Prepositional phrase and main clause subject agreement

I know it would be ungrammatical to say:

After standing up, his cheeks were covered in tears.

Because the subject for "after standing up" should be "he", while the subject in the main clause is "his cheeks".

However, I was wondering if it would also be ungrammatical to say:

This effect could be achieved by refraining from sadness.

I suppose the subject of the sentence is "this effect", but that of the prepositional phrase "by refraining from sadness" should be something like "one" or "you"?

As a contrast, it is perfect grammatical to say (copied the example from dictionaries):

He learned English by listening to the radio.

It would be great if you could let me know your thoughts on the second sentence. Thanks a lot!