tell the pick-up arm to lower itself at the right place and start playing the record.

Beogram 4000
 The turntable’s logic circuits also tell the turntable what speed the record should be played at, depending upon its size (any odd record that does not fit into the conventional format, such as a 7-inch record that should be played at 33 rev/ min has to be manually cued). The automatic playing system, which sets the turntable speed and lowers the playing arm in the right place, does not operate if there is no record on the turntable. And the 11 integrated circuits in the unit’s “mini-computer” will not allow the turntable to obey nonsensical manual orders.

The tone arm is driven by a servomotor. A light beam and photocell arrangement controls the movement of the pick-up arm, which is kept at a tangent to the record’s groove. The arm can deviate from its tangential line by a few degrees. At the arm s base, where it swivels, two photocells look through a pair of slots. When the slots are aligned an electronic bridge circuit is balanced. Misaligned slots unbalance the bridge, which is a sign that the pick-up arm is not at a tangent to the record groove. When this happens the turntable’s servomotor comes into action, moving the arm’s pivot so as to realign the two slots. The positioning mechanism is accurate to within one groove (0 2 mm), and with a slightly eccentric record the servomotor can be seen making a correction to the arm position every rotation.

To this revolutionary departure in turntable design B&O has added other features. The turntable motor, for example, is the usual dynamically balanced synchronous motor, but the power comes from a stabilised oscillator. In this way fluctuations in mains frequency and voltage do not alter the turntable speed. This is the first time that Bang & Olutsen has used a stabilised oscillator power supply, but similar technology has been used in other turntables.

At £160 - which, naturally, does not include tuner, amplifier or speakers - the Beogram 4000 is expensive. But B&O feels that the price is not ridiculously high in comparison with other advanced hi-fi equipment. The real test of a piece of sound reproducing equipment is, of course, what the ears hear. Although many people will be deaf to the improvements given by the Beogram 4000’s tangential arm. B&O says that some people claim to be able to hear the difference.

Perhaps more significant could be the tangential arm’s impact on quadraphonic, four-channel sound. Some of the systems for combining four channels of sound into a single record groove employ a high-frequency carrier wave the CD4 system. for example, has a 30 kHz carrier), and any distortion will, therefore, be more apparent. The tangential arm can reduce this distortion.

Bang & Olutsen is convinced that its engineers have established a pioneering trend. The company feels that, even now, engineers are preparing to gut Beogram 4000s so that they can be copied and tangential arms put into a new generation of record turntables.

(Source: 1973 edition of ‘Design’ magazine)

Beogram 3000

 Beogram 3000 was manufacturered to be used with Beomaster 3000. Similar to the rest of the Beogram 1200 range, Beogram 3000 was fitted with the SP10A pickup and could be used with a separate pre-amplifier. It was particularly well screened against external vibrations and shocks which meant that there was a high level of protection for your records and for your pickup stylus.

The transparent lid on this model was particularly well-hinged in that the hinge was able to hole the lid in any variety of angles. The record deck was provided with the SP10A pickup, the last bang & Olufsen record deck to be sold with an SP-type cartridge.

Fully automatic, this record deck was highly desirable in its day.

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