In a class B amplifier, no DC voltage is used to bias the transistors, so for the output transistors to start to conduct each half of the waveform, both positive and negative, they need the base-emitter voltage Vbe to be greater than the 0.7v required for a bipolar transistor to start conducting.
At this point things can get a bit problematic. Where should the connections be made and how to connect them. I have found that the best place for a central ground to be at the signal input ground. This will be where the connections to other external equipment is made. In a typical audio component there will be left and right channel inputs. Each channel will be via a shielded cable from the source (CD, FM, etc.). At the input jacks I have found that you can connect the two input signal ground returns together (not to the chassis) using isolated jacks provided you do not run shielded cable with both ends connected together internally to the unit. This part is often hard to envision, but you do not want multiple ground paths for the signal. This is likely to cause a ground loop, more about that later. A common error is to connect the signal grounds together at the input jacks and then run shielded cable inside the unit to something like a volume control and connect the shields together there. The shields then acts as conductors and not solely as shields. Use only one end of the shield inside equipment. If needed, run a separate ground wire to the volume control or where ever the signal is going. It will be the ground reference for the signal and the shields will be only shields. This can greatly improve the signal to noise in a piece of equipment.
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