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In Reply to: Tell me what you think posted by Russ57 on July 11, 2006 at 08:57:34:
We are mixing terms a bit here. In theory, NFB is applied as a ratio and therefore should affect all levels by the same percentage. Whether it does in reality is another thing. Obviously NFB can be applied to any type of amp using any tube but you are correct that zero feedback Class A will indeed produce lower distortion at low levels.Regardless of tube type, Class A is inherent in SE but could be used in PP whereas Class AB refers only to PP. AB suffers most on the transitions from A to B and that translates into higher % distortion at low levels due to the fact that all tubes are very non linear near the cutoff point. This could be improved by reducing the applied load but it would have to be much less than would normally be used in the OP stage of a power amp. The problem with this type of distortion, as you rightly put it, is that it occurs at levels where we typically do a lot of our listening.
If there is a consistency in my book it's that those who like PP tend to prefer them for their high power or their sound at higher OP levels. Few seem to choose them for their sound at low levels whereas for SE it's the opposite. This does seem to coincide somewhat to the distortion characteristics of the two topologies ... at least as far as ratios go.
The high output impedance of pentodes compromises the OP stage too much and considerable NFB would have to be used to improve the situation. In this regard my comment was borne from the observation that every time I've triode strapped any tube it sounded better and since no other changes were made I wondered how much of this was due to the fact that triode mode offers lower OP impedance and therefore lower distortion by default. Personally, I think it's not the primary reason and would like to hear other theories.
I also think that NFB is the least damaging to amplifiers that are inherently wide-band before NFB is applied. In other words, I think it can somewhat successfully be used to correct for non-linearities but probably causes too many phase related issues where its primary use is for extending bandwidth.
Naz
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Follow Ups
- Re: Tell me what you think - Naz 01:27:42 07/12/06 (0)
- Re: Tell me what you think - drlowmu 21:13:15 07/12/06 (2)
- Re: Tell me what you think - Naz 21:00:48 07/13/06 (1)
- Re: Tell me what you think - drlowmu 07:12:10 07/14/06 (0)
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