John Curl's Blowtorch preamplifier part II - Page 2039 - diyAudio
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Old 5th February 2012, 08:30 AM   #20381
PMA is offline PMA  Europe
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He loves to refer to Re 1Vrms INPUT. Then he gets for his phono stages fantastic numbers compared to competition, who refer noise to output ....

Pretty confusing, but probably it works.
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Old 5th February 2012, 08:48 AM   #20382
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Quote:
Originally Posted by john curl View Post
[snip]The second, equally difficult problem is the TRANSIENT PERFORMANCE of the regulator. You see, a feedback regulator is essentially unstable, because it sees a capacitive load. Therefore, when you challenge the regulator with a transient, it will not be able to easily suppress it. [snip].
John, the output cap is there for two reasons. First of all, it makes the regulator stable. The reg is definitely NOT 'essentially unstable'. Secondly, it takes the brunt for transient loads.
Both improve transient response.

jan
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Old 5th February 2012, 09:03 AM   #20383
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Hi,

Quote:
Originally Posted by Bonsai View Post
-147dBVA? Referenced to the output? Numbers like that need explaining.
Quote:
Originally Posted by ThorstenL View Post
-147dBVA Ein
The number does need any explanation to anyone familiar with international standard units and electronics. It is complete and clear. It may have been a source Impedance (0R) (bandwidth is covered by the A-Weighting).

However for those who slept through the relevant classes:

Quote:
Originally Posted by Wikipedia
So the above clearly states that the noise is input referred, A-weighted and 147dB below 1V. And no-one with a background in electronics should have any problems with it.

And to answer PMA's charge of being confusing, I use only internationally standardised measurements and ways to state that are unambiguous. But I guess you cannot please some people.

Anyway, if the spec is confusing please take a remedial in electronics or post in the non-technical discussion groups.

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Old 5th February 2012, 09:10 AM   #20384
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Jan,

Quote:
Originally Posted by janneman View Post
John, the output cap is there for two reasons. First of all, it makes the regulator stable. The reg is definitely NOT 'essentially unstable'.
If the regulator is "not essentially unstable" then it is essentially stable. If it is essentially stable it does not require any specific load conditions.

If the regulator requires a capacitive load, then it is not stable.

Most integrated circuit regulators and most discrete or semi-discrete regulators are not stable with the presence of certain capacitive loads and require large capacitive loads.

So your statement in the original is self contradictory.

Either the Regulator is stable and does not need a capacitor to be stable, or it is unstable and requires the capacitor to be stable.

Ciao T
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Old 5th February 2012, 09:30 AM   #20385
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Quote:
Originally Posted by ThorstenL View Post
Jan,



If the regulator is "not essentially unstable" then it is essentially stable. If it is essentially stable it does not require any specific load conditions.

If the regulator requires a capacitive load, then it is not stable.

Most integrated circuit regulators and most discrete or semi-discrete regulators are not stable with the presence of certain capacitive loads and require large capacitive loads.

So your statement in the original is self contradictory.

Either the Regulator is stable and does not need a capacitor to be stable, or it is unstable and requires the capacitor to be stable.

Ciao T
Man, you should go into politics!

The cap modifies the open loop amplitude and phase response to obtain sufficient phase/gain margin to make it stable when the regulation loop is closed. A simple case of compensation.

jan
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Old 5th February 2012, 09:39 AM   #20386
Bonsai is offline Bonsai  Taiwan
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Quote:
Originally Posted by ThorstenL View Post
Jan,



If the regulator is "not essentially unstable" then it is essentially stable. If it is essentially stable it does not require any specific load conditions.

If the regulator requires a capacitive load, then it is not stable.

Most integrated circuit regulators and most discrete or semi-discrete regulators are not stable with the presence of certain capacitive loads and require large capacitive loads.

So your statement in the original is self contradictory.

Either the Regulator is stable and does not need a capacitor to be stable, or it is unstable and requires the capacitor to be stable.

Ciao T

Oh dear.

This such rudimentary stuff Torsten and you should know better. Read the data sheet and study a bit feedback theory and compensation techniques as janneman suggests above. With statements like that, it's no wonder f/back has a bad name - so now it's going to regulators that are no good.

Re your -147 figure ref 1V, how do you arrive at such a low figure? Anything below -120dBVA needs to be carefully qualified. Throwing figures like that around without an explanation is to be rightly questioned. I think PMA is right to challenge you on this. Care to share the details?
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Old 5th February 2012, 09:50 AM   #20387
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Quote:
Originally Posted by ThorstenL View Post
First, resistor tolerances that are better than those of the TL431 are readily available at a cost of fractions of a cent per pcs. So the "improved accuracy" argument is highly specious.
Provide reasoning then, don't just make claims. In my estimation, tolerances typically add as sum-of-squares. Given typical accoracy of el-cheapo TL431 is 2% and resistors are typically 1% this does in fact still degrade the initial 2% tolerance of the TL431 to a (very) worst case 4%. Using 0.1% resistors would improve on that but having no resistors at all remains more accurate. A 0.1% resistor costs about the same as a bog-standard TL431. Unless you can show reasoning to the contrary?

Quote:
I repeat, "stacking" references is a very stupid way to get low noise, as it does not get low noise and only unnecessarily increases component count.
Where did I claim that my aim was to get low noise? I'd certainly steer clear of TL431 altogether if I was serious about noise. The component count of 4 TL431s in series is precisely the same as that of a single TL431, 2 resistors and a bypass cap. QED.

I've attached a plot from Linear Tech's LT317A series regulator - the principle is the same as for the TL431 except the bandgap sits at 1.25V rather than 2.5V. As the output voltage goes up so the error contribution from the resistors increases too.
Attached Images
File Type: png LT317A.png (38.5 KB, 163 views)
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Old 5th February 2012, 10:22 AM   #20388
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Quote:
Originally Posted by ThorstenL View Post
Sy,

Hence stacking references has not netted us an improvement in noise, it has only dis-improved the noise.

Ciao T
No, what SY said makes sense, for the current source shown below. Doubling the voltage makes you increase the degeneration resistance R1, reducing the gain by -6dB. Or as SY put it, increasing the signal by 6dB.

Lower gain means lower noise, since the noise only increases by 3dB, the overall circuit has 3dB lower noise.

Click the image to open in full size.
Attached Images
File Type: jpg CCS_ref.jpg (58.1 KB, 325 views)
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Last edited by Johnloudb; 5th February 2012 at 10:26 AM.
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Old 5th February 2012, 11:05 AM   #20389
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Quote:
Originally Posted by Johnloudb View Post
No, what SY said makes sense, for the current source shown below. Doubling the voltage makes you increase the degeneration resistance R1, reducing the gain by -6dB. Or as SY put it, increasing the signal by 6dB.

Lower gain means lower noise, since the noise only increases by 3dB, the overall circuit has 3dB lower noise.
It also makes sense for a normal voltage regulator that compares the output of a regulator to a reference. The voltage regulator amplifies the reference voltage- drop the gain by two, increase the reference voltage by two, and you have that 3dB noise advantage again. Yes, you could cover that up by incompetently specifying a noisy error amp, but that's the stuff of polemics and cartoons, not engineering analysis.
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Old 5th February 2012, 12:27 PM   #20390
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Jan,

Quote:
Originally Posted by janneman View Post
The cap modifies the open loop amplitude and phase response to obtain sufficient phase/gain margin to make it stable when the regulation loop is closed. A simple case of compensation.
Correct. So the regulator is inherently instable, unless compensated externally.

Simple and short.

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