I want a soundcard with top quality Analog to Digital Convertors.
And it must be cheap , too !!
Any idea ?

Do you already have a phono preamp? What is top-quality for you (what turntable/equipment do you have - that's a good indication)?

Edit: And... what's cheap for you? More info, please

The M-Audio Audiophile 24/96 works well for me for this purpose (you might want to check out the Audiophile USB for a lower noise floor).

My understanding is that ADC's are simpler devices than DACs, thus quality isn't an overriding factor like it is with a DAC. I was previously using a Soundblaster PCI128 to record vinyl to digital, and it added a bit of audible low-level 'whine' from EMI inside the PC -- this is totally gone with the Audiophile 24/96. You might want to consider this card (at around $150 in the USA), among others like the M-Audio Revolution 7.1 at around $100.

I found that the Soundblaster live on my old machine had some odd impulses showing up
on the resulting wav file so that's out for me.

The i810 built into the motherboard on the new machine works well, and I have gotten over 72 dB S/N ratio on digitized LP's, so that's up to the limit of the LP format.

If you go with something over 16 bits (and it really delivers more) the main advantage
is that you don't have to be as careful about setting the gain on recording as you have
a greater range you can normalize without losing anything.

If you want to use anything other than WinXP for your OS, check that drivers are available.
The big downside of the one on my motherboard is that it is too new and won't be fully supported on Linux until the next major kernel release. I can play back, but have to reboot to Windows to record.

thanks to everyone.
In fact, I have a SB 128 soundcard , and I thought
that if I bought a more expensive soundcard (about 100 euros),
I'd get better adc.
I'm very pleased to learn that adc are simplier than dac.
My phono player is a very basic one (toshiba) and it's
output is connected to a mixer before going to the soundcard.
Sound is quite good. Maybe I'll stick to this configuration
after all...there is no audible hiss
or background noise. Anyway, if I got an Audigy instead of my SB128,
would I get better adc ?

If you mean measurably better, then I'd say yes. However, like DonP says, an older 16bit ADC may well be sufficient for LP recording - although measurably worse, perhaps not audibly so.

But I'm not sure if I dare support that statement... Ofcourse, if all noise is just noise, then it doesn't matter to exchange typical LP noise for typical PC noise. But the reason many like the sound of LPs and hardly anyone likes the sound of my old AWE64, is probably that noise generated somewhere in the phono chain sounds a lot sweeter than EMI hum and digital distortions.

Ofcourse, I could send you some recorded LP stuff, but it wouldn't be a useful comparison: my turntable is totally different, for example. Perhaps you can take yours to a friend, and listen if there's a difference?

I guess sometime I take a CD with a good S/N from a digital rip and try recording it
through the analog inputs. If that shows up a lot better than 72 dB S/N, then I could be assured that the LP noise can stand as a feature, untarnished by the soundcard's noise.

One major difference I noticed between my SB PCI128 and M-Audio Audiophile 24/96: When recording from the analog line-in jacks with nothing connected, and amplifying the sound, I get a white-noise type hiss from the 24/96 (which tells me the card is adding dithering, as it should).

From the PCI128 (ES1371, the "good" model) I get nothing but a low-level whine, which indicates to me that from some particular sources, recording from the analog line-in on the PCI128 could sound very bad indeed unless one manually added some sort of dither to the recording afterward. Also, the S/N ratio of the Audiophile 24/96 is significantly better (at least it would be significant with quieter sources).

Other than that, I'm not sure ADC's do anything other than sample at some given frequency and bit depth, and as long as the clock frequency is correct then one should be about as good as another -- I don't think these are ever expensive or high-end devices like DACs can be (someone please correct me if I'm wrong).

I believe that in principle an ADC is more complicated than a DAC. It's some years since I looked at the designs of both, but it was certainly the case then. Now, while some DACs may be more complicated than some ADCs, in general terms it's the other way round.

More complicated maybe, but I think there may be less variation as far as quality (but I could be wrong, I'm just assuming based on some things I've read and since forgotten).

There is most definitely a difference between ADCs, and often times the difference is not subtle. Check out this link for a little more insight. Happy hunting.

http://www.apogeedigital.com/products/rosettaad.php

Thanks... so what are ADC's usually subject to... jitter? Incorrect clock rates?

AFAIK these things just basically sample, and convert loudness levels (electrical voltages) into numbers that represent those levels -- one reason why I've envisioned them as fairly cheap, simple devices.

You would get a better ADC, and probably would gain a even higher signal to noise ratio.

Going from a Live to a Audigy got me another 15db that way, it's pretty quiet.

Of course the Revolution or the Audiophile are even better, for more money.

I look at the ADC or DAC issue as one is the inverse of the other. They're jobs are equally complex, but probably the ADC is more complex due to having an analog input stage, sampling, as well as the digital side logic. I don't see why the nuances of sound wouldn't be affected by the chip that's recording it any less than the chip reproducing it can affect it.

Check the Burr-Brown site for their ADC (and DAC) chips, you will find many types with many levels of specs, sig/noise ratio's, bit depths, etc..

The point here wasn't if one ADC can be better than another. Ofcourse it can. But does it make a difference if you are playing back LPs from what mrbruno calls a very basic Toshiba turntable? Surely, I wouldn't recommend to get an apogee ADC for that...?? And an Audigy - I just don't know. The turntable may well be the limiting factor here, like DonP said.

Fewtch, ADCs differ not only in jitter characteristics; there's also different types of filtering in different designs, and some dither differently, and lots more that I can't think of just now.