help with s/n ratios of preamps and Moving Coil

Guys - i hope someone can clarify this for me. Why is it that most preamps/phono stages are rated at a lower S/N ratio for Moving Coil carts vs their Moving Magnet counterparts. I just ordered a clearaudio pre-amp for a Benz low output cartridge, however i realised that s/n ratio says > 68 db.. Is that enough, dont i want something >80 for MC carts.. i cant find any advice on this on the www

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.
Yes, everything will be plugged in via a AC line filter. I am used to a much cheaper NAD pp2 pre-amp with this cartridge and have no idea what the s/n ratio is but it sounded good to me.

I can only hope the that clearaudio smart phono will be a noticeable improvement. Audio guys at the store said to completely ignore the numbers and concentrate on the overall quality of the preamp and that clearaudio makes really good stuff. I guess, if it doesnt work ill just have to switch to a MM cart, the S/N spec for which is higher on this preamp. Sorry im very new to vinyl, and not having the new preamp for another week or so is making me ponder all this stuff.
oh and btw.. does the S/n ratio include the noise level of the pre-amp itself. for example, some cheaper preamps may create their own noise levels and their s/n ratio's maybe higher taking that into account; while a more expensive preamp may be dead quiet itself and has a slightly lower S/N ratio. Does this make sense ?
The published s/n ratio for the phono stage does not include the s/n ratio of the line stage itself.
I guess the other thing to consider is @ what frequency manufacturers rate the S/N ratio for MC carts. Ive seen some examples rated at 20kHz and some at 1kHz.
1KHz is the more relevant number. at 20KHz the noise is going to be a lot less due to the rolloff of the RIAA curve, I.e. the figure will look better.
Rlawry is correct in explaining that additional gain leads to a decrease in S/N ratio. Typically, the total gain needed by a MM cartridge is around 50 dB, whereas an MC cartridge needs 70 dB or more. Due to complex laws of physics, the amount of engineering effort necessary to maintain S/N ratios with this 20 dB extra gain is enormous.

On the other hand, the EIA published a standard for measuring S/R ratios (among other things), as a common ground to compare the noise performance of preamplifiers. In the case of Phono MC preamps, the standard demands the S/N ratio to be measured with an input signal of 0.5 mVrms at 1 kHz, and an input termination of 10 ohms. Most manufacturers simply ignore this standard, prefering their own way of expressing the S/N ratio trying to favor their product. The problem is, a S/N ratio has absolutely no meaning unless the input level and termination are explicitely defined. For example, a preamp could be described as having 80 db of signal/noise ratio with a 5 mV input signal. The same unit would have a 60 dB S/N ratio with a more realistic 0.5 mV input. A very audible 20 dB difference, no less! This could explain the discrepancies mentioned by Aquaanox.

Some people try to minimize the importance of having very high S/R ratios in audio playback, but I think this is wrong. After all, noise is not part of the music and minimizing it can have a vast improvement in detail and ambience retrieval, leading to more realism in musical playback.