I think the standard max object [matrix~] does what you are looking for, it has an optional ramptime

Hi Timo, I guess matrix will not help me, I need to switch at signal rate.

oops, sorry, misunderstood you. It's not that hard to built, actually I just done it, here it is:

keep in mind that 'smooth switching at samplerate' is not well defined, you need smaller ramp times for really fast switching, and really fast switching is probably never 'smooth'

Thanks a lot for this example, this is a solution. In my case I need to switch between many sources (25), so this solution is a bit messy.

Maybe its time for an mxj~ ..