With an output impedance of 1K for the preamp, a simple voltage divider should work as an offset circuit. Here's a way to do it:
http://interface.khm.de/index.php/lab/experiments/arduino-realtime-audio-processing/.
Some things to think about with regard to that circuit:
- It's not necessary to use a potentiometer to adjust the DC offset level, if you're willing to live with some inaccuracy in your DC offset.
- This scheme uses 100K resistors in the voltage divider. The ATMega328 datasheet suggests an input resistance of 10K or less to ensure that the input capacitors on the ADC have time to charge fully. The equivalent DC input impedance for this circuit is 50K. There might be some impact on accuracy. You could use lower-value resistors, but not too low - you don't want to load down the output of the preamp. A couple of 20K's would yield an equivalent DC input resistance of 10K.
- I'd add a high-value resistor - maybe 100K - between the hot leg of the audio input and ground. If the output of the preamp is capacitively coupled, as I suspect, then that point won't have a well-defind DC voltage. I can't provide a good theoretical justification for that, though; my bias is strictly intuitive.
satercito:
So for sure I need another circuit?
Not absolutely for sure. That preamp could be anything; there's no way to guess its output characteristics. It might have a nicely-biased audio + DC output, centered at 2.5V, just waiting to be hooked to an Arduino, but I doubt it - it'd be an odd preamp. I'll go out on a limb and say, yes, for sure you need another circuit. Doing otherwise risks damaging an analog input to your Arduino, or damaging the the IC generally.