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Phase shift probably isn't the problem it appears to be at first glance...

...the section of the PI under discussion is cathode drive so under ideal conditions the output at the anode is in phase with the input at the cathode. Under ideal conditions, g1 of this section should be at AC ground at all frequencies and phase shift would not be an issue. In the practical implementation with a finite value grid bias resistor and a finite value decoupling cap, there is phase shift approaching -90 deg at g1 of the cathode coupled section. But at all but the very lowest frequencies, say <10 hz, the amplitude of the phase shifted signal applied to g1 is minuscule such that the output at the anode is shifted by just a few degrees and not the -90 deg at g1. IOW, the signal applied to the cathode dominates the output at the anode.

The "long tailed pair" PI certainly isn't perfect but gross phase shift at the non-inverting output is typically not an issue. As you know, this style PI was used successfully in many classic designs including those from Marantz and McIntosh.


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  • Phase shift probably isn't the problem it appears to be at first glance... - Steve O 14:37:23 08/02/17 (0)

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