What would be the typical noise floor of a tube preamp or amp?
Example #1: A quick look at the McIntosh website shows that the S/N ratio for the current MC75 is 110, which is pretty good for a tube amp.
The larger point I was trying to make is outlined in the link provided earlier in the thread, specifically the "distortion under test" area.
"Although this is not intended to be an exhaustive examination of all available semiconductors or tubes, the resulting frequency spectra lead us to some conclusions that experienced audio designers have often remarked upon in the past.
[/size] Transistors operating on low-voltage supplies tend to have higher spectral distortion components than tubes.[/l]
- If we go to high-voltage transistors, operating on supplies comparable to those of the tubes, the distortion products are less objectionable. Unfortunately, the noise floor of such devices is much higher. The IRF822 was very triode-like in distortion yet suffered from a noise floor some 30 dB higher than that of the triode.
- No other active device possesses both the low distortion products and the low noise floor of the medium-mu triode--albeit at the expense of voltage gain.
- The distortion products of transformers are much lower than those of active devices, yet quite different in character. Note that the odd-order harmonic products tend to be higher in level than the even-order products--exactly the reverse of the tubes and transistors.
[/size]It should be obvious that these simple circuit designs can be improved upon, by using differential topologies with constant-current loads and negative loop feedback.
[/size]It should also be obvious that the same techniques can be applied to transistors or to tubes; and if this were done, the triode would continue to enjoy some advantages over the semiconductors--and the pentode, for that matter."