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Cartridge tests - Distortion

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Cartridge tests
Tracking
Separation
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Vertical Tracking Angle
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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.




 

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