Everything you need to know about cartridges: Choice of cartridge

CHOICE OF CARTRIDGE

In the introduction of this brochure, we discussed in general the choice of the right cartridge for the right Hi-Fi system. In this section, our aim is to describe the technical requirement and restrictions that are involved in the interplay between the cartridge and the remainder of the Hi-Fi equipment.

Compliance and tonearm mass

Choosing a cartridge would be a simple matter if all cartridges could be used without problems in all tonearms. This is not the case, however, as even the finest and most expensive tonearms are not necessarily suitable for all cartridges. The majority of tonearms are quite robustly designed. A high degree of rigidity is necessary if the tonearm is to be able to stand up to possible careless treatment. This means that a typical tonearm, with headshell, has a mass of between 15-25 grams. To ascertain the total arm mass, the weight of the cartridge should be added to this value.

When looking at the graph (Fig. 13.) showing the relationship between tonearm mass, cartridge compliance and resonant frequency, it will be noticed that the mass of the tonearm narrowly limits the value of compliance that the cartridge may have in order to give optimum reproduction.

Fig. 13. Diagram illustrating the relationship between cartridge compliance, tonearm mass and the resulting resonant frequency. If the compliance is 20 µm/mN, and the mass is 20 gram, the resonant frequency will lie around 8 Hz.

If we take a typical tonearm with a mass of 20 grams and onto this mount a cartridge weighing 5 grams, the effective tonearm mass will be 25 grams. To achieve a resonant frequency of 10 Hz, it will be necessary to choose a cartridge with a compliance of 10, a value that the majority would consider to be very low. The lowest acceptable resonant frequency of 7 Hz makes it possible to use a compliance value of up to 20.

A truly lightweight tonearm with a mass of 10 grams together with the cartridge in the previous example will have an effective mass of 15 grams. Here, a cartridge with a compliance of 15 would be ideal, and a compliance of 35 would also be acceptable. If a cartridge with a compliance as high as 35 is to operate under ideal conditions, the total mass ( as indicated by the graph) including cartridge would be 7 grams. If the cartridge itself weighs 5 grams, then the mass of the tonearm must be no more than 2 grams! By accepting a resonant frequency of 7 Hz, it is possible with this cartridge to operate with a total mass of 15 grams.

The examples above are intended to illustrate that a cartridge with a high compliance (i.e. higher than 35) should only be used with extremely light and delicate tonearms. On the other hand, this type of tonearm is seldom ideal for cartridges with low compliance. Experiments and listening tests have proved that low compliance cartridges in many lightweight tonearms can easily cause resonances in the audible range. A typical example of such a cartridge is the moving coil, which should always be mounted on a relatively sturdy tonearm.

Measuring the tonearm mass

The majority of tonearm manufacturers today appreciate the importance of tonearm mass in record playback, and several firms now give this information in the data sheets for their turntables or separate tonearms. In many cases, however, it will still be necessary to measure the mass of the tonearm before a cartridge with the correct compliance can be chosen. As arm mass is only significant at very low frequencies, it will be sufficient to simply weigh the mass of the arm as it is registered at the stylus tip of the cartridge. This is done by firstly removing the tonearm’s counterweight - a simple operation that can be carried out without the use of tools on practically all turntables. Then a small spring balance or beam scale weight is fixed to the cartridge headshell as close as possible to the stylus tip. The weight can then be read off. The surest way of measuring the effective tonearm mass is to weigh the tonearm with cartridge mounted. Otherwise, it is necessary ti add the cartridge’s weight to the measured weight. (Fig. 14.).

Fig. 14. The mass of the tonearm with cartridge mounted can be measured approximately by weighing the tonearm without counterweight.

Another method of measuring tonearm mass is with the help of a special test record with subsonic signals (for example Ortofon’s test record DC 0001). Together with the dynamic compliance of the cartridge, which is stated by the majority of manufacturers, the effective mass of the entire tonearm system can be calculated either by using the following formula:-

M = 10 to the power of 6: (f² x (2p)² X C

Where M is the mass in grams, f is the resonant frequency in hertz, and C is the compliance in µm/mN or, by reading the table as below.

Note that the tonearm mass calculated comprises the mass of the tonearm itself, the mass of the cartridge, and the mass of the headshell.

Ex.
a) The tonearm resonant frequency is specified as 10 Hz.
b) The dynamic compliance of the cartridge is known to be 15 µm/mN.
c) From the formula or table, the total mass of the tonearm system can be defined as 16.9 grams.
d) The weight of the cartridge is known to be 5 grams.
e) The tonearm mass (including headshell) can, therefore, be specified as 16.9 grams less 5 grams = 11.9 grams.

It is sometimes possible to reduce the effective tonearm mass a few grams by changing to a very light headshell, and this can be recommended when the cartridge used has a high compliance. Low compliance cartridges should be mounted in a more solid headshell, preferably of a cast light-alloy metal, to avoid resonances in the headshell, which will impair sound quality.

A greater reduction in total tonearm mass can be achieved by replacing a conventional cartridge and headshell by an integrated Low Mass cartridge with standard plug. Such a cartridge can weigh as little as 6.5 grams, which is less than the majority of headshells, and many cartridges. (Fig. 15.).

Fig. 15. Integrated Low Mass VMS cartridge for mounting on tonearms with standard socket.

Table for tonearm mass when the resonant frequency and compliance values are known.

Output voltage

A cartridge’s output voltage should be sufficiently high to utilize to the full the output power of the associated amplifier or receiver. Where magnetic cartridges are concerned, there are generally no problems in this respect as the majority give a signal to the order of 1 mV/cm/s. As the average groove modulation on modern records is in the region of 5 - 10 cm/s, a typical cartridge will give a signal of 5 - 10 mV, which is a good match for the input sensitivity of modern amplifiers, which are typically 2 - 3 mV at 1,000 Hz. A rule of thumb for magnetic cartridges is that their output voltage decreases in line with increasing quality. There are some advanced magnetic cartridges available with an output voltage as low as 0.4 mV/cm/s. Here, an input sensitivity of 2 mV in the amplifier is sufficient. However, such cartridge should never be connected to amplifiers with lower sensitivity, and particularly not older models, as the signal-to-noise ratio can be reduced, and it may be impossible to utilize the full rated output of the amplifier.

The majority of moving coil cartridges have an output voltage which is approximately 50 times below the level for magnetic cartridges. As a result, the output voltage from a moving coil cartridge must be amplified correspondingly before it can feed a conventional amplifier. There are a number of transformers and special amplifiers on the market today for this purpose, but as the output voltage from moving coil cartridges can vary widely from manufacturer to manufacturer, it is generally recommended to follow the cartridge manufacturer’s instructions in order to void matching problems.

As a result of their special construction, moving coil cartridges are particularly sensitive to hum fields from power transformers and turntable monitors. Transformers and special amplifiers for these cartridges are also sensitive to hum pick up. The majority of these hum problems can be solved by connecting the cartridge and its matching device as recommended in the user instructions. However, if the turntable used has a poorly shielded power transformer (which is the case with some direct drive turntables) it can be impossible to achieve reproduction form a moving coil cartridge which is completely free from hum.