help with s/n ratios of preamps and Moving Coil

Usually the lower MC signal-to-noise ratio is due to the additional gain required leading to a higher degree of amplification of noise inherent in the circuit. Oftentimes also, the clipping headroom for MC circuits is lower also. These may not mean anything in your particular application, as you may not be amplifying the noise downstream enough for it to become a problem. A S/N ratio of >68 dB is actually quite good for a MC circuit, especially if it has a large amount of voltage gain, typically about 60 dB.

thanks. the gain on the clearaudio smart phono that i ordered is 54db for MC, and my cartridge is rated at .5mv at 3/5 cm/s. So >68db will be fine ? Its just strange that there are cheaper phono stages than the clearaudio that have a higher s/n ratio >80db for MC. but their THD and channel separation values are not as good as the clearaudio. For example the Cambridge 640p. What gives?

What gives (besides the question about sufficient gain) is that there are other factors that affect phono stage quality/cost besides s:n ration spec.

Slightly lower channel separation and slightly higher THD figures will not be as overtly apparent as a higher inherent noise level, in my view.

How much the low signal to noise ratio will bother you is somewhat dependent on your speaker's treble character and how close you sit to the speakers. Personally, I'd go for a phono preamp with as high a signal to noise ratio as I could afford, especially with a low output moving coil cartridge.

Ultimately, you have to set it up and try it to see if the highish background noise level will bother you in your particular system. Plugging it into an effective AC line filter may help it a bit as well.