How do I measure - 140 dB noise levels?

[PMA]

Just to compare, -80dB from CD

 

[PMA]

and -100dB from SACD

 

[Conrad Hoffman]

1uV below 2V is about 126dB. Rest assured that you probably won't find conventional batteries this quiet. Even Weston standard cells will start to have noise at this level, and the value has to be filtered and averaged to establish the actual voltage. Sure, building a wide band circuit with that kind of noise level is difficult, maybe impossible if the source impedance high and you look at the inherent thermal noise. But, once you narrowband the circuit, it's just not that difficult or expensive. That's why high gain preamps have switchable input filters. The higher frequencies aren't very important in this case, because batteries tend not to generate HF noise, and there's a good chance your circuitry will use bypass caps.

 

[jackinnj]

You can make a preamp with a pair (matched) of 2sk170's -- this article by Dennis Colin has a mistake on the schematic -- R8 should be 51.1k, there is also a missing connector between U2b's output and switch S3 -- I don't know why AX hasn't published the corrections on the website.

http://www.audioxpress.com/magsdirx/ax/addenda/media/colin2764.pdf

Colin has also written a nice article on a low noise phono preamp using the AD797.

You can also use a Linear Tech LT1028.

If you are serious about getting measurements you probably should invest in a USB/PCI/RS232 data-logger. Even some of the Radio Shack DVM's have this.

A-Weighting or CCIR/ARM 486 weighting results in something more applicable to the noise which we actually hear -- but this is beyond the scope of this thread.

FWIW, I use a Christmas Cookie tin to contain the device in which I measure low voltages --