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18th September 2008, 09:31 AM
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#1 |
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diyAudio Member
Join Date: Jul 2001
Location: The Netherlands
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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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18th September 2008, 09:50 AM
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#2 |
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diyAudio Member
Join Date: Aug 2002
Location: Germany
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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
__________________
"I can feel what's going on inside a piece of electronic equipment. I have a sense that I know what's going on inside the transistors." Robert Moog |
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18th September 2008, 12:56 PM
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#6 |
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diyAudio Member
Join Date: Aug 2002
Location: Germany
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I like this wonder!
 Stylish... Rüdiger
__________________
"I can feel what's going on inside a piece of electronic equipment. I have a sense that I know what's going on inside the transistors." Robert Moog |
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18th September 2008, 01:05 PM
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#7 | |
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diyAudio Member
Join Date: Jul 2001
Location: The Netherlands
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Quote:
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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18th September 2008, 06:19 PM
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#8 |
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diyAudio Member
Join Date: Jan 2006
Location: Avalon Island
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What oscillator are you using for -140dB harmonics?
__________________
Just because you can't hear it doesn't mean no one can. |
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18th September 2008, 07:35 PM
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#9 |
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diyAudio Member
Join Date: Jul 2001
Location: The Netherlands
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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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