A "marriage counselor" explores the trouble spots in this sometimes-uneasy union.
by Edward J. Foster
FUNNY TILING, the way some fads come and go and some, by refusing to go away, prove that they weren't fads in the first place. One such is the emphasis that recently has been put on proper phono-cartridge loading.
The dramatic improvement in cartridges over the last few years has made the need to eke out their best performance more-not less-important.
Indeed, the same cartridge can sound subtly, but noticeably, better with one preamp than with another. Switch to another cartridge, and the other preamp might sound better. The obvious explanation is incompatibility between the components rather than essential quality differences in the components themselves. The most common cause of this incompatibility is traceable to the phono cartridge loading-the combination of input resistance and capacitance-provided by the preamplifier's input termination.
The process of engineering a new pickup is like designing a smooth road for a hilly landscape. The ridges must be whittled away and the gullies filled up, and no rock and earth should be left over when you're finished. Thus, in trying to produce a pickup with a flat frequency response, something must be done about the resonance that will occur at a relatively high frequency-generally in the region between 12 and 35 kHz-because of the interaction between the effective mass of the stylus and the compliance (springiness) of the vinyl from which the record is made. There is likely to be a peak in the frequency response at the resonant frequency and a fairly rapid rolloff in response at still higher frequencies. This resonance establishes the upper frequency limit of the cartridge.
The phono-cartridge designer has no more control over the vinyl characteristics than the highway engineer does over the geology of his site. The mass and damping of the stylus, however, are within his power to control. The lower he makes the mass, the higher the resonant frequency. So one way of taming his particular topography is to get the mass so low that the resonance lies far above the audio range. But then he must take care that the stylus assembly will be rigid enough to prevent it from flexing while tracing the groove.
Here is another juggling act involving mass and rigidity-and price, since low mass and high rigidity translate into exotic materials and increased cost.
A second approach to this "topology" is to damp the resonance so that its effect is less apparent.
This takes discretion. Too much damping results in a sluggish cartridge and one with poor high-frequency tracking ability. Yet a third technique compensates (at least partially) for the mechanical resonance with another resonance. Since most high-quality cartridges are of the "magnetic" type and have relatively high self-inductance, the mechanical resonance can be smoothed with an electrical one created by the inductance of the cartridge and the capacitance and resistance to which it's connected. Here's where proper cartridge termination comes in.
The designer controls the internal characteristics of his pickup, but he doesn't control the load.
The best he can do is to postulate likely loads and say, in effect, "If you want this cartridge to work the way I intended, terminate it with X ohms of resistance and Y picofarads of capacitance." It's up to the user to provide that load, which is to say that, for best response, the preamp and tonearm wiring must conform to the expectations of the cartridge designer.
A Question of Standards
It has been standard in the industry for some time that the resistive load a preamp provides for a magnetic cartridge should be 47,000 ohms (though CD-4 cartridges were designed for 100,000-ohm termination). What isn't standardized is the capacitive load. Perusing cartridge spec sheets, you'll find recommended loads of from less than 100 picofarads (usually for CD-4 cartridges) to about 500 picofarads. To complicate matters further, relatively few preamps have heretofore carried input-capacitance specifications, although generally they have conformed to the 47,000-ohm resistive requirement. Nor have many tonearms carried a wiring-capacitance spec.
Some help is on the way: The recent IHF amplifier-measurement standard includes characterization of phono input impedance as a "primary" specification for preamplifiers and integrated amplifiers. Furthermore, it requires that both the input resistance and the input capacitance normally be specified. Presumably, the standard will make its presence felt, and disclosure of these parameters will be more widespread, in the future.
The sleeper here is that not all phono-preamp circuits can be "modeled" by a simple two-element circuit containing just a resistor and a capacitor. Some circuit configurations exhibit an equivalent input capacitance that varies with frequency, so the ideal load cannot be presented to the cartridge at all audio frequencies even if some portions of the range are well matched. Such a load certainly was not what the cartridge engineer envisioned, and it can induce frequency-response aberrations that are even more substantial than those caused by a slight capacitive mismatch.
According to the IHF standard, such circuits are to be characterized by a specification of input impedance at 1 khz alone, without the separate specification of resistance and capacitance-a warning to the consumer that all may not be well. Since the IHF document is relatively new, however, and since providing only the 1-kHz spec was the old way of doing things, you can't really tell whether a manufacturer has merely ignored the new standard or indeed does have an atypical input load and is properly indicating that fact. (Starting with this issue, HF is reporting the phono input impedance in accordance with the new standard.) You're likely to see more and more phono circuits that present a classic load to the phono cartridge. Certainly the many preamps that now afford user control over the input capacitance suggest that greater attention is being paid to input characteristics and reflect the fact that their designers are more or less in the same boat with the cartridge engineers: Neither knows which product will be used with his, so neither knows what capacitance values to design for.
----The effects of incorrect capacitive loading can be difficult to
predict. With this particular pickup, the correct loading (450 picofarads)
produces the fairly flat response curve represented by the solid line.
When loading is reduced to 150 picofarads (dotted line), not only does
a nasty peak appear at the top end, but the treble below the peak is
noticeably weakened.
----Some preamps (like this, in a Pioneer integrated amp) have phono-loading
controls visible on the front panel.
-----Others (this is Holman design for Apt) hide controls at the back as a set-and-forget onetime adjustment.
------ DB Systems offers color-coded capacitor-loading plugs.
-------- Berkshire does it with a single multiple-value capacitance loader for each channel (above). It also offers capacitance meter (below) so you can measure values already present in your system.
What Can You Do?
The new genre of adjustable-load preamps affords valuable opportunities to act as marriage broker between pickup and preamp. To do so, there are two facts you must know: the optimum load for the cartridge you are using, and the cable capacitance of your tonearm or turntable. The first is almost always given in the cartridge's specification sheet; the latter is harder to come by.
Few current tonearms or turntables have specified values of capacitance, though a common value is "under 100 picofarads" (thanks to pressure from the CD-4 lobby, whose subcarrier tends to get lost at higher values). When in doubt about how much capacitance the cabling adds, contact the turntable or arm manufacturer for the specifics. The preamp's capacitance should then be set to equal the desired load for the cartridge minus the tonearm-cable capacitance.
There are several products on the market with which you can increase the capacitive load on the cartridge. One is the DBP-6 Phono Equalization kit ($24.95) from DB Systems. In consists of a set of "Y" connectors and pairs of phono-plug-mounted capacitors that add 100, 150, 200, 300, or 400 picofarads to the system. Another product is the Match-Maker ($49.95) from Berkshire Audio Products.
This device is connected in the phono-cable line and can add capacitance from 50 to 350 picofarads, in 50-picofarad steps. ( Berkshire also offers a capacitance meter--the $119.95 Model CCN--so you can measure your leads.) The cheapest way (if you're handy) is to buy capacitors of the appropriate values in a radio-parts store and solder them directly across each phono-input jack.
In each case, you will need to know the capacitive load already present in your preamp. Lacking test data or specs, you will have to contact its manufacturer. The total capacitance is the sum of the parts; add the input capacitance of the preamp to that of the tonearm cabling and subtract this sum from the optimum load required by the cartridge.
Make up that difference with the closest value at hand. You needn't be precise; if you're within 20% of the optimum value, that should be good enough.
Let's say, for example, that your pickup is rated for a 400-picofarad load, your tonearm leads at 100, and your preamp at 130. You would need about 170 [400-(100 + 130) = 170] picofarads more, and a 150-picofarad value should be dandy.
If the capacitance of the system already exceeds that desired by the cartridge, no external "fix" will help.
When you've corrected your system's capacitance, what difference in sound can you expect to hear? That will depend upon the cartridge you're using. Some are quite tolerant of mismatch; others are not. Remember that there are several options open to the cartridge designer to achieve a flat response. To the extent that a particular design relies upon a specific electrical termination, the sound of your cartridge can be improved substantially by providing that termination. (Improperly terminated cartridges have been known to exhibit 5-dB variations in frequency response.) To the extent that the cartridge does not rely greatly on external compensation, the load is less critical-although, in any case, an excessive capacitive load will depress the high end.
Oddly enough, adding capacitance may make a cartridge sound brighter. Some pickups (for example, those made by Shure prior to the V-15 Type IV) exhibit a depression in the region between 3 and 10 kHz when the load capacitance is insufficient. This depression is accompanied by a high-frequency resonant peak somewhere near 20 kHz. With proper loading, the upper-midrange output is increased and the peak brought under control. Since there is more musical energy between 3 and 10 kHz than at the resonance frequency, the cartridge sounds brighter (and smoother) when properly terminated.
Proper capacitive termination is most important when using fixed-coil magnetic cartridges of relatively high impedance--those with moving magnets or moving iron (variable reluctance). Moving-coil cartridges have relatively low impedance (inductance), and their frequency response cannot be smoothed by capacitive loading. (Their designers are left on their own to solve the problem.) In the opinion of some critical listeners, however, terminating a moving-coil cartridge with the proper resistive load is as important as the capacitive load is to a fixed-coil cartridge.
Why this recent concern with the phono-cartridge/preamp "marriage"? Surely, if its effect is so dramatic, the problem (and solution) would have surfaced earlier. Perhaps part of the answer lies with the quest for better high-frequency phono-cartridge tracking ability. In some cases, this may have been achieved by a reduction in internal damping and a greater reliance on external loading to assure flat response. But, undoubtedly, it has been the very progress made in phono-cartridge design that has exaggerated--or even created--the problem.
As long as phono styli were so massive as to resonate in the region between 12 and 15 kHz, one could only damp the resonance in the pickup and make the best of it. And, in any event, early styli geometries were incapable of accurately tracing signals of much higher frequency; even if such signals were on the record, you'd never know it. But today's styli--with their narrow tracing radii and exceedingly low effective mass--can reproduce the highest tones on a disc. And, in high fidelity lore, if it can be done, it will be done.
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AND TONEARM AND CABLE CAPACITANCES OF FAMILIAR TURNTABLES, ARMS, AND PHONO CABLES
The table immediately below shows preferred resistive and capacitive load values for familiar phono cartridges that are likely to be used with the "regular" (moving-magnet, etc.) phono inputs in good home installations. Where a range of values is indicated, relative insensitivity to loading values is implied-though, since specification practice varies, some forgiving pickups may show only single values. The table that follows shows capacitances for familiar tonearms, turntables, and connector cables; instructions for using these figures are given in the article's text.
The information is drawn from a number of sources, including manufacturers' data and measurements at Diversified Science Laboratories. We are particularly indebted to Thomas Holman for permission to use data compiled for and supplied with the Apt Holman preamplifier.
PHONO CARTRIDGES
Load Resistance In Ohms Load Capacitance in pF
Manufacturer Model
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Also see:
TRACKING-ANGLE ERROR: A NEW SLANT
Osawa High Performance Phono Group (ad, Jun. 1979)