Is my Heatsink big enough?

[littlerick]

Will this Heatsink be enough for my bridged LM4870's?

I have 2 of them but would prefer to use one if I can. I have read about the chip getting hot and dont want to under-do it.

[Minion]

Nice heatsink ... That should be plenty big enough ....

[littlerick]

Nice one, should be able to make a pretty small unit all being well.

[fastasulike]

Take a look at this.... ESP - Heatsink design and transistor mounting

[godfrey]

Make sure it's lifted so the holes aren't blocked at the bottom. You need free air flow through it.

[AndrewT]

is that two channels of single bridged 4780? i.e. br100+br100?
What is the speaker impedance?
What is the +-DC supply voltage?

from National datasheet

A direct consequence of the increased power delivered to
the load by a bridge amplifier is an increase in internal power
dissipation. For each operational amplifier in a bridge configuration,
the internal power dissipation will increase by a
factor of two over the single ended dissipation. Thus, for an
audio power amplifier such as the LM4780, which has two
operational amplifiers in one package, the package dissipation
will increase by a factor of four. To calculate the
LM4780’s maximum power dissipation point for a bridged
load, multiply Equation (2) by a factor of four.
This value of PDMAX can be used to calculate the correct size
heat sink for a bridged amplifier application. Since the internal
dissipation for a given power supply and load is increased
by using bridged-mode, the heatsink’s θSA will have
to decrease accordingly as shown by Equation (4). Refer to
the section, Determining the Correct Heat Sink, for a more
detailed discussion of proper heat sinking for a given application

[littlerick]

My plan is to have the 2 amps, both bridged.. I will be running a 300va 2x12v torodial. I would like to fit it all into a 8" wide box. However it is my first project and I'm kinda makin it up as I go along.

I will be happy to have a working kit at the end, then I can have a go at another.

[AndrewT]

what is the open circuit voltage of the 12Vac transformer when fed with your nominal mains supply voltage?

Look at the graph p10 bottom right, 4r0 and +-20Vdc.
at 1.5W into 4r0 the single channel dissipates ~20W to the heatsink.
the 4780 chip in bridge mode will dissipate ~40W to the heatsink when it delivers 3W into 8r0.
It will be even hotter driving a real speaker.

That 40W from a 4780 through the thermal resistance of 0.8C/W Rth j-c raises the junction temp about 32Cdegrees above the Tc temp.
The Rth c-s of ~0.9C/W raises the case temp above the sink temp by about 36Cdegrees. Now Tj~Ts+68C
A sink with two 4780 has to dissipate ~80W for an output of 3W+3W.
If you select a sink with Rth s-a of 2C/W then Ts will be about 160Cdegrees above ambient temperature
If that sink you have chosen is inside your chassis, the ambient temperature may be anywhere between 40degC and 100degC.

Now, let's add these temperature drops up:-
Ta=40degC, Ts-a=160Cdegrees, Tc-s=36Cdegrees, Tj-c=32Cdegrees giving Tj~268degC when the two chips are delivering 3W+3W into 8r0+8r0 from +-20Vdc supplies.

Find a bigger heatsink. Do the sums for the proposed giant heatsink. Tell us what you find about Tj when output power is 3W+3W into 6ohm speakers and supply is +-17Vdc.

I get the impression you have not checked any arithmetic relating to your bridged proposal.

I have a temperature standard for a chipamp.
I must be able to hold my finger on the chip without discomfort when it is operating at typical SPL levels with my typical music using my speakers with my typical audience @ my typical ambient temperature.