Bridged LM4780 driving 2,5 way speakers...

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2.5Way speakers.
How many bass drivers are there?
What is the impedance of the multiple drivers?
Are the multiple drivers in parallel?
What is the range of impedance of the whole speaker?

A bridged amplifier behaves as if each half of the bridged Pair had a half impedance load attached.

If you have 8ohm speakers then the bridged amplifiers think that have a 4ohm load attached.

If the speakers are 4ohms to 8ohms then the amplifiers think you have a 2ohms to 4ohms load attached.

The dual chip 4780 will not like driving a 2ohm equivalent for the Bass/Mid drivers.

You must investigate your speakers.
 
Normal 2.5 way loudspeakers end up having ~4 ohm bass and mids.

The 4780 is capable of driving 8 ohm loads given 35 volt rails for around 60 watts output power.

Now when driving 4 ohm loads the 4780 is still only capable of delivering ~60 watts as it is current limited, this is why it is recommended to lower the power rails.

If you were to use two 4780s as in a bridged configuration, when driving a 4 ohm load, it would be as if each 4780 were seeing a 2 ohm load. Now given the 18 volt rails each 4780 would be capable of providing around 60 watts so ~120 watts total into a 4 ohm load.

The trouble here is that the protection circuitry in the 4780 might cut in with loads much below 2 ohms and could end up severely limiting the power one can get from the amplifier.

There is however an easy solution to this. You will notice in the datasheet that national gives guidelines on how to use the 4780 in a parallel configuration, this is what you should be looking at.

When used in parallel the current demand placed upon each chip is halved, in this way, if driving a 4 ohm load, each chip sees half the nominal 4 ohm load and it's as if the chip is now driving an 8 ohm load. As can be seen from the 4780s datasheet this is beneficial in more ways then one as the chips performance is better into higher ohm loads.

For your given application using two LM4780s in parallel, per channel, when using 35 volt rails, would make far more sense then using two LM4780s, bridged with 18 volt rails.
 
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