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In Reply to: RE: LCR Phono stage posted by walge on May 01, 2021 at 11:49:16
Just measured a PEC potentiometer 50K with a output source of the amplifier of 500 Ohm. Capacitive load 100pF.
Correction: the 100 pF is wrong, i measured with my 1:10 probe which is 15-17pF
This is the worse setting at 3 o'clockMeasured at 12 o clock setting
And this is measured at the 9 o'clock setting
Looks "bad" but look at the scale, flat within 0,3dB.
Edits: 05/01/21 05/01/21 05/01/21 05/01/21 05/01/21 05/01/21 05/01/21 05/01/21Follow Ups:
The problem with measurements and sims for that matter is they only show you what the presenter wants you to see. You were trying to show how perfect a 50K pot was but fed it into an unrealistic 15pf load. Using the 1X probe and its ~100-150pf capacitance is much more realistic of what may happen in the real world. In the plot below a 50Ω source is driving a 50kΩ pot set to -6dB. The green trace is 15pF, blue 30, red 60pf cyan 125pf Violet 250pF and gold 500pF.
Granted this represents the absolute worst case setting scenario (-6dB) and explains why many opt for a 10K pot for attenuation which makes things appear much better and 1K would be nearly perfect but then we are back to the question of how to properly drive a 1K volume control and the answer is obviously a cathode follower loaded with a CCS and we are back on topic. :-)
10K pot set to -6dB into varying capacitances
The approach the the tests are done just to show what the presentr want is out of the reality.
When we test the equipmente on Audioreview magazine these tests follows ( in part) the intl indication then there are other test but are well documented and replicable.
Otherewise the reputation will be lost.
Also when I present on some forum the circuit and measurement.
The REAL test are replicable and there aren't nothing to manipulate.
And, in my opinion, the simulation are the main problem that we can met in forums; there are thousand of "designer" that presents lot of circuit tested only with simulations.
And is is almost impossible to see the proto of that strange things.
Some of them are a strange mix between tubes and sand.
After that I will return on topic
Walter
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.
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
Please send the test at - 6dB from the max position of volume.
Which brand is the pot?
Walter
Edits: 05/01/21
Btw, the test was not done with 100pF load but with my 1:10 probe which is about 15- 18pF. So the 3 o'clock setting is about what you asked for (-4dB, almost -6dB )Walge, the capacitance load of the potentiometer largely determines the frequency response. So the capacitance load of the next stage is very important.
The great advantage of a normal potentiometer is the absence of all kinds of resonances at higher frequencies. At the same time, the impedance of a normal potentiometer is still high, that's is why professional equipment with tubes usually use lower values and also compensation circuits.
Edits: 05/01/21 05/01/21 05/02/21
Hi
I know well the problem and, when possible I use a low value of pot.
In the last numbero of Audioreview magazine there is a presentaion of a potential kit of pot with 6 rel่ and a (almost) costant impedance ( done by a collaborator).
Will be the possibility to change the value desired.
Walter
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