Johann (Georg) Andreas Stein, (Heidesheim, 16 May 1728 - Augsburg, 29 February 1792) was an outstanding German maker of keyboard instruments, a central figure in the history of the piano. He was primarily responsible for the design of the so-called "Viennese" fortepiano, for which the piano music of Haydn, Mozart, and the early Beethoven was written.

The Stein piano dynasty:

Stein was the founder of an important piano-making dynasty. His daughter Nannette (Augsburg, 1769 - Vienna 1833) was a skilled builder, and continued the family business under her husband's name, Streicher. The Streicher firm built pianos for Beethoven and played an important role in the technological development of the piano. Run by family members, it endured until 1894, still making (much larger) pianos with the basic action design from Stein.


Fortepiano by Johann Andreas Stein (Augsburg, 1775) - Berlin, Musikinstrumentenmuseum

  His contributions to the piano:

The Prellmechanik action

 
Stein's most important innovation, his piano action, was perfected around 1781. This is the so-called Prellmechanik with escapement. In this arrangement, each hammer was mounted on top of the key, with the head on the end closer to the player, a traditional arrangement in German pianos of Stein's day. The hammers were like small, asymmetrical levers, with the hammer head far from the fulcrum, and a small upward-facing hook ("beak") on the other side of the lever, much closer to the fulcrum. When the player depressed the key, the whole hammer assembly would rise. The beak would engage an escapement hopper attached to the keyframe. The escapement hopper pulled down on the beak as it rose, in turn causing the hammer (the other end of the lever) to fly upward and strike the key. The escapement hopper was hinged and sprung; this permitted the beak to move downward past it as the key sank back to rest position.

Latcham (see Grove reference below) calls this invention "a breakthrough in the piano's history;" it "offer[ed] the player a remarkable control of the hammers, especially when playing softly, and [wa]s astonishingly responsive to the player’s touch."

Pedal-controlled dampers:
 
Stein may have been the first to produce a fully functional damper pedal, in which the player can lift all the dampers from the strings. Such a device had been devised by Stein's predecessor Gottfried Silbermann, but Silbermann's device required the use of the player's hands to work, and thus could only be deployed during pauses in the music. Stein's device was controlled by a knee lever, pushing upward on the lower surface of the instruments, permitting the full equivalent of modern piano pedaling

Stein's acquaintance with Mozart
 
Mozart visited and befriended Stein in Augsburg in 1777, during the (unsuccessful) job-hunting tour that took him also to Mannheim and Paris. The enthusiastic letter that Mozart wrote to his father Leopold is informative concerning Stein and Mozart's own preferences in pianos.[9] The letter is very widely quoted. The following translation, by Emily Anderson, is taken from Broder 1941:
 This time I shall begin at once with Stein's pianofortes. Before I had seen any of his make, Späth's claviers had always been my favourites. But now I much prefer Stein's, for they damp ever so much better than the Regensburg instruments. ... In whatever way I touch the keys, the tone is always even. It never jars, it is never stronger or weaker or entirely absent; in a word, it is always even. It is true that he does not sell a pianoforte of this kind for less than three hundred gulden, but the trouble and the labour which Stein puts into the making of it cannot be paid for. His instruments have this special advantage over others that they are made with escape action. Only one maker in a hundred bothers about this. But without an escapement it is impossible to avoid jangling and vibration after the note is struck. When you touch the keys, the hammers fall back again the moment after they have struck the strings, whether you hold down the keys or release them ... He guarantees that the sounding-board will neither break nor split. When he has finished making one for a clavier, he places it in the open air, exposing it to rain, snow, the heat of the sun and all the devils in order that it may crack. Then he inserts wedges and glues them in to make the instrument very strong and firm. He is delighted when it cracks, for he can then be sure that nothing more can happen to it. Indeed he often cuts into it himself and then glues it together again and strengthens it in this way ... The device too which you work with your knee is better on his than on other instruments. I have only to touch it and it works; and when you shift your knee the slightest bit, you do not hear the least reverberation.
This letter dates from slightly before the Stein action was fully perfected; presumably, the version that Stein had developed by 1777 was good enough to impress Mozart at the time.
 
During the years following his move to Vienna (1781), Mozart made enough money to buy a fine piano, and he bought one not from Stein, but from Anton Walter, a Viennese builder who followed Stein's principles. It is not known whether Mozart actually preferred Walter's pianos to Stein's, or simply wanted to buy locally rather than having an instrument shipped all the way from Augsburg.