What is the difference in a CPAP and an auto machine?

Update- My Primary changed his mind and wrote a prescription for me. Now my decision is which one to purchase, another S-9, or something less pricy like this- IntelliPAP Standard Travel CPAP Machine I don't use humidifier, but the rep at CPAP.com said this was a quiet machine, and lists for $308 (vs...

Thanks for all the help!
I found another website, http://www.1800cpap.com , and called them. They will take my sleep study in lieu of a prescription, so I've requested that from doc's office. There prices seem reasonable too.

Thanks again!
Brett

My S9 Elite no longer works. The medical supply company I always used says I need a new prescription from a doctor...they say it's a new law. My sleep study was 2011. I know my pressure setting was 11, I didn't use the humidifier. I just want a new (or used) CPAP, I don't want another doctor's visit...