Looking, QST construction article about measuring noise of a resistor - Page 3

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Re: Looking, QST construction article about measuring noise of a resistor
On 2/25/2019 1:05 PM, Winfield Hill wrote:
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  I'll do that, I need to pull your book of the shelf more often.

My curiosity started when I watched a video of a guy using an LM386 as a  
  amp for a parabolic reflector and a microphone. I tried to tell him  
his circuit was noisy and he could do better with a quiet front end. He  
argued it was very quiet. I gave him a search with 100 links to how  
noisy the LM386 is, he didn't budge on his it's very quiet.
  Hard to tell from his video, but it's seems like his electronics are  
noisy to me. >
https://www.youtube.com/watch?v=-dpMFqeQrC4




                               Mikek

Re: Looking, QST construction article about measuring noise of a
amdx wrote...
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 Haha, using a speaker amplifier as a low-noise preamp!  But
 I suppose if you have a parabolic reflector to concentrate
 the noise, sorry, the signal, it might be more noisy than
 the amplifier.


--  
 Thanks,
    - Win

Re: Looking, QST construction article about measuring noise of a
On 2/25/2019 3:29 PM, Winfield Hill wrote:
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  I wonder how parabolic his reflector really is? It's a lamp shade.

                                 Mikek

PS, Parabolic reflector is what they use at football games, would a  
exponential horn be better? Even if unwieldy.
  Ok, did some reading on exponential horns, they work, but have their
problems like a parabolic dish.

Re: Looking, QST construction article about measuring noise of a

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shade.
their

  Get Mr. Klipsch to fashion one for you.

https://www.klipsch.com/heritage-speakers .

  IIRC one of his early models had 36 feet of tuned enclosure cavity.

  Except instead of an output transducer, he makes a acoustic  
gathering horn for you.

  Remember those toothless old men with the little horn tube pressed  
into their ear?

Re: Looking, QST construction article about measuring noise of a
On 2/25/2019 6:46 PM, amdx wrote:
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Horn Construction article,
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                                       Mikek





Re: Looking, QST construction article about measuring noise of a resistor
amdx wrote:

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** What about the electronics INSIDE the electret microphone ??  

 Likely to generate more noise then the LM386 input stage does.  

FYI:

 Low noise microphones typically have large diaphragms and are true condenser types - not some cheap electret capsule.  



....  Phil  






Re: Looking, QST construction article about measuring noise of a resistor
On 2/25/2019 3:33 PM, Phil Allison wrote:
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  That's what I'm finding as I read about low noise microphones. But it  
seems like you need to spend several hundred if not thousand dollars to  
get one of those low noise large diaphragm microphones.
  The guys recording nature sounds seem to have interest in and info  
about low noise mics.
                             Mikek


Re: Looking, QST construction article about measuring noise of a resistor
amdx wrote:
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** The Australian built Rode NT1-A is one such and not too expensive.  

   But you will need a supply of phantom power for the mic.  


....    Phil

                  

Re: Looking, QST construction article about measuring noise of a resistor
On Monday, February 25, 2019 at 10:22:11 AM UTC-5, amdx wrote:
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I  think the article was in " Nuts and Volts  " magazine.

                                      Dan

Re: Looking, QST construction article about measuring noise of a resistor
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"Build the JCan to Measure Resistor Noise" appears in the July 2007
issue. The author, Joe Geller, kindly makes it available for free at his
website. http://www.gellerlabs.com/ACV/JCan%20NV%20article.pdf


Thank you, 73,

--  
Don Kuenz KB7RPU
There was a young lady named Bright Whose speed was far faster than light;
We've slightly trimmed the long signature. Click to see the full one.
Re: Looking, QST construction article about measuring noise of a resistor
Don Kuenz wrote...
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 I was surprised at Joe's using a BF244A JFET for the input stage.
 This VHF/UHF amplifier JFET isn't what I would call a low-noise
 JFET, but then I saw that he's measuring noise voltage on up to
 (and above?) 1 megohm resistors, and super-low input capacitance
 was one of his goals.  1meg and 3pF has a 50kHz response rolloff.

 My answer to that situation would be to measure noise current,
 rather than voltage.  Yes, the Johnson noise current from a 1M
 resistor is only 0.13pA/rt-Hz, or 13pA for a 10kHz bandwidth,
 but it's not hard measuring into a short with TIA amplifiers.
 I've got free RIS-617 PCB boards for anyone who wants to try.
https://www.dropbox.com/s/6pjvndsnjah606k/RIS-617E_A-D%285%29_E-versions%285%29_12pgs.pdf
 Choose the version of bare PCB you'd like, customize component
 choices, and warm up your iron.  email: snipped-for-privacy@yahoo.com


--  
 Thanks,
    - Win

Re: Looking, QST construction article about measuring noise of a resistor
On Tuesday, February 26, 2019 at 7:19:01 AM UTC-5, Winfield Hill wrote:
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Wow! Awesome.  
Say in the second one (RIS-617B-1)  
The photodiode boot strap (Q1) goes through the cap multiplier
low noise bias supply (Q2)?  Does there have to be some gain there?  
I'm trying to sort out the various R and C values.  
(Gain needed to overcome C10/C15 current loss?)  

Thanks
George H.    
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Re: Looking, QST construction article about measuring noise of a resistor
George Herold wrote...
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 The bootstrap gain is nominally 1.0, but actually slightly
 less, which helps to insure it doesn't oscillate.  This is
 to remove most of the bootstrapped sensor's capacitance
 from the amplifier's e_n-Cin noise, i_n = e_n Cin 2pi f.
 See AoE-III section 8.11 and page 538.  The bootstrap gain
 value isn't critical.  A source follower running at 3mA,
 with 4.7k load, and emitter follower should be above 0.95.
 If it's only 0.8, then 80% of that noise source is removed.
  


--  
 Thanks,
    - Win

Re: Looking, QST construction article about measuring noise of a resistor
On 26 Feb 2019 04:18:52 -0800, Winfield Hill

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There are a couple of better opamps around, like AD8067 or LT1793.
Maybe something even better.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Looking, QST construction article about measuring noise of a resistor
On 2/25/2019 9:53 PM, Don Kuenz wrote:
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  Very good, thank you for finding that.
                                 Mikek

Re: Looking, QST construction article about measuring noise of a resistor

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  The first article I looked at in that mag was the one about making  
in inband gated synch video decoder for the early analog video  
"scrambling" methods.  I think it was like Feb '82.  I know it was a  
February issue.

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