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In Reply to: RE: LCR Phono stage posted by dave slagle on May 02, 2021 at 06:06:44
Nice plots Dave, and you are right about the 15pF for a normal tube circuit although if we use a ecc88 the total capacity is less then 60pF and if it is a cathode follower even a lot less.
Follow Ups:
but what about the cable capacitance that is added to the Miller of the tube?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 05/03/21
If you know how to design the cable capacity is not a big issue.
If you are using a pot for a passive preamp (and some people do) and you have a cable between it and a power amp, that cable will have at least 20pf per foot. That can be a big issue.
That what I was referring to.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I already posted that that is a stupid thing to do and the plots from Dave shows what actually happens when you add to much capacity to a volumcontrole.
Also transformer coupled volume controle have issues with to much capacity.And if people insists on a direct connection potentiometer to the next amplifier, keep de value of the pot low, LOW + the length of the cable short.
You can calculate (or measure) the hf roll off
Edits: 05/04/21
"Also transformer coupled volume controle have issues with to much capacity."Mine don't.
Edit, I should have said anything circuit or device has issues with too much capacitance.
With the normal capacitance of a tube input plus some cable capacitance my AVC doesn't have issues.Let me ask you a question. Does a signal level step down output transformer have trouble with a normal amount of capacitance? What is a TVC if not a signal level step down output transformer?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 05/04/21 05/04/21 05/04/21
Well, a normal transformer has not the high L value as a inductive volume controle and also the winding capacity is totally different, actually the whole design is different. So why you compare them that way?
The small signal output transformers that I am familiar with in vintage tubed pro studio gear are not gapped like an output transformer for a SE power amplifier. They are used in what one might call a "parafeed" arrangement (no DC in the primary) and I see no reason that they wouldn't have relatively high inductance. Also I see no reason that the winding capacitance would necessarily be high.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Correct and the opposite is the normal line output transformer with low induction and high capacity.
Totally different designs.
But i would be glad to see the measurements so we can see how the transformer in real life behaves with a capacitive load.
Edits: 05/05/21
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