A guide for prospective buyers based on our test reports.
by Edward J. Foster
[ Edward J. Foster runs his own audio-testing and technical service, Diversified Science Laboratories, and is a frequent contributor to these pages on audio-related subjects. ]
TEST REPORTS IN HIGH FIDELITY are intended to give a basic idea of the characteristics and peculiarities of a product. They are based upon both lab oratory measurements-technical data that are considered to reflect a product's strong and weak points-and listening tests. The correlation be tween the data and actual performance is not al ways direct; therefore the audio editors must interpret the figures. But to get the most out of a report the reader should understand why the various measurements are made and something of their relative importance.
Details of the test procedures need not concern us here, though in some tests they may differ markedly from the procedures used by at least some manufacturers in determining product specifications. We'll take up the more significant divergences as we go. The point to keep in mind is that, while HF takes pains to be consistent in its approach to competing models both on the test bench and in practical use, other sources of information about components may well represent different viewpoints or different measurement techniques--even where a measurement standard exists in the interest of uniformity of data.
Turntables and Tone Arms
Record-playing equipment usually consists of the turntable/arm combination and the pickup or phono cartridge. Generally HF reviews each separately, but sometimes a record player comes with its own cartridge, in which case the reviews are combined. A few turntables are marketed without arms-and, naturally enough, arms without turn tables.
The lab tests on the turntable and arm are of two types: those that indicate the "convenience" features and, more important, in our view, those that measure basic performance. Since the primary purpose of the turntable is to rotate the record at a constant speed and introduce no noises of its own, HF considers the flutter and rumble measurements paramount. Flutter (or wow and flutter) is a measure of the short-term speed stability of the turn table. If the drive system were perfect, the turn table would spin at its nominal speed (say 33 rpm) without variation. But imperfect bearings and jerky delivery of power by the motor ("cogging," as it is called) create minor wavers in speed from instant to instant. The change in speed, divided by the average speed and expressed as a percentage, represents flutter.
The currently accepted standard (the so-called IEEE/ANSI standard) specifies a weighted peak measurement. Weighting is used because listeners are most annoyed by speed variations that occur at a rate of about 4 Hz; thus variations that occur at this rate count more heavily in the measurement than those occurring at less irritating rates. Flutter, by its very nature, is not constant. For this reason HF generally reports two figures: an "average" value (an interpretation of what the flutter meter reads) and a "maximum" (the highest level reached on the meter). The latter corresponds to the above standard.
What flutter "sounds" like depends upon the rate at which it occurs. Slow speed variations--individual "wobbles" lasting from one-sixth of a second to two seconds and therefore said to have a frequency of from 0.5 to 6 Hz-cause instability in the pitch of reproduced tones, a sort of "wowing" sound. (These slow variations used to be measured separately as "wow.") They are most annoying on sustained tones, such as those produced by piano or woodwinds. Faster speed variations ("flutter" in its more limited sense) from 6 to 200 Hz cause a roughening of the sound rather than pitch variations. Equipment of high quality will generally have average peak weighted flutter figures of ±0.1% or less.
Rumble, sometimes called the turntable's signal to-noise ratio (or simply S/N), is the measure of the noise introduced into the music by vibrations in the drive system. The phono cartridge responds to the wiggles of the record groove, but it also is sensitive to vertical and lateral vibrations induced into the turntable platter by the motor or drive train. Since these vibrations are generally very low in pitch, they produce a rumbling sound.
Again, weighting is used to make the measurement reflect the amount of annoyance caused by the rumble. HF employs the ARLL (audible rumble loudness level) weighting, which is in widespread use in this country, although the DIN B and DIN A (German), the JIS (Japanese), and the NAB (an older American standard) standards frequently are used elsewhere. (There are no direct conversion factors relating these systems.) The ARLL curve counts rumble most heavily at 500 Hz.
Rumble low enough to produce an ARLL measurement of -60 dB or better (e.g., -61 dB, etc.) is a prerequisite of high performance.
Lateral and vertical friction in the arm pivot is an important consideration. Excessive friction (more than, say, one-tenth of the tracking force needed for the cartridge used) will necessitate raised tracking forces, which can result in excessive record wear. Fortunately, in equipment re viewed today in HF, bearing friction usually is so low as to almost vanish.
The frequency of tone-arm resonance is an important parameter in that it relates to the playing of warped records. At the resonant frequency, the sensitivity of the system-both in terms of the out put voltage from the cartridge and of susceptibility to mistracking-increases. The tonearm resonance actually involves the total effective mass of the tone arm and cartridge and the cartridge's stylus compliance. But in measuring a turntable/ arm combination, HF must use some cartridge, and measurements are therefore valid only for that cartridge (or for others with the same mass and compliance). For convenience, HF has standardized with the Shure V-15 Type III for testing all high-quality arms-unless, of course, the turntable system comes with its own cartridge, in which case HF measures the combination and the results are then valid for the system.
The ideal range for arm resonance is 10 to 15 kHz. This is sufficiently below the music band so that the increased output from the cartridge at resonance will have a negligible effect on the tonal balance, and sufficiently above the frequency band at which record warps are most prevalent and severe (about 1 to 6 Hz) to minimize the chances of mistracking on a warped disc. The less boost at resonance (reported as a rise of so many dB) the better, but since the tolerable boost depends in a fairly complex way on the resonant frequency, the reviewer normally assesses the importance of this data.
It may seem odd that HF rates absolute speed ac curacy as less important than, say, tonearm resonance. Speed accuracy is important, because it affects the over-all pitch of the music, but very few listeners can tell the precise pitch of what they hear without a reference for comparison. An en tire piece could be as much as a quarter-tone sharp or flat without bothering most people. The exceptions are people with perfect pitch or those who wish to play the piano along with the record. Absolute speed accuracy within, say, 0.5% (one twelfth of a semitone in pitch) is sufficient for any musical need.
Variation in speed with changes in line voltage can cause perceptible shifts in musical pitch, as when the switching of an air conditioner causes a short-term low-voltage condition. HF measures the speed at line voltages of 105, 120, and 127 volts and reports the discrepancy, if any. In practice "120-volt" AC lines may fall anywhere within this range and tend to vary; typical home power lines average 110 to 115 volts.
Many modern high-quality turntables include a speed control. In such cases, HF reports the range of the control. A variation of 6% (say, ± 3%) is equivalent to a semitone and should be more than adequate.
Most pivoted tone arms now include an anti-skating control that compensates for the tendency of a conventional pivoted arm to move toward the center of the rotating record. Without antiskating "bias" compensation, the stylus force on the two groove walls is unbalanced by the skating force, producing a tendency toward mistracking of the outer (right-channel) wall. (Straight-line-tracking arms do not develop this unbalanced force.) Since the skating force develops from friction between the stylus tip and record groove, the amount of compensation required depends on the stylus shape. Frequently, several scales are included on the antiskating control to indicate the proper set ting for various styli. HF checks to see that the antiskating forces are reasonably close to the empirically "correct" values. We also verify that the minimum tracking force required to activate the arm-cycling mechanism on automatic and semi automatic turntables is well below the smallest value that would ever be used.
Some matters on which we report are of varying significance to different users. A cueing control that skips ahead or back by more than one or two grooves when you use it as a temporary "pause" can be very annoying-if you use it that way. Similarly, a slow cueing cycle in an automatic (more than 12 seconds or so) can be galling to some users, unimportant to others. And while accuracy of the stylus-force setting is worth measuring, we wouldn't turn down a player because the gauge was a bit off or because calibrations are no finer than quarters of a gram. For a few dollars you can buy a gauge and set the vertical tracking force as accurately as you want.
Pickup Cartridges
When it comes to pickups, frequency response and channel separation are cardinal factors. Frequency response that is flat from 20 Hz to 20 kHz is necessary for the cartridge to reproduce the entire audible spectrum without coloration. This ideal is seldom realized. Typical anomalies are a trough between 5 and 10 kHz, followed by a peak between 10 and 20 kHz. The peak (which occurs at a higher frequency in CD-4 pickups) is caused by a resonance between the stylus tip and the groove and can vary according to the record used. A response curve that is flat within ±2 dB from 30 Hz to 15 kHz is typical of cartridges with very good performance. Responses are plotted for both channels. They should be identical for best performance, but a match within 1 dB is considered very good.
The channel separation curves are plotted on the same graph with the frequency response curves. Again, the separation is plotted for both channels. The degree of separation (in dB) can be read directly from the scale on the left; typically, it will be best in the midrange and at its minimum at the resonant frequency. You will never see a "flat" separation curve-desirable as that might be-nor are the curves likely to be identical for the two channels. Generally, a separation of 20 dB from about 100 Hz to 15 kHz in the worse of the two channels would be considered adequate; other things being equal, the greater the separation and the greater the frequency band across which it ex tends, the better.
When testing CD-4 cartridges, HF extends its upper limit of response and separation measurements from 20 to 50 kHz and plots the results on a separate graph. The response is not usually as smooth, nor the separation as great, in this region.
Fortunately, depending on the design of the particular CD-4 demodulator, anomalies in this, the CD-4 carrier region, are relatively tolerable.
Smoother response and wide separation are still the ideals, however.
Coequal with frequency response and separation is the "maximum tracking level" test. For this test, the tracking force is adjusted to the center of the manufacturer's recommended range, and measurements are made to determine the highest recording levels that can be traced without significant distortion of the signal. Different signals are used to check the tracking performance at different points in the band: sine waves for the lower frequencies and a one-octave noise signal for the top band. The measurements are carried out to a level of + 12 dB above the RIAA 0 VU at 300 Hz, to + 18 dB at 1 kHz, and to -5 dB in the 10- to 20-kHz band. These levels correspond to the highest levels normally encountered on modern discs. Preferably the cartridge should track the highest level on each test (indicated by > + 12 dB, > + 18 dB, and >-5 dB, respectively, in the data). A cartridge that can come within 3 dB of these levels is still acceptable. HF also measures the minim um force required to track a series of certain low-frequency glide tones. This force is usually less than that recommended by the manufacturer and can be taken as an indication of the precision with which the recommended value need be maintained.
Second-harmonic distortion (at various frequencies from 1 to 10 kHz) and intermodulation distortion (400 Hz and 4 kHz) are measured. Distortion, of course, is a very important criterion, for it is related to the "cleanness" or clarity of the re produced signal. The correlation, unfortunately, is not direct, so the significance of particular distortion measurements is assessed by the reviewer after auditioning the cartridge.
The importance of the low-frequency resonance point of the cartridge/arm system was mentioned when we discussed the turntable/arm tests. HF is faced with the same problem here--having to mount the cartridge in a particular arm. We've standardized with the SME 3009 arm, but the performance in another arm is likely to be different.
Keeping in mind that increasing either stylus compliance or effective arm mass (which, of course, includes the weight of the pickup cartridge itself) will lower resonant frequency, while de creasing either will raise it, and that the optimum resonance-frequency range is about 10 to 15 Hz, you can draw some conclusions from our reports.
The Shure V-15 Type III, with which we test tone arms, happens to produce a 6.5-Hz resonance in the SME arm, with which we test pickups. (Resonance is, of course, higher in the most recent SME, which Shure says the Type III was designed to complement.) This means that pickups for which we show a higher resonance frequency in the SME arm will also deliver a higher resonance than the Shure in other arms; those that measure lower in the SME will be lower in other arms too. This in formation can help you avoid poor pickup/arm matches.
Let's say, for example, that you're considering a pickup that we report produces a 10-Hz resonance in the SME arm and want to use it in an arm that checks out at 8 Hz with the Shure. Since this pickup produces a somewhat higher resonance than the Shure, it should be just about in the "ideal" range with this arm.
The output voltage measurement, given in millivolts per centimeter per second (of groove velocity) at 1 kHz, indicates the compatibility of the cartridge with your preamp. Rarely do problems arise in this area, and when they do they are normally discussed in the report. This figure gives a clue to channel balance: HF would look for a match within 1 dB (10%) between the two sensitivity measurements.
The 1-kHz square-wave photo provides some indication of the degree of damping of the stylus assembly. Ideal reproduction of the square wave would imply a flat top, a vertical rise and fall, and no overshoot, although this is only approached in practice. Some would claim that good performance is indicated by a rapid rise and fall time, no more than a 30% overshoot, and less than two cycles of ringing, though by that standard some very good-sounding cartridges do not reproduce square waves well.
The ideal vertical tracking angle for a cartridge is the same angle as that at which the record was cut. When the two match, distortion is minimized.
Over the years, record companies have adopted a number of different "standards." The current RIAA recommendation calls for a 15-degree angle.
In practice, any tracking angle between 10 and 20 degrees is acceptable.
Every stylus is examined microscopically to determine its geometry, size, orientation, and polish.
HF reports these results as a general indication of the quality of the stylus.
The amount of lab data measured for an HF re view is quite exhaustive, and rarely does any one device turn up spectacular figures in all of the tests. Moreover, the data represent necessary rather than sufficient conditions for good performance on the part of the product. To put it another way, while a product can flunk by coming up with a very bad number in any one of the tests, passing all of the lab tests we know of will not guarantee that it sounds good when used to reproduce music.
And for HF, that is what matters most-making music. So in the final analysis the lab data collectively represent a guide: useful, to be sure, but only a guide. The final judge is the ear.
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Ten Aids in Choosing Record-Playing Equipment
Selected Guideposts to the Highest Quality Turntable and Arm
1. Average peak weighted flutter of no more than 0.1%.
2 Rumble low enough to produce an ARLL weighted measurement of -60 dB or lower.
3. No variation in speed when line voltage changes.
4. A cueing control that sets arm back down with in a groove or two of where you lifted it.
5. In an automatic turntable or changer, a cueing cycle or change cycle not so slow as to bother you.
6. Arm resonance to cause a boost of no more than 1-2 dB in the 5- to 10-Hz range, and 3-4 dB in the 10- to 15-Hz range.
Cartridge
7. A response curve flat ±2 dB from 30 to 15,000 Hz and with the channels matching within 1 dB.
8. Separation of channels no less than 20 dB between 100 and 15,000 Hz.
9. Trackability of + 12 dB (above RIAA 0 VU) at 300 Hz, + 18 dB at 1,000 Hz, and -5 dB through the 10,000- to 20,000-Hz band, all within a 3-dB tolerance.
10. Second-harmonic and intermodulation distortion as low as possible.
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(High Fidelity, Apr. 1977)
Also see:
Turntables -- What You Should Know About / Today's Designs [Apr. 1975]
Understanding Tonearms (Audio, June 1980)--part 1
Understanding Tonearms (Audio, June 1980)--part 2
Tone Arm Damping--The Overlooked Feature (High Fidelity, Jul. 1975)