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Old 6th May 2014, 02:46 PM   #101
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Did you consider this arrangement? I only put up the SNR graph for entertainment purposes. Obviously, noise would be much worse than shown. The pot should be a wire wound type, but they are expensive.
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Old 6th May 2014, 02:47 PM   #102
popilin is offline popilin  Argentina
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Originally Posted by dirkwright View Post
OK, then I hope you realize that your spice models have limited utility when it comes to modeling circuit noise. Generally, it's best to use the lowest value potentiometer (or resistor) that you can get away with in any particular circuit if you want to minimize noise.
This thread is not intended to be a treatise on noise, its only contribution, if any, is to put the volume pot on a place where it does the less harm.

I already did say this somewhere, but I repeat it again

Valves are high impedance devices, instead of with semiconductors, you can't put ridiculously low value pots, especially on preamp input where them affects input impedance.

Even more, because of dielectric anisotropy, the constitutive relation

D = ε E

Needs that ε must be a tensor, and the curve D=f(E) is called dielectric hysteresis curve, then, in order to preserve linearity we must avoid the use of enormous capacitors, so a 100 K volume pot is a good compromise between noise and capacitor size for the proposed scheme of post#1.

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Allowing a gain stage to be exposed to the full input voltage from a source such as a CD player is bad engineering practice because the distortion will be far higher and there is the potential for overloading the gain stage. This is why the volume control is at the front of the preamp.
As a designer, you must know voltage levels you work with, this is why you can compensate CD input level with a resistive divider, as a CDP/DAC has very low output impedance, this is the best place to put very low value resistors, simple right?
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Last edited by popilin; 6th May 2014 at 02:54 PM.
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Old 6th May 2014, 03:07 PM   #103
popilin is offline popilin  Argentina
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You'll have to talk to Merlin about whether or not "factory conditions" plays a large enough role to invalidate his equations. I have the article but I don't have time to find a particular reference to this issue in there right now.

Also, don't take everything you read in a book as gospel. Use your head and question everything. Authors are merely people who were able to convince a publisher that their work was worthy of publication. It does not mean that everything they say in their work is the absolute truth, or the be-all, end-all of discussion about the subject.
If you need a "gospel", here it is, one of our forum superheroes, the reference and the last resource for almost everything.

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Flicker noise can't easily be calculated as it varies so much from sample to sample; it has to be measured.
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Old 6th May 2014, 06:30 PM   #104
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Yeah, I'm not responding to this thread any more. I don't think you're interested in what I have to say. Good day.
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Old 6th May 2014, 07:42 PM   #105
sbrook is offline sbrook  Canada
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Harking back to an early post in this thread by ruffrecords where he commented on the concept that the goal is (by my interpretation ... it was lots of posts ago!) to accurately reproduce the original, not to outengineer each other to no effect.

We all tend to forget the one part of the listening chain that can make all this one-upmanship quite pointless ... It's the one part we can't do a lot about! Our ears and sinus cavities.

Most people's ears vary from hour to hour, day to day, year to year ... So much so that the chance of reproducing what you heard in your brain in a studio vs what you heard in your brain in your home two quite different things. My ears are particularly bad for that, even though as I near retirement age, I've lost very little top end. Some days I suffer from horrible "noise" ... the doctors call it tinnitus ... to the point it drowns out all manner of superfluous sound. My sinuses have different amounts of yuck in them which varies the resonances in them which can actually impact my listening pleasure and may actually cause "buzzing" at selected frequencies. All quite normal for most people according to the hearing specialist doctor.

So chasing down that last bit of noise, that last nuance of phase shift is a pretty pointless exercise when your own ears can upset the applecart. It's a part of the diminishing returns paradigm.
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Old 6th May 2014, 08:35 PM   #106
Merlinb is offline Merlinb  United Kingdom
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Originally Posted by popilin View Post
Even more, because of dielectric anisotropy, the constitutive relation

D = ε E

Needs that ε must be a tensor, and the curve D=f(E) is called dielectric hysteresis curve, then, in order to preserve linearity we must avoid the use of enormous capacitors,
Not sure what you're getting at here. It is well known that capacitor distortion is minimised by minmising the signal voltage across the capacitor. Therefore, enormous capacitors are actually better. Even the worst capacitor dielectric will introduce unmeasurable distortion in the audio band if it is enormous enough... Of course, they may be other reasons to avoid large capacitances.
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Old 6th May 2014, 08:55 PM   #107
sbrook is offline sbrook  Canada
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Pardon???? How does putting an enormous capacitor minimize the signal voltage across it?

And then you say that "even the worst capacitor dielectric will introduce unmeasurable distortion" ... That's would be like saying a capacitor in my amplifier remembers the entire 1812 Overture. And you'll ask what do I mean? And I'll say "Prove it doesn't"!

If it's unmeasurable how do you know it's there? You can hear it? I can hear the 1812 from my speakers ... even though they're unplugged!

Sorry but that just makes no sense to me!
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Old 6th May 2014, 09:09 PM   #108
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Pardon???? How does putting an enormous capacitor minimize the signal voltage across it?
Basic ac circuit theory. Any coupling capacitor forms a potential divider with the source impedance driving it and the load it is connected to. The larger the value of the capacitor, the smaller its impedance and hence the smaller the signal voltage across it.

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Old 6th May 2014, 09:29 PM   #109
sbrook is offline sbrook  Canada
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D'Oh! Of course. I wasn't thinking that way! (puts his dunce cap on with the "of course I knew that" on it! :-) )

Still it doesn't eliminate the second part of my argument. If it's not measurable is it there or is it significant.
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Old 6th May 2014, 09:34 PM   #110
Merlinb is offline Merlinb  United Kingdom
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Originally Posted by sbrook View Post
If it's unmeasurable how do you know it's there?
You start with a smaller cap so it is measurable, then gradually use bigger and bigger capacitances. You will see the distortion trace slowly disappear off the edge of the graph as bigger capacitors are used, so when you get to really big capacitors it is safe to assume it is 'there' but buried below the noise floor.
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