A single transistor audio power amplifier with no external components except a 9V battery

At your risk. Speaker may be blown since there is class A current passing though it. A protective fuse must be in series with battery for fire hazard protection too. Other equipment connected may be blown too. This is the Pleiades 2N3055 amplifier in a 2 cigarette packet size box. Today the 10K bias resistor from collector to base was removed. It immediately worked much better as there was overbias due to direct coupling to the Sony CD Walkman line output. Purely by chance due to the particular 2N3055 Hfe, no series resistor is needed for volume reduction. So no capacitors are used, no resistors, just a 9V batterry, actually 8V due to use. Quiescent current through speaker when said series resistor is 0Ohms is 47mA. Sound quality although of low volume is amazing on voice and many instruments including percussion. When Rs=10K is used to reduce volume, Ic becomes 33mA which is handy for saving battery energy. Signal path: Ingenue - KD Lang CD - Sony D-EJ758CK sn5048123 CD Walkman line out -Pleiades 2N3055 - Pleiades linoleum Auratone type 5C speaker with Tesla 5in drive unit fullrange