[LAU] Measuring preamp performance (was: USB audio interface support)

On Thu, 26 Nov 2009, fons(a)kokkinizita.net wrote: /snip/ /snip/ Fons, thank you for your clear explanation! I got quite an education from that one post alone. Makes me want to go out and measure all the preamps in my environment. So how do I do it? Just off the top of my head, I'm thinking of something as simple as placing a 150ohm resistor across the input of the preamp, turning the gain all the way up, and measurnig the p-p voltage at the output. Comparing that to the '0dB' reference level would give the EIN. Am I even close? On a related note, what is the maximum dynamic range of a 24-bit a/d converter? Each 'bit' of precision represents a doubling of the maximum input voltage, and I remember being told somewhere along the line that 3dB represents a doubling, but I don't remember if that's voltage or power. 3*24 is 72dB, but that sounds too low. I remember being told that 16-bit CD audio is capable of something like 100dB dynamic range, so I would expect 24bit to be capable of something more like 124dB. And the '0dB reference level' isn't the full-scale 24-bit clipping level, is it? So the 'working' dynamic range would be somewhat less. I realize I'm babbling on... I guess the ultimate question is how good is good enough? WHat is the EIN figure that would be unmeasurable with a 24-bit A/D converter? What level (expressed in dB wrt '0VU' is required to make the transition from 0x000000 to 0x000001? I'm displaying my total ignorance here. I don't even know if audio is represented by signed integers, or unsigned integers. Is a shorted input represented by 0x000000, and a minimal signal would oscillate between 0x000001 and 0xFFFFFF, or is it represented unsigned, where a shorted input would be 0xEFFFFF, and a maximum signal would oscillate between 0x000000 and 0xFFFFFF? What's the binary representation of the '0VU' reference level? -- Rick Green "Those who would give up essential Liberty, to purchase a little temporary Safety, deserve neither Liberty nor Safety." -Benjamin Franklin "As for our common defense, we reject as false the choice between our safety and our ideals." -President Barack Obama 20 Jan 2009