Trends in Cartridge design
Modern trends in cartridge design
During the past couple of years a quiet revolution has occurred in pickup design. Since the introduction of moving coils they have been extolled as 'the sound' to aspire to. Unfortunately, hi-fi is a field where subjective impressions often take the place of hard data.
What should happen is that a large range of cartridges should be directly compared with a live recording. If such a test were properly conducted and then subjective impressions fed into a computer together with data on the construction techniques, some correlations should be found.
Armed with this information some of the myths surrounding cartridges could be either dispelled or proven. We would then be in a better position to judge what is the 'real' sound.
Nevertheless some very real improvements have been incorporated into recent cartridges. Five years ago a tip mass of 0.3mg was unheard of. Normally a tip mass of 1mg would be encountered, resulting in a resonance in the audio band. Moving coils with their lower moving mass generally have higher frequency tip resonances, often well outside the audio band.
Some of the hard cutting edge apparent from some moving coils can be directly related to a tip resonance at the edge of the audio band. Such cartridges show a gentle upward trend in their response curves above 10kHz.
Another interesting development concerns stylus shape. The basic stylus shape is conical and when a record is cut the cutter has a wedge shape. If the stylus also were to be wedge shaped, theoretically the playback should be perfect, but a stylus of this shape would bump along the bottom of the groove producing horrendous noise, not least because large amounts of dust and garbage accumulate at the bottom of the groove.
The conical stylus is seldom seen these days because it is incapable of tracking hf modulations. The standard elliptical type is much more successful from this point of view and has become standard on all mid priced cartridges.
New profiles have also been tried, with various degrees of success, which are designed to increase the contact area with the groove, thus ensuring less record and stylus wear, and also tracking ability. Another far more contentious area of performance is concerned with the cantilever; even if the styli follow the groove modulation perfectly they still have to be accurately coupled to the generator. Any mechanical structure is prone to resonate and mechanical energy can be lost in the form of heat etc. Cantilevers of pure Boron or, in the case of the Shure V15 V, a Beryllium tube, have been employed. Which of these operates best is a matter of conjecture, or rather, personal choice.
At the output end of the cartridge things have also been improved. For years cartridges of the moving magnet variety have had a high inductance; this poses problems at hf.
Another interesting fact is that the resistive part of the cartridge impedance limits the ultimate S/N ratio of disc reproduction. All resistors generate noise, the amount being proportional to both the resistance and absolute temperature. With a cartridge impedance of 1k, a normal value for many MM designs, the ultimate S/N ratio is limited to about -65db even assuming noiseless amplifiers! The new wave of MM designs feature a very much lower impedance, stretching the ultimate S/N ratio towards -70db. As a personal comment, I would like to see balanced line techniques used with cartridges to eliminate hum pickup from connecting wires; at any rate I have recently had the opportunity to hear some of these cartridges, and although I intend to examine them at some length at a later date a few preliminary observations would be in order.
First the moving magnet design. Grado's 'Gemini Gold' cartridge which retails for around £89. The sound balance of this model is superficially similar to the Ortofon VMS20E11. However, the sound is much more detailed with an exceptionally fine top; the tip resonance is an extraordinarily high 65kHz and the stylus profile is a 'true ellipsoid' to quote their own words.
It is a pity that more technical information is not available, for instance, a frequency response curve. Although no problems were encountered with the cartridge mounted in my aged 'Acos Lustre' arm, the same could not be said when mounted in the Rega. A slight but disturbing edginess occurred on high level passages, due to mistracking.
The second moving magnet in this 'reviewette' is the flagship model from Shure, the V15 V. This cartridge is unique in many ways, not least because of its amazing tracking ability. Like its predecessor the V15 IV, a carbon fibre brush stroke stabiliser is an integral part of the design.
Subjectively, the unit tracks everything you can throw at it at 1gm tracking force. It has an incredibly neutral and detailed midrange, with a slight recessed top and rather bland bass. For £160 one would expect a very good cartridge, and overall this is.
Lastly, a cartridge with a totally different modus operandi, the MC82 from Coral, distributed by Videotone. This is a moving coil unit fitted with a Van de Hul stylus. The stylus is 'grain orientated' for minimum wear. Indeed this is the only cartridge I have ever used which works perfectly even with a ball of fluff on the stylus! Subjectively I found this cartridge the best I have heard so far. The frequency response is flat within 1db between 20Hz-20kHz and the sound is totally neutral throughout.
The most striking aspect is the transient response; instruments start and stop very quickly. Unlike the MC81, its predecessor, this cartridge continues to track the most tortuous passages without mistracking. An enthusiastic welcome for this device then; the cost is about £100, to which must be added a step up device.
As a final word on the subject, perhaps I should mention the AT1000 from Audio Technica. With its specially matching toro-dial step up transformer this will cost you £1,400; I'm still trying to get a review sample!