S/N ratios

[KevinL]

I've built 2 of ea of the pre's except for the T15. I just installed a LynxTwo converter which has 6 inputs and the meters have a scale of -114db. Anyway, I connected the N72's, J99's, and A12's to the Lynx's inputs. The N72's are closest to the power supply, then J99's then the A12's. With all gains at minimum, and no input to the pre's, the Lynx's meters are showing the noise level of the N72's at about -80db, the J99's about -95db and then the A12's even lower. I know the visual meters aren't an accurate measurement but the noise level falls off as each module gets further from the PS. The noise isn't really audible but I thought it was interesting to see and was wondering if this is normal. If I turn off the power to the preamps, all visual noise on the meters disappears.

I didn't look at the C84's because I have them connected to a different pair of converters that don't have the metering of the Lynx. They are actually the closest to the supply. I'll check that out tonight with the Lynx.

Long question short....what is the S/N ratio of each type of pre? Is this noise generated by the PS or the preamps themselves?

[dkatz42]

If you leave the inputs floating, any measurements you make will be meaningless. They need to be properly terminated.

[KevinL]

Terminating the inputs showed a negligable difference. I'm using the crude metering of the Lynx's mixer panel. This is what I'm reading for each pre in order from the power supply across:

C84's<114db
N72's~85db
J99's~110db
A12's~114db

I'm surprised that the A12's have less noise than the J99's. C84's had the least amount of noise and closest to the supply so I think I can rule out supply noise.
Again it's just number's. These pre's are all very quiet when it comes to audible noise.