Thanks for pointing out this very important fact, Steve. Yes, I am indeed using an RC filter on the amplifier's input. I tried first to see how well it would work without the filter, but the motor was not running smoothly.
I've been working with microcontrollers for almost 15 years now, and on this project I'm trying out one of the Arduino compatible boards. From what I can see so far, "Arduino" could be replaced with "Microcontrollers for Dummies" because it greatly simplifies and glazes over some of the more difficult parts of microcontroller development.
Not that I'm complaining. It enabled me to get up and running very quickly with minimal screwing around with initialization and figuring out which bits I needed to tweak this way and that. While I was collecting information for this, I saw a posting online where someone said they were _surprised_ to learn that analogWrite() didn't actually output an analog signal, rather a square wave (aka PWM). Though I think it's good that more people are making things using microcontrollers, I'm afraid that there's going to be a lot of mistakes made because of false assumptions due to a lack of knowledge.
It's sort of like programming for a PC these days. Even I don't care much what's going on behind the scenes, though I do understand a lot of what goes on behind the scenes.
Thanks for putting up with my rant.

-Sparky