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In Reply to: RE: Frequency Compensation posted by 91derlust on March 3, 2015 at 19:43:01:
Yes.
A high pass filter, like bypassing a cathode, will shift phase one decade above the -3db point. So if the cathode is bypassed to 2Hz the amplitude and phase will be correct at 20Hz. If the cathode is only bypassed to 7Hz then the phase is shifted all the way up to 70Hz.
A low pass filter, like the output impedance of a driver stage vs. the Miller capacitance of the output tube, will shift phase a decade below the -3db point. If the low pass (output impedance vs. Miller capacitance) has a -3db point of (let's say) 50kHz, the phase will be shifted all the way down to 5kHz.
If we want the amplitude to be flat and the phase to be not shifted, 20-20kHz, then all the low pass filters need to have their -3db points at about 2Hz and all the high pass filters need to have their -3db points at about 200kHz.
Of course there are other causes within a system that limit the frequency response and cause phase shifts. Like the output transformer, speakers, etc. But that's not an excuse to design our stages wrong.
I believe we should take care of the filters that we can take care of and not look for excuses not to.
Loss of amplitude and phase shift within a system will add. Minimizing them, where we can, will pay off.
Tre'
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Follow Ups
- RE: Frequency Compensation - Tre' 21:15:56 03/03/15 (1)
- Thanks for the clear explanation Tre. (nt) - 91derlust 23:39:54 03/03/15 (0)
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