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In Reply to: RE: preamp vs power amp tube distortion posted by ph5y on August 11, 2014 at 08:05:32
There's more to this question than meets the eye.
In terms of quantum, one would expect an OP stage to produce appreciable more distortion, particularly at levels nearing overload simply because the load that the OP tube is subjected to, ie it needs to be optimised to deliver power into a speaker. Of course, topology plays a major part and the harmonic content can vary significantly. Not to mention the effects of NFB if utilised.
In preamp stages it's much easier to design to minimise distortion of any kind simply by ensuring that the tube is very lightly loaded. It would be extremely rare to ever come even close to signal overload conditions in any half decent design. So, unless you are specifically looking for distortion as in a guitar amp then it can be effectively ignored.
Generally speaking, the harmonic spectra of triodes (Pre or OP) displays a waterfall effect within the normal operating range, 2ndH dominating. At very low signal levels almost all distortion is 2ndH relative to higher order harmonic distortion. Nearing overload, 3rdH tends to dominate and this is quite audible. Higher order harmonics also increase significantly relative to 2nH, not normally desirable.
Naz
Follow Ups:
It would be extremely rare to ever come close to signal overload conditions in any half decent design .
Would that instance be if the grid goes positive?
Generally speaking, yes although grid current can begin before the grid goes positive. Gain stage tubes are normally only required to handle very small PP swings and the bias point should not be close to allowing the positive peaks to reach 0V.
Another way of looking at it is from the plate's POV. A good operating point would allow the plate to swing reasonably equally both pos and neg before clipping,(which in a tube is really more like compression). Other than for driver stages I can't think of too many applications where you'd need a plate swing anywhere near the point where an overload condition would be reached, typically well over 100V PP in a linear design with only modest B+.
Of course, a tube can still be quite linear even on the grid's transition
to grid current if the driving stage is up to it and the right coupling is used, so we are really generalising here.
Naz
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