WHY do these 4 transistors work good in parallel?

[gary350]

I asked the wrong question.

A push pull amp signal moved the speaker in both directions, + and -. Is the positive negative voltage at the speaker considered to be AC?

I know + & - voltage is pulsing DC.

What name is the pulsing voltage called?

Amp that is not push pull pulses DC and pushed speaking in 1 direction only what is the name of this voltage?

 

[audioguru]

Yes, a push-pull audio amplifier drives a speaker positive and negative which is called AC.
A single transistor can drive a speaker in only one direction with a DC pulse to make a POP or CLICK sound which is not voice nor music.
If the single transistor has a resistor as its DC load then it can use an output coupling capacitor to drive a speaker with voice or music.

But a single transistor as a class-A audio amplifier heats all the time even when it is not making sounds.

 

[Nigel Goodwin]

But a single transistor as a class-A audio amplifier heats all the time even when it is not making sounds.

Not just a single transistor, any class A amplifer outputs masses of heat ALL the time (more heat than full audio output), regardless of music or not.

 

[unclejed613]

speaker voice coils, especially the "dry" type (which is about 99% or more of the speaker drivers available) do not handle DC well at all. a voice coil cools itself by the airflow created past the pole pieces when the speaker cone is in motion. if DC is applied to the coil, heat is generated but with no motion (except the initial thump), and so, no air flow. the heat has nowhere to go, and either the voice coil overheats and burns out, or the voice coil heats up and melts the glue holding the voice coil to the cone and surrounds. if you look at the "audiophool" amplifiers, you will find even they aren't foolish enough to connect the speaker directly as the load for the transistor, they use a transformer or RC network to couple the speaker.

there are speakers that can handle a lot of power, and have the added advantage of being more efficient drivers (sensitivity figures around 90-100db@1W@1meter). the secret with these drivers is that they use ferrofluid to fill the gaps between the voice coil and magnets. this does two things, it carries heat away from the voice coil to the magnet, and it extends the magnetic force on the pole pieces to the surface of the voice coil, which greatly increases the efficiency of the voice coil.