Struggling with acetaldehyde

I recently started brewing and have completed three batches so far (Amber/Pale Ale). However, each batch seems to have that distinct green apple flavor to it. It's gone down a bit with each batch but even as my latest batch conditions I still smell a small amount of it. I've been reading forums and researching possible causes and still can't seem to fix this issue. Here's a summary of the latest batch:

-Grains steeped in 170F and boil done using malt extract.

-Wort aerated with a stirring stick for about five minutes until a thick head of foam appears. OG reading of ~1.050

-Wyeast smack pack is now ready after ~4 hours to get started, pitched in to ~70F wort in a plastic bucket.

-Airlock activity starts ~12 hours later and lasts for ~24 hours before stopping.

-A thick krausen forms and lasts for about a week before settling back down. ~10 days after pitching the FG reads ~1.010. Acetaldehyde smell is present by this point.

-Wort is re-racked to a secondary for conditioning. It's been seven days since re-racking and I'm going to add the dry hops tonight.

What can I do to improve my process? Am I adding enough yeast? Should i use pure O2 for aerating? Would switching to an all-grain brew help? I've been meticulous about sanitizing with StarSan so I don't think sanitization is my issue. Any advice would be welcome, thanks!