By Julian D. Hirsch
Phono-cartridge Loading
A READER writes to ask about the significance of capacitance in cartridge-loading specifications. He wonders just how important it is and what can be done to modify his turntable or receiver to take it into ac count for the best results with any particular cartridge.
It is not practical, in this limited space, to go back to the fundamentals of electrical theory to define capacitance and show in detail how it affects the compatibility of phono cartridge and preamplifier. I will therefore assume that the reader has at least a rough understanding of the term "capacitance" (or is willing to look it up) and limit myself to discussing its practical implications for record playing.
The frequency response of a magnetic phono cartridge is largely determined by two factors: its physical response to the record-groove modulation and its electrical frequency response. Both of these involve the phenomenon of resonance. This resonance can be placed (by the cartridge designer) within a wide range of frequencies, depending on the effective values of the cartridge's electrical characteristics of inductance and capacitance and its physical characteristics of mass and compliance. Taking the sources of the mechanical resonance first, if a mass is suspended on a spring (compliance) and excited by an external vibration at various frequencies, the physical movement of the system will be maximum at the resonance frequency of the combination. The moving mass of a phono cartridge consists of the stylus-tip jewel, the cantilever that supports it, and any generating elements attached to or contacting the cantilever (such as coils, magnets, or iron armatures). Since the mass is distributed over the length of the assembly and each part has an effect proportional to its distance from the stylus tip, it is customary to refer to an equivalent or effective tip mass; this is the single value of mass that, if located at the stylus-tip position, Would produce the same resonance frequency as occurs in the actual cartridge-moving system. In most modern cartridges, the effective mass is less than 1 milligram.
The compliance or "springiness" of the cartridge's moving system is usually sup plied by the elastomeric cantilever pivot. There is also another compliance involved, one that is not within the direct control of the cartridge designer: the compliance of the disc material itself. This can differ be tween makes of records according to the composition of their vinyl compounds. The combined effect of these elements on the mechanical resonance of the cartridge can be quite complex, but for most good cartridges the high-frequency stylus resonance is above the main part of the audible-frequency range. Typical frequencies are 15,000 to 25,000 Hz, and in some cartridges with very light moving systems the resonance can be as high as 35,000 Hz or above.
The result of this resonance is an increase in cartridge output at that frequency, and to some degree over a range of frequencies surrounding it. There is not likely to be much recorded program content near the resonance frequency, but random noise is always present, and, if it is unduly accentuated by an increase in resonance, it is possible for the cartridge output to overload the input stages of some preamplifiers (this is particularly true of some moving-coil de signs which may have large undamped resonances at very high frequencies). Therefore, it is common for the cartridge designer to use mechanical damping (analogous to resistance in an electrical circuit), generally in the pivot material, to reduce the amplitude of the mechanical resonance.
Excessive mechanical damping can impair the tracking abilities of a cartridge as well as degrade its transient response, so with most non-moving-coil cartridges the remaining resonance peak is compensated for (equalized) by the electrical resonance between their coil inductances and the external load capacitance. The midrange response of a cartridge can be determined by a proper choice of internal mechanical and electrical characteristics. Often there re mains a fairly well-damped mechanical resonance in the vicinity of 15,000 Hz which could raise the cartridge output by several decibels, with the output dropping off rap idly above that frequency. Tuning the electrical resonance to a somewhat higher frequency can compensate for that drop, resulting in a virtually flat response.
As with the mechanical resonance, the amplitude (and bandwidth) of the electrical resonance can be controlled by the proper amount of resistive damping (loading) in the external circuit. Since most cartridges are designed to be terminated in the standard 47,000-ohm preamplifier input resistance, only the capacitance can be varied by the user to control the cartridge response. Usually, a slightly high capacitance value will increase the cartridge's output in the frequency range up to its mechanical resonance but will cause it to fall off more rapidly at higher frequencies.
Each cartridge manufacturer specifies (or should specify) the value-or range of values-of load capacitance with which his product will deliver its rated frequency response. This is usually in the range of 100 to 300 picofarads, with a few cartridges being designed for operation with 400 to 500 picofarads. In most cases, a moderate departure from the recommended value (say, ± 50 percent) will have only a minor effect on the frequency response. This is fortunate, for it is frequently impossible for the consumer to know the actual capacitance in his phono-input circuit since it consists of the turntable's total wiring capacitance plus the input capacitance of the phono preamp.
Our test reports state the load capacitance we find gives the flattest frequency response with each cartridge, and we usually indicate the effect of variations about that value. We also report on the measured cable capacitance of the record player and the phono-input capacitance of amplifiers, which is about all we can do to provide guidance in a difficult area.
A number of amplifiers are now equipped with switchable phono-input capacitance (and sometimes resistance as well). This makes it easy to determine the audible effects of capacitance changes without worrying about the characteristics of the system's components. I might add that the response flatness of some cartridges can be improved by departing from the recommendations of the manufacturers.
Several accessory manufacturers make "add-on" capacitors that can be plugged into the phono inputs to adjust the total cartridge load. Or you can try adding capacitance experimentally across the cable terminals beneath the record player or even in the preamplifier. I do not recommend soldering capacitors into the preamplifier (even if you can find the correct locations in the circuit), since this will probably invalidate the warranty. Another way to avoid the question of cartridge loading completely is to use a non-inductive cartridge such as an electret or moving-coil type.
Also see:
The Shorter Guide to Cartridge Shopping
Source: Stereo Review (USA magazine)