Modern pickups come in in two main types, moving magnet and moving coil. In theory whether you generate electricity by moving a magnet in a coil or a coil in a magnetic field the results will be exactly the same. Iin practice the two designs produce noticeably different results. The crystal cartridge was very popular in the sixties but is very rare now and is not covered in this article.
Moving coil cartridges are constructed by making a very small coil of extremely fine wire that is attached to the stylus via a cantilever. If the coil is very light it will have a very small mass and will be able to move quickly and have a very good transient response. The required magnetic field is produced by a fixed magnet. The down side of this design is that the electrical output will be very low at around 200µvolts. A typical amplifier will require 10 to 20 mvolts to produce a noise free output. The low output impedance means that a transformer can be used to increase the output to the required level. The advantage of this design is that all moving parts are very light and produce little moving mass at the stylus tip.
Some modern moving coil cartridges are designed to have a higher output, this is achieved by using more turns of thinner wire. This increases the moving mass at the stylus and will reduce the advantage of the original design.
Moving magnet cartridges use a very powerful magnet positioned at the end of the cantilever (where the coil was in the m/c ). This has to be as small as possible to minimize the mass at the stylus. A fixed coil of many turns is positioned close to the magnet and the result is a much higher output than was possible with the m/c design. The down side here is the compromise necessary in magnet weight to achieve a good level of output whilst keeping the moving mass as low as possible.
Many variations in design exist to overcome this problem, it is possible to use a fixed magnet which is positioned to induce a magnetic field in another piece of ferrous material which would have a smaller mass than the magnet. The B&O mcc (moving micro cross) design is a good example.
A very unusual design of a moving iron cartridge was produced by Decca in the early 1960's. This involved attaching the stylus directly to the moving iron. In practice, this meant that the coil had to be very close to the stylus and therefore the disc surface. This had the effect of producing a superb transient response because the cantilever had been eliminated. Unlike all other cartridge designs which used individual left and right channel coils the Decca cartridge was also notable for having separate sum and difference coils, their outputs being mixed in the cartridge to produce accurate stereo. This cartridge was one of the most respected ever made because it produced the most dynamic and lifelike sound available at the time.
The stylus is the most important part of the cartridge, they are made of industrial diamond and are available in various shapes such as conical, elliptical and proprietary eg. fine line.
Typical stylus diamensions are:-
Conical mono 40µ (0.001 in.)
Conical stereo 20µ (0.005 in.)
Eliptical 15 x 7µ (0.0006 x 0.0003 in.) or18 x 5µ to (0.0007 x 0.0002 in.)
78rpm 108µ (0.0027in.); 128µ (.0032in.) typical values
When used to play vinyl LP's, a good stylus will last for about a thousand playing hours, the best life being achieved if tracking distortion is kept low by using a playing weight close to the upper limit. Records should be cleaned before playing.
Early mono micro groove records had a deeper groove and hence the wider stylus. 78rpm records were made with several groove widths so the best compromise would be about 110µ. You should note that playing records with a different shaped stylus to the one normally used will often give improved noise and distortion results. This is because the stylus is using a less worn part of the groove wall.
All the early types were conical which present the same width from any angle. However, as the cutting stylus is chisel shaped (it is wider head on), the lateral cut will be slightly less wide when it cuts a high modulation groove. For this reason, it was thought that an elliptical shape would improve tracking by maintaining better overall contact with the groove. In practice the benefits are very small and offset by increased record wear and a tendency to collect dust from the record surface.
The best shapes are fine line. This means that it is shaped so that it has a longer contact with the groove wall greatly reducing wear and improving trackability.
Dave McGhee
Ganymede Test & Measurement
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web: http://www.wavecor.co.uk
1st October 2001