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Old 18th September 2008, 09:31 AM   #1
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Default Ultra low noise measurement amp

Hi all,

A fellow member of DIYaudio.com requested the schematic of a very low noise preamp I made. I made it a year ago, maybe more, to measure harmonics, after a -100dB notch filter. Feeding this to my sound card enabled me to measure THD down to -140dB.

Here goes, any comments?
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Old 18th September 2008, 09:50 AM   #2
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Hi,
I would find it extremly instructive if you would post a photo of the stuffed board, since the physical layout is for sure one key for the impressive performance of your circuit!
Rüdiger
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Old 18th September 2008, 10:56 AM   #3
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no worries.... it's not a very sophiscicated layout...
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Old 18th September 2008, 10:58 AM   #4
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inside this wonder of technology..
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Old 18th September 2008, 12:54 PM   #5
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Input impedance is probably pretty low as each high gain JFET has about 30pF input capacitance.

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Old 18th September 2008, 12:56 PM   #6
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I like this wonder!
Stylish...

Rüdiger
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Old 18th September 2008, 01:05 PM   #7
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Quote:
Originally posted by jackinnj
Input impedance is probably pretty low as each high gain JFET has about 30pF input capacitance.


Yeh... 4 x 30pF x gain in 1st stage... hmmm about 1.2nF?.. but if you want to measure low noise, your output impedance has to be very low to begin with..
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Old 18th September 2008, 06:19 PM   #8
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What oscillator are you using for -140dB harmonics?
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Old 18th September 2008, 07:35 PM   #9
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DIY offcourse!


I never really reached -140db, but close, somewhere around -130dB... again using a passive parrallel-T notch filter to filter out the 2nd and 3rd harmonic. I routinely reached -120dB but the last 10dB take some tweaking and patience...

The amplier does enable me to reach down to -140dB but I don't have a pure enough oscilator.. that explains my interest in the Uber-pure-oscilator (see Jackini) and ultra high feedback opamp in general...

jackini are you reading this? How about a super-opamp/LM3875 combo? Can we put togher a 180dB feedback chipamp?
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Old 18th September 2008, 10:07 PM   #10
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Looks good.
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