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In Reply to: RE: Frequency Compensation posted by Triode_Kingdom on March 01, 2015 at 15:10:02
does this differ from what DF and JM are doing with multiple bypass caps? are they going to have a simliar effect when paralleled with multiple values?
Follow Ups:
TK is only bypassing the cathode at the high frequencies. By my calculations, about 5kHz and up. That stops the current feedback for the high frequencies so the gain at those frequencies is higher.
What DF does is different. DF bypasses the cathode at all but the lowest frequencies.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Does that mean that the phase shift issues mentioned elsewhere occur higher up frequency, perhaps where the ear is more sensitive?
regards,
91
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'
Have Fun and Enjoy the Music
"Still Working the Problem"
.
So essentially, if I understand correctly, each of the many bypass caps DF uses would have a simliar effect... (except bosting a multitude of frequency ranges)
No, the total uf value of all of DF's caps added together is the cathode bypass capacitor value.Using smaller "side caps" is a different thing.
Bypassing a tube's cathode is one thing, "side capping" the cap used to bypass a tube's cathode, in an attempt to change or "fix" the "deficiencies" of capacitors, is another.
We also see side capping used on power supply caps and coupling caps in a attempt to improve some attributes of the main cap.
Bypassing a cathode (to the lowest frequency of interest) is about preventing local current feedback and restoring the tube's plate resistance.
Bypassing a cathode to less than the lowest frequency of interest is "Frequency Compensation".
Again, side capping is a different thing.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 03/02/15
Thanks Tre'!
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