Originally Posted by
RothwellAudio
I agree that it is preferable to have a shunt resistor in the network. There are two problems with using only a series resistor.
The first problem is that you need to know the input impedance of the power amp (or whatever it is you're feeding the signal into) to know what series resistor is required, and you will only be able to use the attenuator with that power amp (or one with the same input impedance) to get the specified attenuation.
The second problem is that the input impedance of the power amp (or whatever) may be listed as, say, 47k in the spec sheet, but is it really 47k? Probably not, at high frequencies anyway. More likely than not there will be an RF filter at the input, and raising the source impedance may turn the RF filter into a "top end of the audio band" filter.
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Actually, there's a third problem with the series resistor - Johnson noise. All resistors have a noise voltage called Johnson noise - and the bigger the resistor, the greater the noise. It may not be significant with 10k resistors and signals at line level, but get up to 10M and the noise can't be ignored.