Simple question about output fuse on power amplifier

output speaker fuse

Hi,
i have a power amplifier with 3 MJ15015 on positive and 3 MJ15015 on negative power supply.
The amplifier is B class and the power supply is +/- 36 V.
This amplifier have not electronic speaker protection but a simple fuse.
The question is: how to calculate the fuse value?
Second question: what's the output power of this amplifier?
Thank

fuse power formula

The output power is set by the load resistance. The worst case is when the amplifier is driven into square waves. You get the v squared/r power equation where V is the supply voltage and r is the speaker impedance real part which is usually 80% of the rated impedance. There may occur a current limiting by the output transistors which will reduce the output voltage.

Almost all of the electrical power is turned into heat in the speaker coil. The speaker coil has a longer thermal time constant than the transistors. Several forms of damage may result.

1. Your ears from the intense sounds
2. The serenity of your neighbors who may be tempted to perform acts of violence on your person.
3. The displacement of the speaker cone may exceed the elastic limit and rupture.
4. The heating of the speaker coil will start a fire.
5. The output transistors will overheat and quit working.

You select the fuse rating for the maximum current that you want the load to draw. The fuse will usually blow at twice the current. The time to blow is set by the thermal mass of the fuse. Even if the fuse blows, many of the enumerate damages could still result. I would suggest electronic current limiting on the power supply or amplifier output.

calculate fuse amplifier

Rough estimation of power (@8Ω speakers) ≈ 150W
For that power I would use ≈10A fuse - slow blow.
Regards,
IanP

power amplifier using mj15015 circuit

Hi,
question 2 (power output):
i can use a signal genarator and an oscilloscope to measure it, but i don't want this, i want to calculate it with formula.
I have estimated as : max output volatge to load = power supply (72 volts)
so the max output is (Vout*Vout)/Rload (72*72)/8 ohm= 648 w, but an amplifier B class have max 70% of rendering so the max output power is 70% of 648w (450 w on 8 ohm).
Question 1: i have repaired the amplifier so i don't want to build a speaker protection but i want use the fuse but i don't know his original value.
Wich formula to calculate it?

how to calculate speaker fuses

The 70% value you refer to is probably the efficiency rating of the amplifier. For an ideal class B amplifier, this value is approx 78.5%. Most class B amplifiers are actually class AB (The quiescent current is slightly greater than 0), which may account for your 70% value. This has nothing to do with the maximum power that the amplifier can deliver to a load. In any case, the maximum undistorted sine wave output that you can expect is the value where the peak of the output voltage is just below the clipping value. At this point the RMS voltage will be 0.707 times the Supply voltage = .707 X 36V = 25.42. The maximumum power, then, would be 25.42^2/Rl, where Rl is the load resistance. In the real world, of course, the speaker does not represent a purely resistive load, so the power delivered to the speaker will be less than this value. The RMS current for a resistive load would then be 25.42/Rl. The fuse value would be slightly higher than this.