You guys are correct of course, but it works fine for this application (I designed and sold a few hundred 40khz ultrasonic alarm units in about 1980 using 741's and never had any problems) so I didn't see a problem using it here. I figured the op did not want a 10vpp 40khz output (which is not required in something like this), but rather a 10vdc output when the tranducer was receiving the 40khz signal. Newbies often ask questions in the 'wrong' format so it can be helpful sometimes to try to give them what they need and not what they want.