I recently purchased a ONKYO HT-S3300 receiver and speakers set. The set comes with a passive subwoofer, and I'd prefer a powered sub but can't currently afford one (even a cheap one), so I was thinking about making my own powered subwoofer using some car audio equipment that I have lying around. It will cost me far less than buying a sub and seems like a fun project. I've already seen that you can connect car audio to home systems using a computers power supply. My issue is a very specific problem, my car amp has RCA inputs, while my subwoofer output on the receiver is regular +- speaker wire. I'm wondering if there is any danger of destroying my receiver, amp, or sub if i connect it using a RCA-to-speaker wire converter.
if there is a headphone jack , that may work as the range of a headphone jack is quite close to the range of a pre-amp output , i have seen people use a discman with a cord that has a pair of rca's to test car amplifiers . as for using the speaker level inputs if you have them , remember that an indash unit will be 45-50 watts peak and closer to 8-18 watts rms , the typical speaker output of a av receiver is much higher than that so it isn't likely that speaker level inputs would be suitable for this use . i would try to see if you could make use of the headphone output jack for this purpose , if that don't work , you may have look at passive subwoofer options , some car subwoofers are good for this use , jl w0 comes to mind .
Your receiver contains the amp for the subwoofer since it has its' own speaker terminals. Connecting a different amp may not make any difference but if you want to play with it use a speaker to line level adaptor to connect your homebrew amp to the speaker terminals on the sub.