Originally posted by Joe Gwinn
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In the original definition, Thevinen's theorem applies to voltage sources, not both voltage and current sources and Norton's theorem is the more general one. But neither theorem is required to merely combine impedances. (Each theorem does imply that you can combine impedances, but more is involved when there are sources. The combination of impedances follows from nothing more than the fact that such networks are described by simple linear differential equations.)
2. Especially in tube amplifiers where the ac voltage across a capacitor can be large, one of the key concerns about the sound of capacitors is their lack of perfect linearity. In a situation involving non-linearities, you cannot simply add impedances in general. So even the weaker condition which you mean, combination of impedances, does not apply.
" It does not work that way." It certainly does; how you combine non-linear circuit elements is important.
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