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Phono stage tests - Overload

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OVERLOAD



WHAT IT TELLS US

It is remotely possible, in a few circumstances, for a cartridge to overload a phono stage, causing distortion. We test for this possibility.


Generally, modern transistors and integrated circuits have enough headroom to easily avoid being overloaded by any cartridge, but there are some limitations.


Most external phono stages use integrated circuits, fixing maximum output swing to 10V, usually because they are limited by 15V supply rails. In some designs this sinks to 6V, possibly because the designer thinks this adequate (0.5V is enough to drive most amplifiers) and it makes for a cheaper power supply. When gain for MM can be freely set, however, a gain of x200 – quite common – would put such a stage into output overload with a modern Ortofon 2M Red or Blue cartridge, able to deliver 35mV peaks.


This tells us that with MM cartridges a phono stage should ideally be able to accept 40mV input before overload. Output swing usually sets the overload ceiling, and the input limit is the output divided by gain. So if the output limit is 10V and the gain x100, the input limit is 0.1V, or 100mV. This is more than enough for modern cartridges. However, if the output limit sinks to 6V and the gain used is x200, then the input limit becomes 0.03V or 30mV, in which case overload becomes a possibility. So overload isn’t likely, but it is possible.


None of this applies to valve phono stages. Their output swing is up to 45V, and valve phono stages rarely, if ever, go into overload.


Overload conditions are rare with Moving Coil cartridges, although not impossible. Maximum output from an Ortofon Cadenza Red, which produces a healthy 0.7mV at 5cms/sec rms, is around 3.5mV.  Feeding a phono stage with a low x1000 (60dB) gain gives 3.5V output maximum. It would be possible, using higher gain to exceed 6V, if rare.


HOW WE MEASURE IT

We input a 1kHz test tone and increase level until output overload, seen as peak clipping, occurs, as seen on an oscilloscope, connected to the monitor output of our Rohde & Schwarz UPL spectrum analyser, The input applied is measured direct across the input terminals (so is not a generator emf) and the output monitored for distortion on the UPL so where soft clip occurs, the limit can be set by using 3% distortion criterion. This usually only applies to valve phono stages.





 

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