I am designing guitar amplifier with TDA7294 output stage, but i cant find anywhere how big input signal is needed for this IC. I have designed preamp with gain of 6 so with 100 mV peak-to-peak input signal from guitar i got 600mV output signal (i know guitar signal can be bigger). I want to make 40 Watts amplifier and this chip is for 100 Watts max. How do i limit output power to 40 Watts when my volume is at max? I cant find anywhere correlation between input signal and output power. I am scared that if i put transformator of 40 Watts and i got too much gain in preamp that i will get max power at lets say 1/2 turn of potentiometer, or that gain is too low and i won't get 40 Watts at all.

And second question is should i put fuse of 200mA on main or should it be bigger?

 

The supply voltages (dual polarity) and the speaker impedance will dictate the maximum power available, but the available power before severe clipping distortion occurs will be less than that maximum.
The maximum input signal level without clipping will depend on the gain chosen for the amp, as set by resistors R2 and R3 in the circuit shown in the datasheet.
As for the fuse, what is your mains voltage?

 

My mains voltage will be +-30V. So with resistors value 22k and 680R i got gain 32, so i need more than 2V peak to peak from preamp to have clipping (which i dont want). Preamp has gain value of 6. So gain of preamp and amp is 192. Maximum input voltage without clipping will be 0.3V peak to peak. Is this ok?

 

The datasheet says differential input voltage is +/-10mV, so you're well ok, you could just put the guitar in directly.

 

The datasheet says differential input voltage is +/-10mV, so you're well ok, you could just put the guitar in directly.

That's minimum voltage that TDA can take as input, but the question is is the gain enough or is it too low/high? If the gain it is too high there will be clipping and if it is too low it won't give full power. I think my guitar output is 100mV but I am not sure. I don't know which input value should i use as reference for projecting amplifier because there are also guitars that give 1V on output.

 

It's all about the voltage available and the speaker impedance. P = E^2/R
For instance, 40 watts RMS into an 8 ohm speaker is 17.888 volts RMS.
The peak voltage of 17.888 RMS is 25.3 volts. (RMS times the square root of 2)
If your power supply can provide +/- 30 volts and all 30 volts can get to the speaker, you might produce 56.25 watts, RMS. (But you can't make an amplifier with no voltage loss.)
That's how it is done.

Other considerations include the fact that music typically has peaks at 10X the RMS voltage value.
If you design for 30 volts peak, your undistorted output signal will resemble 3 volts peak (0.56 watts RMS), give or take how much distortion you feel comfortable with. The need for headroom is inescapable in music amplification. Trying to guarantee the speaker can not be harmed is difficult because you must allow for headroom, but connecting a 300 watt speaker to a 30 watt amplifier is expensive and usually results in a speaker that is too heavy to produce the necessary bandwidth with only one speaker. This is where a fuse is useful. My 50 watt Onkyo amplifier came equipped with 3 amp fuses on the speaker outputs. If you want good sound quality, you build that first, then add some sort of protection circuits so nobody can crank it to the max and melt your speakers.

 

From what #12 is saying, it looks like a 2A fuse in series with the speaker would offer resonable protection for both the amp and the speaker.