You won't get 60 W into 8 Ω unless you bridge the two channels of the LM4780.
For calculating the power consumption, I suggest taking a look at the spreadsheet I link to near the bottom of my Taming the LM3886 - Power Supply Design page.
~Tom
For calculating the power consumption, I suggest taking a look at the spreadsheet I link to near the bottom of my Taming the LM3886 - Power Supply Design page.
~Tom
60W into 8r0 requires an output of ~31Vpk
Using the numbers given by National, expect the loaded supply rails to be around 4V to 5V above the peak output voltage.
i.e. loaded supply rails of +-35Vdc to +-36Vdc should give 60W into 8r0.
The quiescent supply rails will need to be substantially higher, expect at least 3V of rail sag at full power, but more likely 4V to 5V of sag, requiring the quiescent supply rails to be around +-39Vdc to +-41Vdc
Now comes the problem.
The chipamp is limited to +-42Vdc when signal is present.
The quiescent supply rails must not exceed +-42Vdc, when the mains voltage is at maximum supply tolerance.
Now to current.
A speaker is a reactive load.
The maximum current demanded by the reactive speaker when subjected to fast transients is NOT the same as a that demanded by a resistor.
expect maximum speaker demand current to be at least 1.5times the equivalent resistor load and design for at least double the resistor current.
Many speakers demand currents exceeding 3times the nominal resistor current.
I design for and test my power amplifiers to meet this 3times.
Applying this to a 60W into 8r0 load you get 30.98Vpk/8r*3 = 11.6Apk.
Most chipamps can't achieve that peak transient current. But they can easily drive an 8r0 test load.
Using the numbers given by National, expect the loaded supply rails to be around 4V to 5V above the peak output voltage.
i.e. loaded supply rails of +-35Vdc to +-36Vdc should give 60W into 8r0.
The quiescent supply rails will need to be substantially higher, expect at least 3V of rail sag at full power, but more likely 4V to 5V of sag, requiring the quiescent supply rails to be around +-39Vdc to +-41Vdc
Now comes the problem.
The chipamp is limited to +-42Vdc when signal is present.
The quiescent supply rails must not exceed +-42Vdc, when the mains voltage is at maximum supply tolerance.
Now to current.
A speaker is a reactive load.
The maximum current demanded by the reactive speaker when subjected to fast transients is NOT the same as a that demanded by a resistor.
expect maximum speaker demand current to be at least 1.5times the equivalent resistor load and design for at least double the resistor current.
Many speakers demand currents exceeding 3times the nominal resistor current.
I design for and test my power amplifiers to meet this 3times.
Applying this to a 60W into 8r0 load you get 30.98Vpk/8r*3 = 11.6Apk.
Most chipamps can't achieve that peak transient current. But they can easily drive an 8r0 test load.
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