How we test a Phono Cartridge (Aug. 1972)

TELLING READERS how we evaluate various elements of high fidelity component equipment appears to be on the road to becoming a series-and after enough articles in the series, everyone will be just as able to test equipment as, presumably, we are. In June, we described the steps followed in making PROFILE tests on turntables, and a year and a half ago we described testing methods employed with stereo headphones. Now it's cartridges which are to come under our scrutiny.

As has often been said before, over the years, we do not print a PROFILE on a piece of equipment which, in our opinion, does not come up to the standards we feel should be met for high fidelity equipment, or at least, one which does not come up to the manufacturer's specifications. It has long been our policy to pass the information on to the manufacturer in the hope that he may agree with us that the particular item tested was in some way faulty and will substitute another unit which does meet specifications. If a second or third unit fails in the same tests, the item is returned and publication of the profile is cancelled.

In case readers should think that the absence of a particular manufacturer's products might indicate that we do not think they are worthy, let us clear up that misapprehension for once and all. Some manufacturers do not feel that profiles are of any value to them, and equipment is not offered for testing. This we cannot prevent, although often we do request an item, and it may or may not be sent, depending on the manufacturer. But the absence of profiles on any product does not necessarily indicate that it is not considered good enough for these pages.

Cartridge Test Requirements

The important parameters of the performance of a cartridge are, in probable order of importance, the following:

1. Frequency response,

2. Separation,

3. Signal output,

4. Tracking ability,

5. Intermodulation distortion,

6. Stylus-force requirements, and

7. Effect of lead capacitance.

Of these, the easiest to measure is frequency response. This may be done in a number of ways, and many years ago it meant measuring the output of the cartridge at 20 or 30 discrete frequencies as reproduced from a typical test record--for example, CBS STR-100. Because of the time required to make such a series of measurements, and the possibility of some disturbance originating anywhere between the frequencies available on the test record, the writer developed a device which would provide a continuous graph of frequency response from a sweep record and which was modest enough in cost for our budget. Originally this was done using the same CBS STR-100 test record, but this particular source had some disadvantages. First, it covered the range only from 40 to 20,000 Hz, and second, the recording characteristic was a straight line from 40 to 500 Hz, at a 6-dB-per-octave slope (constant amplitude), and another straight line from 500 to 20,000 Hz (constant velocity). There is nothing wrong with that approach, providing the corrective network is the complement of that curve. But such a complement cannot be produced readily using RC networks, which are almost universally used in component equipment. Nor, of course, does this characteristic conform to the RIAA equalization.

A little search, however, turned up another test record, Brüel and Kjaer QR-2009, which covered the range from 20 to 20,000 Hz, and which did conform to the RIAA curve from 20 to 1000 Hz, and flat above that point, thus making it possible to produce a graph of the response of a cartridge with a simple and conventional RC equalization network. Consequently, the graphic recorder was redesigned to accommodate the curve of the QR-2009 record.

So, using the graphic recorder, the frequency response of the cartridge is first recorded over the range from 20 to 20,000 Hz, first with the left channel of the cartridge and then with the right. Then the separation curves are made, with the left channel playing the right-channel recording, and again with the right channel playing the left channel band. The record also provides a lateral band and a vertical one for a considerable flexibility in testing requirements.

Now, having the frequency response curves, we observe them carefully to determine if there are any peaks in the response. Actually there are often some such peaks, so further graphs are made-on both channels--with additional capacitance across the leads. The normal capacitance of typical turntable leads is of the order of 250 pF, and we add increments of 50 pF across the leads and make further series of curves which show the effect of added capacitance.

We have previously remarked on the importance of reading the instructions which accompany any electronic equipment. It is of no less importance with cartridges, for the manufacturer is likely to suggest the optimum capacitance into which the cartridge should work. Even if he doesn't, however, we find it out for ourselves. If it is found that a flatter response is obtained with a specific value of capacitance, we so indicate, and all further testing is done with the value in the circuit.

In addition to reducing possible peaks, the proper value of load capacitance also helps to remove the "swayback" that often occurs in the range from 5000 to 8000 Hz.

The next step in testing a cartridge is the photographing of its square-wave response. Another CBS record--STR111--is pressed into service for the next two operations. The 1000-Hz square-wave band is played through a flat preamplifier-one without either low- or high-frequency equalization--with a gain of 40 dB-and the output applied to a scope. Right, left, lateral, and vertical bands are also available on this record for various tests, but we normally use only the left channel for this step. The display on the scope screen is photographed, using a 105-mm lens on a 35-mm camera. This usually requires an exposure of 1/15 of a second at f/4.0 with Panatomic X film--and to identify the film at any later date, a card is placed at the side of the screen with the name and model number of the cartridge written on it with a felt-tipped pen.

Distortion Measurement

The same CBS STR-111 test record is used to make measurements of intermodulation distortion. This record has a number of IM test bands at different levels and with different frequency combinations, and in lateral and vertical modes also. These combinations are: lateral, 400/4000 Hz at levels of +6, +9, + 12, + 15, and + 18 dB; lateral 200/4000 at the same five levels; vertical 400/4000 at +6, +9, and + 12 dB, and vertical 200/4000 at the same three levels. While we make and record distortions at all 16 conditions, we chose only two to list in profiles--lateral +9, and vertical +6, both at 200/4000 Hz. These seem to be indicative of overall performance.

Returning to the use of the STR-100 record, we measure the output for a stylus velocity of 3.54 cm/sec in the left-channel mode and then in the right channel to compare outputs between the two channels. Next we go through the various steps of levels recorded on the STR-100 record, watching the scope for any evidence of mistracking at the normal (manufacturer-specified) stylus force. We then reduce tracking force 1/4-gram at a time and observe the results.

Note that the order in which the measurements are made do not coincide with the importance we assigned them earlier. Our order of making the various steps was simply the result of minimizing the changing of leads and equipment, as well as the records themselves. Intermodulation measurements are made on an EICO 902 Harmonic and IM Distortion Analyzer. The "flat" preamplifier used for taking the photos of the square-wave response is the one used in the description of "Testing Turntables" as described in the June issue.

For many years we have been using a turntable never sold in this country-the PE Studio 33, which we acquired in Germany during a visit there in 1964. It has apparently gotten "tired," so we have substituted a Thorens 150 with a Rabco SL-8 arm, and the results are somewhat more consistent.

Frequency-response curves are recorded on the new Justi-Meter III, the latest model of the original graphic recorder designed by the writer some three years ago. Since the magazine provides its own form of graph paper to ensure consistency in appearance, we re-plot the machine-run curves on the forms used in these pages, using a light blue pencil. The drafting department retraces the curves in black and adds any necessary legends to the forms before including them in the made-up pages for the magazine.

The write-up of the observations, together with a listing of the manufacturer's specifications which precede all our profiles, is finally prepared, with the last few paragraphs devoted to a subjective evaluation of the performance of the cartridge on a number of records with music on them-as contrasted to all the measurements which are made with no music whatever, nor with any listening, just viewing on the scope screen in some instances, and reading meters and making runs on the sweep record.

To the purist, the listing of the qualifications of a cartridge are of some importance in his selection. To the music lover-the kind who has no technical interest in equipment whatever--the subjective aspect is the more important. It has been our experience, in most instances, that the cartridge that measures well also sounds the best, even though we have long subscribed to an axiom of the late C. J. LeBel that "If it measures good and sounds bad, it is bad."

Measurements of inductance and d.c. resistance are usually made somewhere along the line. If these values fall within the usual range for the particular type of pickup, they are seldom mentioned--it is only when one model deviates appreciably from the norm for cartridges of its type that it is mentioned at all. About the only effect of inductance differing from the usual could be a variation in the equalization, provided the equalization were of the passive type ahead of the preamplifier--practically a rarity in consumer equipment. A low d.c. resistance might improve the noise figure of the first transistor amplifier stage-again an unlikely difference in consumer amplifiers and receivers. Regardless of their intrinsic inductance or resistance, most cartridges are designed to work into an impedance of 47,000 ohms, and this value is standardized in most equipment. Some high quality items offer an extra pre-preamp stage into which may be fed low-impedance cartridges, such as the moving coil types.

It is possible that a lower inductance could possibly reduce pick-up from external sources, but this again is unlikely. Practically every good pickup on the market is sufficiently well shielded against hum fields.

As with practically all components in a high fidelity system, the final selection should be made on the basis of how it sounds to the buyer. No two speaker systems sound exactly alike, nor do any two cartridges. Nor, for that matter, do any other two components. You end up by choosing the one which sounds best to you, preferably in your system at your own home with your own two (or four) speakers. But a thorough perusal of the actual measurements may give you a hint as to the ones you should compare before you buy the one of your choice.

-C. G. McProud

(Audio magazine, Aug. 1972)

Also see:

Phono Cartridge Survey (Feb. 1971)

How a Turntable is Profiled (June 1972)

A Practical High-Frequency Trackability Test for Phono Pick-Ups (Aug. 1972)

Phono Cartridge Q's and A's--Mainly for Beginners (Aug. 1972)