Apollo Ensemble, a period instrument chamber orchestra composed of leading period instrument players in North America, was founded by John Hsu in 1991 for the primary purpose of performing the ca. 50 symphonies that Joseph Haydn composed between 1762 and 1775. Hsu has devoted many years of study to these works and the 126 Baryton Trios composed during the same period.

With the aim of recapturing as much as possible the tonal qualities and ballance of the orchestras for which Haydn composed these symphonies, the Ensemble duplicates both the instrumentation and the size of the orchestra at Esterhaza during that time. In its performance, the Ensemble also takes into account eighteenth century musical precepts and ideas relating to performance practice of the period, especially those that are crucial to an understanding of the stylistic and expressive content of Haydn's symphonies. Such information ranges from the meaning of certain musical notations and their proper execution to the question of tempos for the different kinds of menuets and the dramatic intent of many symphonic movements.