Ehmer has joined Preston on loan until the end of the season.
Preston boss Graham Westley said: “This is the time of year when you have to bring players in who can fill slots in the squad.
“Max gives us versatility, he can play centre-back or in the midfield holding role. He has got plenty of experience at League One level and for those reasons he gives us an extra pair of legs.
“I’ve seen a little bit of him over the last couple of years and he is certainly a steady player who can make a strong contribution to this side.
“He knows this level of football and I know QPR think very highly of him.”