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Old 4th February 2012, 11:09 PM   #20361
SY is offline SY  United States
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OK, I thoroughly don't get your point.
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Old 4th February 2012, 11:20 PM   #20362
gerhard is offline gerhard  Germany
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Quote:
Originally Posted by Joshua_G View Post
You are taking about 3 *identical* devices in series. Identical devices wasn't mentioned in the post I replied to. When talking about LEDs as reference voltage, possibly different LEDs have different noise and possibly a certain LED of higher voltage may have lower noise than another certain LED of lower voltage.
No, he wasn't. He was talking about a *4 voltage gain that happens when the
feedback resistor ratio is 3:1 in a non-inverting amplifier.

With regard to LEDs, red ones usually are the best (noise voltage normalized to Vf) and
blue ones are the worst. Red ones have lower Vf than blue ones.

Disclaimer: I have not yet tested infrared etc.

regards, Gerhard
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Old 4th February 2012, 11:35 PM   #20363
gerhard is offline gerhard  Germany
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Originally Posted by john curl View Post
Gerhard, you have NO idea of what I have to do to make a quiet power supply, THAT ALSO sounds good.
That retreat to your personal taste does not come unexpectedly.



best regards, Gerhard

The Silence Of The Amps, what could one desire more?

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Old 4th February 2012, 11:45 PM   #20364
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Quote:
Originally Posted by john curl View Post
Gerhard, you have NO idea of what I have to do to make a quiet power supply, THAT ALSO sounds good.
well, thank you John! Now you're talking business. I don't believe in this 'low noise super regulator' nonsense as well.
Please elaborate on line rejection, load rejection, stability requirements etc..
I would love to read your opinions and insights!
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Old 4th February 2012, 11:49 PM   #20365
gerhard is offline gerhard  Germany
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Quote:
Originally Posted by SY View Post
That's the input-referred noise voltage density spec. Will that be your output noise?

I would assume that you'd double the input current noise density as well, which will make the reference slightly trickier.
For high enough loop gain, that will be the output noise (assuming the ref is clean).
The input error is regulated to zero.
And the noise of a power transistor in the loop will be killed, too.


best regards, Gerhard
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Old 5th February 2012, 12:28 AM   #20366
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OK, I thoroughly don't get your point.
So be it.
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Old 5th February 2012, 12:34 AM   #20367
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Quote:
Originally Posted by SY View Post
OK, I thoroughly don't get your point.
Wait a bit SY, he means some classified devices, but we'll get him soon!
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Old 5th February 2012, 12:34 AM   #20368
Bonsai is offline Bonsai  Taiwan
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A quiet reference is easy to make with a noisy Zener (you don't need ppm drift performance for audio). Just filter it with an RC. 10k and a 100uF or values that give a low cutoff frequency. Then buffer and off you go. So what's all the fuss about?
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Old 5th February 2012, 12:41 AM   #20369
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Quote:
Originally Posted by Joshua_G View Post
You are taking about 3 *identical* devices in series.
No, if you read what I wrote you'll see I was talking about 4 identical devices in series.

Quote:
Identical devices wasn't mentioned in the post I replied to.
And indeed its not absolutely essential for the point. But its fairly poor engineering practice to put a high noise device in series with a low noise one. If you care about noise you won't **** away the advantages of the lower noise (read expensive) device by combining it with a higher noise one. OTOH TL431s are dirt-cheap so if you don't care about noise so much you've got yourself a very cost-effective reference.

Quote:
When talking about LEDs as reference voltage, possibly different LEDs have different noise and possibly a certain LED of higher voltage may have lower noise than another certain LED of lower voltage.
Yes, agreed. Orthogonal to the original point.

@gerhard : Red LEDs also had the lowest dynamic impedance, when I tested.
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Old 5th February 2012, 12:54 AM   #20370
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Quote:
Originally Posted by Bonsai View Post
A quiet reference is easy to make with a noisy Zener (you don't need ppm drift performance for audio). Just filter it with an RC. 10k and a 100uF or values that give a low cutoff frequency. Then buffer and off you go. So what's all the fuss about?
I guess, the fuss is about secret knowledge of filtering and buffered discovered by some genius back in 1050'th. "Learn and grow!" (C)
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