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Cartridge tests

Article Index
Cartridge tests
Tracking
Separation
Output
Distortion
Vertical Tracking Angle
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ortofon-2m-red

Frequency response of a modern Moving Magnet pickup cartridge, an Ortofon 2M Red.


FREQUENCY RESPONSE



WHAT IT TELLS US

Pickup cartridge frequency response is a good guide to tonal balance and accurately tells us what to expect, since correlation between measurement and this aspect of sound quality is good. Although a transducer, like a loudspeaker, a pickup cartridge is far simpler. An electrical generator feeding a load, it offers a perfectly flat frequency response up to 2kHz or so.


cart-circuit-1

cart-response-1

Typical MM pickup cartridge generator and, below, its frequency response. Simulation by LTSpice.



Old (1970s) Moving Magnet cartridges, like the Shure M97xE still available today, had falling high frequency output into the standard 47k load, producing a warm tonal balance and an easy, amenable sound.


shure-m97

Shure M97xE has falling treble and a warm sound.


Modern cartridges like the Ortofon 2M Red shown above have rising midband output and a more forward and detailed sound as a result. Frequency response measurement reveals such differences.


What is called ‘tip mass resonance’, a mechanical resonance between stylus tip effective mass and vinyl compliance, occurs around 18kHz and peaks up treble sharply. In old cartridges this restored upper treble. In modern cartridges, notably Moving Coils, it makes for a bright sound overall and this can emphasise distortion from poorly recorded / cut or damaged LPs, and on inner grooves.


audiotechnica-atf3-fr

Audio Technica AT-F3 Moving Coil cartridge with treble peak caused by tip mass resonance.



HOW WE MEASURE IT

We measure frequency response using the JVC TRS1007 test disc. Cut at half speed, it is known for its accuracy and is commonly used by manufacturers, so our results compare well. Most other test discs give significantly less accurate results, often wildly so, and results are neither correct nor comparable. TRS1007 is played through a custom designed preamplifier and equalisation network, as specified for the disc (test discs are rarely cut to RIAA curve). The preamplifier also provides a range of capacitive loads from MM cartridges and resistive loads for MCs so we can assess load sensitivity.



jvc-trs1007freqresptest

JVC TRS-1007 test disc, cut at half speed and very accurate.



Our published curve, recorded by the one-sixth octave analyser of a Clio measurement system, shows lateral frequency response close to outer grooves as a greet trace, and the same plot on inner grooves is shown in red. Tracing loss caused by the stylus being unable to fully read the narrow mechanical wavelength of inner grooves causes output to fall away above 10kHz. However, with a good modern stylus inner groove loss can be very small, just a few dB.




mistracking

Mistracking, here of an Ortofon 2M Black on maximum 90µm peak amplitude track of CBS-STR112 test disc.


TRACKING


WHAT IT TELLS US

Tracking is the ability of the stylus to stay in contact with the groove on loud signals. If it fails to do so, then the sound becomes strained and eventually a regular short buzz can be heard as the stylus momentarily loses contact on music peaks, or mistracks. At worst the stylus can jump right out of the groove and the arm skate across the LP surface.


Not only does mistracking affect sound quality, it also causes groove damage. If subsequently played by a good cartridge that doesn’t mistrack the imprinted distortion sounds like mistracking.


The stylus assembly moves on a compliant hinge of rubber or springy tie-back wire, that latter providing better location. If the compliance of this hinge is insufficient the cartridge will mistrack large, low frequency excursions, like a loud bass drum.


The stylus has an effective tip mass too and if it is excessive the will not be able to accelerate fast enough to follow the groove at higher frequencies. This commonly produces mistracking on loud vocals having a strong sibilant content.


Budget cartridges have their stylus mounted on a small pedestal called a rondel. This allows it to clear detritus on the disc surface. But it also adds to tip mass and degrades high frequency tracking. A small diamond without a rondel (nude) is common on quality cartridges but fluff can gather under the stylus and cause mistracking.


HOW WE MEASURE IT

We measure tracking at 300Hz with CBS STR-112 test disc and at 1kHz with Bruel & Kjaer 2010 test disc. Both channels are fed to an oscilloscope via a special measurement preamplifier without RIAA. The distortion harmonics generated by mistracking are not attenuated by the 75uS high frequency characteristic of RIAA equalisation, giving a clear picture of the onset of mistracking. Mistracking is obvious as a distortion of the waveform. Tracking force is set to the manufacturers recommended figure and bias set so that both channels mistrack simultaneously.


cbs-str-112-test-disc-

CBS STR-112 test disc yields meaningful results for distortion and tracking tests. We also use B&K 2010.



cart-separation

Channel separation of -27dB recorded by Rohde & Schwarz UPL set to third-octave mode, using Shure TTR-109 test disc.


SEPARATION


WHAT IT TELLS US

This is the amount of separation between the two stereo channels, or how much right channel is in the left, and vice versa. It tells us about the degree of stereo that can be achieved. With high separation the cartridge will produce a wide sound stage. As separation decreases the sound stage narrows until at zero separation the channels are fully mixed and a mono central image results.


Cartridges can be relatively poor in this area, and the LP too, often with just 20dB separation, a pathetically low figure compared to other sources (VHF/FM radio 40dB: CD 110dB). Yet the stereo illusion is still subjectively maintained. One reason is that the human head manages only 20dB or so separation between the ears and is used to working with this amount, so around 25dB-30dB of separation is adequate in practice for a good sense of stereo.



rs-upl


HOW WE MEASURE IT

We measure separation using Shure test disc TTR-109. It was cut with a 1kHz test tone having correctly aligned modulation axes, specifically to provide an accurate separation figure, and it is also cut on both sides so the affect of a dished disc can be averaged out, and from outside to inside also to give some idea as to the impact of dishing may affect results. We ensure the disc lies flat and measure the signal residual in each channel with a spectrum analyser to remove any contribution from warps, noise etc. The result published is the average of the two channel residues.


Separation is affected by the azimuth of the cartridge in the headshell. Headshells can be often be rotated (tilted left or right) slightly to ensure a cartridge sits perfectly upright. Unfortunately, many cartridges have poorly aligned internal generators and when upright give unbalanced channel separation figures. Adjusting azimuth to get a balanced result shows the degree of generator misalignment and can result in a visibly non upright cartridge. See our Adjust+ measurement system review for more on this.



output-2mblack-3.54cms_sec

Output of 6.4mV at 3.54cms/sec peak of Ortofon 2M Black, on Shure TTR-109 test disc.


OUTPUT


WHAT IT TELLS US

Pickup cartridges produce a very weak signal. Moving Magnet (MM) types output a few thousandths of a volt when playing (i.e. a few millivolts) and Moving Coils around one-tenth as much, or a few hundred microvolts (millionths of a volt). The output from MCs is so low, hiss from preamplifiers is an issue, although this is not the case with MMs.


Measurement of output level tells us whether hiss will likely be an issue. For example, the Goldring Legacy MC produces a very low 0.26mV and hiss is audible with many preamplifiers. Only a low input Z transformer with a high turns ratio can cope with this, such as that of the Luxman E200 preamp (see our August 2010 issue). The Lyra Dorian produces 0.84mV, or 10dB more, and hiss is rarely an issue.


It also tell us whether preamps will have enough gain, and sufficient overload headroom. High output from a modern cartridge is delivered by the Ortofon 2M Red and Bronze, which deliver 7mV at 5cms/sec peak. Our tracking tests show they can clear 22 cms/sec peak so a very high 30mV is possible. This will give 3V from a x100 preamp and 6V from a x200 unit. As 6V-10V is the maximum output from a majority of (silicon chip) preamps a cartridge of this output needs no more gain than x200, and where gain is adjustable it should be set low to avoid the possibility of output overload.



rs-upl



HOW WE MEASURE IT

We measure output using the Shure TTR-109 tests disc, which carries a 1kHz tone cut at 3.54cms/sec rms (5cms/sec peak). The output into 47k Ohms for MM and 100 Ohms for MC is measured direct by a spectrum analyser, through a screened feed from our special measurement preamplifier, to avoid hum and noise.




cart-lat-distortion

A distortion value of 0.656% recorded by our Hewlett Packard HP3561A analyser, from an Ortofon 2M Black tracking +12dB / 45µm lateral modulation track of CBS STR-112 test disc.


DISTORTION


WHAT IT TELLS US

There is quite a lot of distortion in the mechanism of music reproduction from LP. Our measurements show 1% is typical at normal music levels. Luckily, it is undetectable second harmonic distortion. Around 5%-10% of second harmonic must exist before it is detectable and then only as a lightening of timbre - not a nasty sounding effect. Other distortions can appear, however, third harmonic as a magnetic effect and, with the Decca London, as a geometric tracing problem.


As a cartridge nears its mistracking threshold distortion rises rapidly too, but remaining correlated it is not heard as a buzz divorced from the music itself, but as a sense of strain in the sound related to a high but correlated harmonic content. Cartridges that track well suffer less from this.


cart-vert-distortion

A distortion value of 3.56% recorded by our Hewlett Packard HP3561A analyser, from an Ortofon 2M Black tracking +12dB / 45µm vertical modulation track of CBS STR-112 test disc.


Below 1% it becomes difficult to attach distortion to differences in sound quality. There is no consistent link between  measured distortion and perceived quality of sound, top moving coils measuring no better than less expensive Moving Magnet designs. However, providing distortion hovers at a reasonable level of around 1% maximum with second harmonic dominant a measurement of distortion shows that there is no particular problem in this area.



HOW WE MEASURE IT

We measure distortion using the +15dB lateral and vertical tracking tests in CBS STR-112 test disc. These are low distortion cuts and allow a minimum figure of around 0.4% to be measured. Distortion is measured by a HP3561A spectrum analyser, via a measuring preamplifier without RIAA, so harmonics are not suppressed by the 75uS time constant treble cut imposed by equalisation. The cartridge is carefully aligned to be at a tangent to the test grooves to avoid tracking error and the distortion it produces.


hp3561a-1

Hewlett Packard HP3561A analyser provides distortion figures, in conjunction with CBS-STR112 test disc.



Lateral distortion (i.e. distortion from lateral modulation, that affects central images) ranges typically from 0.5% - 1.2% under these conditions and should go no higher.


Vertical distortion (i.e. distortion from vertical modulation, that affects left and right images) ranges from 1% when Vertical Tracking Angle is correct (22 degrees) up to 8% second harmonic, usually as a result of VTA error.



vta-2m-black

Vertical tracking angle measurement is made by recording intermodulation sum at 1kHz of 370Hz and 630Hz test tones on DIN 45 452 test disc, with tracks that progressively increase in cut angle.


VERTICAL TRACKING ANGLE


WHAT IT TELLS US

Vertical tracking angle, if it is wrong, is a source of distortion. It is the angle subtended by the cantilever to the record surface, as our diagram shows, and it is meant to be the same as the vertical cutting angle, set to 22 degrees. If it strays from this value, distortion results. Few cartridges actually achieve 22 degrees or anything close, most measuring out at 30 degrees or higher. The reason is that achieving 22 degrees means using a long cantilever and suffering limited disc clearance, causing the cartridge body to hit disc warps.


What does this tell us? It means high distortion on vertical modulation, affecting left and right images, but not centre images. The amounts can be large, from around 1% second harmonic if VTA is right, up to 5% or more with incorrect VTA, as measured on the +15dB vertical cut of CBS-STR112.


It has hard to pin down the subjective impact of low order distortions but incorrect VTA does seem to slightly mess the sound, degrading a sense of clean precision. As VTA is also linked to cartridge azimuth (see our Adjust+ measurement system review) there is quite a lot going on here.



HOW WE MEASURE IT

We measure VTA using the German DIN 45 542 test disc. The 1kHz sum product of 370Hz + 630Hz test tones, cut at successively increasing angles, is measured with a spectrum analyser. The result is graphed to find where the sum tone reaches a minimum, which gives the VTA figure.


We also measure distortion on vertical modulation with CBS STR-112 and this correlates well with the vertical tracking angle error provided by DIN 45 5452.


vta-graph-2

Vertical tracking angle is determined by plotting measurements on a graph. Shown are results for a Denon DL103R MC cartridge, showing it has correct VTA of 22 degrees.

 

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