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Phono stage tests - Noise

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NOISE

regaphono-unwtd-noise

The noise spectrum of a phono stage follows the RIAA gain curve.


WHAT IT TELLS US

This is a measure of hiss and, with valve phono stages, of hum too. It tells us whether an annoying steady hiss or hum will spoil enjoyment of the music. With ordinary Moving Magnet cartridges, both transistors and valves are quiet enough for hiss to be inaudible at a typical listening position, say 3m from the loudspeakers, with volume up. A large number of commercial Phono stages use ultra-quiet ‘audio preamp’ silicon chips, purpose designed for such work.


This is not necessarily the case with Moving Coil cartridges, some of which have very low output (e.g. Goldring Legacy). Then hiss will be audible with all except the quietest Phono stages. Although many MC preamps also use silicon chips, which have adequately low noise, input transformers better exploit the low impedance of MC cartridges and give the lowest noise figures (e.g. Luxman E200).



HOW WE MEASURE IT

We measure output noise, IEC A weighted, in mV or µV, across the audio band (A weighting defines a band around 1kHz). This figure is then divided by the gain to give an input noise value. Values of 0.3uV - 0.8uV for MM inputs are typical with this measurement method, when the input is shorted to ground.


It’s little known that MM cartridges produce quite a lot of noise, around 3uV IEC A weighted our measurements show, enough to swamp the input noise of phono stages, so cartridge hiss dominates in practice.


This is not the case with MC cartridges. They are virtually silent so phono stage input noise dominates.



 

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