Ampliphers what dose btl mean?

Question

theres a switch that changes the sound

Answer 1

Maybe "Bell Testing Laboratory." They are the one that invented transistor.

Answer 2

The initials b.t.l. stand for "bridge-tied load."

It is a method of connecting a single load (loudspeaker for instance) to the output of two similar amplifiers, one of which drives the load with an inverted signal. It results in much greater power to the 'speaker. (Theoretically up to 4 times as much as one of the amplifiers alone, assuming that the power supplies can deliver the increased current demand.)