Hi all,
I have never owned a dedicated mic-pre/preamp, such as the Universal Audio Solo 610, Focusrite ISA One etc. What is the best way to connect a preamp to the computer? do you use AD converter? or do you use a specific type of soundcard?
Right not I have Focusrite Scarlett 6i6 which is very clean/transparent, I'm considering to buy a preamp with more color.
BR

You would connect the audio out of the mic pre into a line level in on your focusrite (which bypasses it's mic pre).

dwardzala
You would connect the audio out of the mic pre into a line level in on your focusrite (which bypasses it's mic pre).

Thanks for the reply. I read on several other forums that the Scarlett never fully bypasses the mic pre (Focus is supposedly have said that to a user). You could dial the gain way down to make the impact small as possible. So I was thinking of other options, and perhaps even better sounding options. Wondering how people in here do it :)

The 3 and 4 input on the back are not pre amps and do not colour the sound.  That's where you plug it in for sure. I have a Joe Meek 3Q pre amp and it was a huge improvement over the 6i6 front panel preamps. 
I find the scarlett pre amps to be hyper sensitive to peaks. You end up turning them down below a reasonable level to avoid overs. So I bought the Joe Meek. Problem solved plus other benefits. 
Make sure and set the 3/4 input to "line" in mix control. 

Cactus Music
The 3 and 4 input on the back are not pre amps and do not colour the sound.  That's where you plug it in for sure. I have a Joe Meek 3Q pre amp and it was a huge improvement over the 6i6 front panel preamps. 
I find the scarlett pre amps to be hyper sensitive to peaks. You end up turning them down below a reasonable level to avoid overs. So I bought the Joe Meek. Problem solved plus other benefits. 
Make sure and set the 3/4 input to "line" in mix control. 

I see.. the Scarlett I mentioned above (for the other guy) is actually 2i2, so I guess they are different in that way.
Thanks. That's good.

A mic pre raises the microscopic electrical output of a mic to line level.  So it is best practice to connect it to a line level input, such as your 3/4.
 
However, many interfaces only have a single stereo input that serves for both line/mic inputs - the mic signal gets routed into the line.  So you do pass a line signal through the mic amp, but for the most part it has no discernable impact on the sound.  Or I've never found it a problem.  I mean, does your mic pre cause your mic signal to suck?  Most pres today (even combo interface pres) are pretty clean, esp. when they don't need to add any gain to a line signal.  There are about a million other things for most home studios to worry about than the infinitesimal change a few circuits add to your signal when plugging a line into an interface pre. 

Oh, if you own the 2i2, then your going to have to upgrade to an interface with more than just those two combi inputs. You were correct that even using the line input setting the signal is passing through the pre amps.  It will work so go ahead and see what happens. But really if your going to spend more money you'd be better off looking at an interface like the Tascam uh 7000 which has good pre amps already. Or even better  an RME.  

Cactus Music
Oh, if you own the 2i2, then your going to have to upgrade to an interface with more than just those two combi inputs. You were correct that even using the line input setting the signal is passing through the pre amps.  It will work so go ahead and see what happens. But really if your going to spend more money you'd be better off looking at an interface like the Tascam uh 7000 which has good pre amps already. Or even better  an RME.  

Thanks for the suggestions :-)

My dream preamp at this moment is the Bae 1073 (single channel). 
 

Oh I see now what you were saying about the 2i2 was what you had read about the pre amps, and that you actually do own the 6i6.. good.. problem solved.
Myself I see it as a mistake to purchase the 2i2 and the 2i4 over the 6i6. For not much more cash you get a heck of a lot more functionality and features. I can see using one for a mobile rig but not as my main interface.
 And for the same money you can get a Tascam us 2x2 which has better features and probably way better pre amps.
 But the Tascam is 2x the size so not very portable.