Quote:
Originally Posted by Trevor
i think you are over-simplifying the cartridge the input will see impedance not resistance and this will be a lot higher because of the inductive element of the aforementioned cartridge
Trev
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Well sure - if you are doing the complete analysis you have to take account of the 500mH inductance of the cartridge, which progressively isolates the 47k load resistor and leaves that is the dominant noise source.
Assuming current noise is low enough (you have to try quite hard for it to dominate), all these effects are common whatever the inherent noise of the amp is.
As a good example, the 5532 opamp has an input noise voltage of 5nV/rootHz, which is the noise of a 1500 ohm resistor. Even so, when loaded with an M75ED Shure cartridge, and after RIAA EQ and 30x gain at 1kHz, but not weighted, gives a measured -88dBu at the output, which is very quiet indeed.
Now the BC109 has an effective noise resistance of around 300 ohms - so 1/5 that of the 5532 - which is amply good enough.