What to do?
Rumor has it that output impedance of amplifier might be of the order of 70Ω but speaker is just 4Ω. Impedance mismatch means reflection of energy going to speaker back to the amp. A nice electromagnetic wave travels along the speaker cable at almost the speed of light. Until it arrives it does not know what impedance it will find. When it arrives it feels the almost short circuit compared to 70Ω and voltage then drops? Transistor is ignorant yet of what is happening until almost short circuit of 4Ω wave arrives back to it. Surprise surprise. Then voltage across transistor drops due to 4Ω load.
Where does the 70Ω estimate comes from?
Possibly roughly from deciding Vb to current. When Vb is say 10V current measured is about 100mA, 10V : 100mA is 10V : 0.1A is 100 V/A is 100Ω.
How to impedance match?
One option:
Rewinding a Tesla speaker coil with more turns and less diameter.
A patient tortoise might do it.
Or sing many BD139 Ttansistors in parallel?
Or using an output impedance matching transformer?
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