Albert Barnes Commentary Psalms 33:2

"Give thanks unto Jehovah with the harp: Sing praises unto him with the psaltery of ten strings." — Psalms 33:2 (ASV)

Praise the Lord with harp – For a description of the “harp,” see the notes at (Isaiah 5:12).

Sing to him with the psaltery – For the meaning of this word, also, see the notes at (Isaiah 5:12), where the word is rendered “viol.”

And an instrument of ten strings – The word “and” is supplied here by the translators as if, in this place, a third instrument was referred to, distinct from the harp and the psaltery. The more correct rendering, however, would be, “a psaltery (or lyre) of ten strings.” The same construction occurs in (Psalms 144:9). In (Psalms 92:3), however, the two words are separately used as denoting different instruments.

The “lyre” or psaltery was probably not always made with the same number of strings, and it would seem that the one that was made of “ten” strings had something special about it as an instrument of uncommon sweetness or power. Therefore, it is particularly designated here; and the idea is that instruments of special power and sweetness should be used on this occasion in the service of God.