Vintage

Our version of B&O’s inaugural Beogram turntable was fitted with the later synchronous AC motor with fine speed control achieved via a conical pulley (rather than a centrifugal governor)driving the intermediate jockey wheel. This rubber-edged idler, in turn, drives the belt around the deck’s sub-platter. The inevitable mechanical uncertainty between the idler’s edge and position on the conical pulley explains the complex speed variations seen on the W&F spectrum [see Graph 1] where discrete sidebands between ±3-6Hz contribute to a high 0.25% peak-wtd wow. These same peaks are also revealed on the unwtd rumble spectrum [not shown here] occupying 10-25Hz. Incidentally, the platter strobe is fairly accurate, realising a slight positive speed error of +0.16%.

Audible rumble is another story. Fortuitously, as these ~20Hz noise components fall within the DIN-B weighting curve, the through-bearing figure is a fine –69.0dB (20Hz-20kHz re. 1kHz/5cm/sec) while the through-groove figure is a relatively spectacular –73.2dB. Because the platter mat only supports the LP over ~50% of its surface, that latter rumble figure will also depend on the weight and flatness of the vinyl, and the proximity of the deck to sources of vibration. The effective mass of the combined tonearm/cartridge body is arguably a little too high for the 15cu compliance of the SP 7 pick-up (achieving a 6-7Hz fundamental resonance) while the rubber grommets used within the arm’s bearing necessarily give rise to a degree of compliant ‘play’. Otherwise, the alloy tube tonearm has a moderate main bending mode at 110Hz [see CSD waterfall, Graph 2] followed by a complex series of harmonic, twisting and other modes stretching from 180Hz-1kHz. Fortunately, these are all at a relatively low level and, as a rule, swiftly quelled. PM