Modular Multichannel Stereo Audio Mixer

I was really bothered about the frequency response, so I decided to take some measurements.

I used REW (Room EQ Wizard), and got a very flat response for a direct cable.. but a (I think) quite bad for my mixer :(.

(blue is -48 dBFS, pink Phase)

I will try to get more adequate Op Amps, and maybe tweak the dc coupling. 

Also somehow I think I do not output enough volume... and I used 15 volts for that.... so maybe I will have to redesign it all?

Suggestions are appreciated!

I received my set of 78xx linear regulators and did some fast test to check if it removes the USB power noise.. and it does!

So I set together a fast protoboard to try it live with the full mixer, and the noise level is now non existant! Amazing!

Now I have another problem to solve and is that I'm filtering out low frequencies. Quite probably is the DC bias removal. Need to investigate.