As you may have seen on the Arduino page, the sine wave quality with the Arduino Motor Shield is pretty poor. However, if you want to just run the MagicQuartz software to see if the real unit will be working with your turntable, I will come up with a new and very simple (and cheap!) solution that produces a very clean sine wave.
The whole thing consists of an Arduino Nano and a digital power amplifier. You can either use the L6203 (which is also used in the “real” MagicQuartz) or a more breadboard-friendly L6202 chip … and some boost caps and resistors. That’s all!
Below are some pics – please excuse the quality, they were taken with an el cheapo smartphone with a fixed focus.