Everything you need to know about cartridges: Key specifications for Hi-Fi cartridges

KEY SPECIFICATIONS FOR HI-FI CARTRIDGES

It would be wrong, or at least risky, to choose a cartridge on the strength of its technical specifications alone. Good technical data are, of course, necessary for good sound, but even the most detailed data sheet gives only a rough indication of the sound of the cartridge, and says very little about hie a particular cartridge will suit a person’s taste. The choice of cartridge is further complicated by the fact that it is not sufficient merely to aim at obtaining the best possible technical data. As it is necessary to consider a cartridge in relation to the tonearm on which it is mounted, thought must also be given to the tonearm’s characteristics so that problems of resonance or mistracking in playback are avoided. Therefore, it may be necessary to compromise certain of the cartridges specifications in order to gain other advantages.

In the following paragraphs, we will discuss the specifications that have direct influence on the cartridge’s practical performance in a Hi-Fi system. Many more specifications may be included in the data columns of cartridge brochures and the most general of these will be discussed briefly after the key specifications: frequency response, channel separation, compliance, tracking ability, and output voltage.

Frequency response

As with the other links in the Hi-Fi chain, the cartridge must be capable of transmitting the entire frequency range. This means that reproduction of the frequency range from 20 Hz to 20,000 Hz must be as linear as possible. It is important here to note the difference between frequency range and frequency response. (Fig. 10)

Fig. 10. Frequency response diagram for a good (A) and an inferior (B) cartridge. Both cartridges cover the same frequency range, but (A) has a more linear frequency response.

A frequency range of 20 Hz to 20,000 Hz merely tells that the cartridge is able to give a signal at and between these frequencies; whereas a frequency response of 20 Hz to 20,000 Hz +/- 1dB indicates that the cartridge’s output signal will vary a maximum 2 dB across the entire frequency range. A 1 dB variation in sound level is marginally discernable.

The manufacture of a cartridge with a completely flat frequency response is extremely demanding on construction, production and quality control. Such a cartridge is, of necessity, relatively expensive. Where magnetic cartridges are concerned, the frequency response figure stated can only be achieved when the cartridge operated with the recommended load impedance. This load always consists of two elements: a resistance and a capacitance. For stereo cartridges, the load resistance is nearly always 47 kohm, with 100 kohm for CD-4 cartridges. On the other hand, the recommended load capacitance has not been standardised, and can vary between 100 picofarads (pF) for CD-4 cartridges and 500 picofarads for some stereo cartridges. If too high a load capacitance is used, the treble will be subdued. If the capacitance is too low, the treble will be too sharp. Some amplifiers are fitted with switches for both load resistance and load capacitance, and the correct combination can be found with the help of listening tests. In other cases, however, it is wisest to receive advice from a Hi-Fi specialist shop so that optimal matching between cartridge and amplifier or receiver can be achieved.

Channel separation

A good channel separation, i.e. the cartridges ability to keep the two stereo channels separated throughout the entire frequency range, is essential for good and stable reproduction of stereo records. Channel separation is normally only stated at 1000 Hz, where values of 25 dB or more are not unusual.

When assessing the quality of a cartridge, it is useful to know its channel separation throughout the entire frequency range, for example by consulting a graph. (Fig. 11.).

Fig. 11. Frequency response and channel separation for a good (A) and an inferior (B) cartridge. Not only has (A) a more linear frequency response, it also has a wider channel separation over a wider frequency range.

A high channel separation figure in the treble range is particularly advantageous, as the stereo image in recordings will be reproduced with greater clarity.

Compliance

Cartridge compliance is an expression for the elasticity of the cantilever suspension. Compliance is measured in cm/dyn or µm/mN. In practise, compliance values for Hi-Fi cartridges lie between 10 µm/mN and 40 µm/mN, and a high degree of stylus elasticity is represented by a high compliance value. There is a close correlation between a cartridge’s compliance and its tracking force and tracking ability. The greater the elasticity of the stylus suspension, the lower the tracking force that is required for the stylus to playback powerful bass passages. Most record collectors prefer to play their records with a low tracking force, as this gives less record wear. The majority of cartridge manufacturers have, therefore, made great efforts to obtain the highest possible compliance values for their cartridges, with the result that some of the better known cartridges are bale to operate with a tracking force of less than 1 gram. Recent cartridge research has shown, however, that it is wrong to choose a cartridge merely on the grounds of a high compliance value.

When a cartridge is mounted in a tonearm it can be compared to an oscillating system consisting of a spring and a weight, where the compliance is the spring, and the tonearms mass is the weight. (Fig. 12)

Fig. 12. Oscillating system consisting a spring (c) and a weight (m). The spring can be compared with the compliance of the cartridge’s cantilever, while the weight represents the total mass of the tonearm and cartridge.

When such a system is set into motion, it will oscillate at a certain frequency - the resonant frequency - which is determined by the elasticity of the spring and the mass of the weight. Both high elasticity and high weight will give a low resonant frequency for the oscillating system.

When the cartridge plays back a record it will, of course, be influenced by the music that has been recorded whose frequency range extends from 20 Hz and over. As records are never completely flat, warps and other surface irregularities will also influence the cartridge in the frequency range around 5 Hz and below. It is extremely important that the resonant frequency for the cartridge and tonearm lies in the range between 8 Hz and 15 Hz. If the resonant frequency is below 5 Hz - 6 Hz, it will easily be excited by warps and other irregularities in the record, and in extreme cases the complete tonearm will vibrate visibly. Tracking ability will be impaired, reproduction coloured by the vibrations, and there is a risk of acoustic feedback. These problems plague many Hi-Fi enthusiasts who, with the very best of intentions, have chosen a cartridge whose compliance is too high in relation to the mass of the tonearm. On the other hand, modern cartridge/arm combinations with a resonant frequency approaching the audible range are scarce.

In the majority of cases, a new cartridge has to match a specific tonearm with a mass that cannot be altered by the consumer. Thus, it will be necessary to choose a cartridge whose compliance, together with the mass of the tonearm, gives an acceptable resonant frequency. If it is an absolute must that the cartridge have the highest possible compliance, it will often be necessary to change to a special tonearm in order to avoid problems in reproduction. In many cases however, by using one of the new Low Mass cartridges, the total tonearm mass can be reduced so much that it will be possible to operate with a compliance that is higher than would be possible with a conventional cartridge. In the section “Choice of cartridge” we will illustrate the correlation between compliance and arm mass with some practical examples.

Tracking ability

Tracking ability is an expression for the ability of the cartridge to track the high groove amplitudes at a given tracking force without losing contact with the record groove. Tracking ability can be expressed in two ways. In µm, by the oscillating ability of the stylus, or in mm/sec as measured by the speed of the stylus in the record groove. The former is the most general means of measuring and here, tracking ability is measured by playing a 300 Hz sinus tone that is adjusted at levels from 20 µm to 100 µm. As mentioned in the previous section, tracking ability at 300 Hz is decided first and foremost by the compliance of the cartridge, and should be sufficiently high to handle the full range of recordings. A value of 50 - 60 µm will be sufficient to obtain playback of all conventionally recorded records without distortion. Only special test and demonstration records are recorded with a greater groove modulation in the low frequency range.

In the treble range it is the effective stylus tip mass - and not compliance - that determines tracking ability. A 10,000 Hz tone can be recorded so powerfully on a record that the cartridge has to make accelerations of more than 2,000 G in order to reproduce it correctly. (Astronauts experience an acceleration of 7G when their rockets take off from the earth’s surface). To be able to perform such rapid movements, the stylus must have a minimum of mass. If it is to reproduce the 10,000 Hz tone mentioned above at a tracking force of 1.5 gram, the effective stylus tip mass must not exceed 1 milligram. Better quality cartridges use a microscopic, nude diamond stylus, which can bring the effective tip mass down to 0.5 milligram or less. Cheaper, more robust cartridges generally use a bonded diamond that comprises a diamond tip held by a metal shank, which does increase the stylus tip mass somewhat.