I need to get four discreet simultaneous audio inputs into my DAW (for two mics and two line inputs from guitars), which currently has a Steinberg VSL 2020. My system works flawlessly right now and I'd like to keep it that way. Is there a way I can get some sort of input mixer to take advantage of the 2020's lightpipe inputs and get what I need? Any help is much appreciated. I would prefer not to have to switch out my soundcard, as my system works so well with all my sample libraries, etc.
If you need 4 discreet simultaneous inputs, you can achieve it a few different ways. You can get any digital mixer than has ADAT i/o (either optional or built-in). Mackie, Tascam and Yamaha have some as well as some other companies.
If you want better quality and don't necessarily need a mixer, you may be better off going with a dedicated A/D converter. There are also a variety of choices there too. There are a variety of options from Alesis, Behringer, Swissonic, RME, Lucid Technology, Apogee, etc. You basically just need to set your budget and find an appropriate unit.
This will typically give you the option of up to 8 simutaneous inputs (not including the rca i/o inputs you already have on the card. If the card's s/pdif i/o is not a mirror of the analog i/o then you may be able to get 4 simultaneous inputs by getting a stereo a/d converter into the s/pdif connector and using the analog inputs for the other 2.
If budget is a concern, Alesis is probably a better unit than the Behringer but, the higher-priced units are certainly better quality. My recommendation would be for an ADAT a/d converter (instead of the mixer option) since it would give you 8 simultaneous inputs for less money or 8 simultaneous but high-quality inputs for the same or less money.