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In Reply to: Re: Common Mode ( long ) from Dennis posted by mikeyb on December 4, 2006 at 09:36:12:
Exactly what parts of music are 'common mode' and what are 'differential mode'?Neither. However, PP tubes will be mismatched due to varying capacitance at given frequencies. Matching is usually a static measurement, but also a fundamental assumption behind a PP gainstage. This intrinsic variation between tubes is a natural consequence of the manufacturing process, and has an impact on the gain of those stages at critical frequencies. This variation is in effect differential mode rejection.
This only considers the gain stage in isolation, with a perfect power supply. if the powersupply isnt perfect, then you will have a variation of one output tube due solely to the modulation of the powersupply, in the other tube, which is inverting in relation to the signal. This is common mode rejection, and could be sufficient to reject some smaller details.
The essentials of good layout, are taking appropriate measures to prevent hum (tightly twisted heater wires, routed to avoid inducing hum into the signal wiring) and as far as possible, signal path lengths that are the same on both arms of the PP gainstage. It goes without saying that action taken to isolate each 'arm' of the PP gainstage will reduce power supply modulation, and will help to let more of the music out.
Choosing well matched parts, and tubes will further help the cause along.
Owen
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Follow Ups
- Re: Common Mode ( long ) from Dennis - Owen 11:32:10 12/04/06 (1)
- Thanks - mikeyb 12:29:05 12/04/06 (0)
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