Question about audio output transistors and power out

[AndrewT]

96Vdc across the supply rails is the same as +-48Vdc.
This can never give 288W into 4r0.

Go and read ESP site on power amplifier design.
Read Nelson Pass for more on ClassA power amplifier design.
Read Leach Lo Tim for design details of all stages in a power amplifier.

 

[cbdb]

Andrew I think you meant 20 V rails. At 48v V^2/4 *.707 ( for RMS power) =407 watts. The other things that need mentioning; power rating of tranies is average or RMS power while current and voltage ratings are peak. tranies can handle over current spikes as long as there short enough but voltage is an instant destroyer. Your tranies have to operate below all 3 of these limits.

 

[AndrewT]

Hi C,
where did 20V come from?
I certainly did not mean to quote that.

 

[AndrewT]

Using two pairs, paralleled should drive 4Ohms. There are a couple of issues that must be addressed here first.
1) In order to ensure equal current sharing between paralleled transistors, they need to be matched as close as possible, beta particularly.

no,
it is far more important that the bias current through each output transistor is as near matched as possible when at or very close to quiescent state. This requires that Vbe of the NPN pair is accurately matched at the planned bias current. Similarly the PNP pair must also be Vbe matched.

Loose, say +-5% matching of hFE plus the effect of adequately sized Emitter resistors will help to ensure the transistors pass very similar currents when output currents are well above the quiescent bias value.

 

[TonyTecson]

I'm kinda confused. I always thought that the output transistors determines the output wattage into a speaker load. But has i read more i have noticed that the output power is determined by the rail voltages. I also noticed that the output transistors only determines the current through the load. M i right or wrong?

Also When designing the output stage how do you determine the amount of transistors you will need for the rail voltages to get a desired output to the load?

it is the load that determines how much power is delivered, coupled with how loud you play your music....

now the power supply and the ouput SOA then determines if your amp can deliver the power demanded by the load and input signal....