It was in the early 1710s, in the middle of his days in Weimar, that J.S. Bach first became acquainted with Antonio Vivaldi's remarkable instrumental concertos; they would prove to have a tremendous influence not only upon Bach's own instrumental concertos, but also upon his style as a whole. One of the ways that Bach learned and absorbed new music was by making adaptations and transcriptions of it. So he did with many Vivaldi concertos during his Weimar residency, recrafting them, along with concertos by other composers, into series of concertos for harpsichord and for organ (in both cases without orchestral accompaniment). Bach's Concerto for organ No. 2 in A minor, BWV 593, is an arrangement—or a condensation and refinement—of Vivaldi's famous Concerto for two violins in A minor, RV 522, No. 8—from the volume L'estro armonico, Op.3, which first appeared in print in Amsterdam in 1711.
Bach's reduction of the concerto for two soloists, strings, and basso continuo to a work for just two hands and pedals was a process that requireed some alteration of the music. Bach went beyond simple necessity, and in fact tightened and enriched the very contrapuntal fabric of his source piece. BWV 593 is in three movements, the second of which Bach gives to the manuals alone. It is this second movement that I re-arranged non-standard Woodwind Quartet (Flute, Oboe, English Horn and Bassoon) and is best played using the "GeneralUser GS.sf2" Soundfont by S. Christian Collins Software (http://www.schristiancollins.com/generaluser.php).