Basic LM4780 amp

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Hello all

I am planning on making an amp around the LM4780. It will be driving a pair of full range audio nirvana 12" speakers (8Ω)

Do i want to use a speaker protection circuit, or does the lm take care of that?

If instead of an automatic delay when turning it on and off, i use a second switch to manually connect/disconnect the speakers, is that a good idea? will it work to avoid the on and off thump? for a laugh i connected one of the 12"s to a silly tda2030 based amp and when i turned it on the cone went about half an inch and my heart skipped a beat. poor speaker :(

Does anyone know the output impedance of the lm4780?

Is a power supply based on a set of deep cycle 12V car batteries viable?
 
First of all, u will need six batteries to drive the amp, or u will have to construct a DC-DC circuit that will convert 12 or 36 volts single voltage to dual supply required for 4780.

Looking at your driver, u better construct some transistorized amp as it will do better justice to your driver. However the issue of dual supply will have to be tackled if u do not want to power it from mains.

Gajanan Phadte
 
LM does not provide effective protection for the speaker.
Buy or build a speaker delay.
This can also incorporate DC detection and activate isolation. and over temperature of the heatsink and isolate.
A manual switch will work. But you will tire of using it, or forget.

You don't need to know the output impedance of the chipamp.

The chipamp has a minimum voltage from which it can operate. It will shut down if the voltage drops too far.
This will probably require four 12V batteries for a nominal +-24Vdc supply. I would say, not recommended

The datasheet shows under-voltage activates at 9V. If your two battery supply for +-12Vdc never allows the chip to see <9V you may find this acceptable.
 
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i thought the lm has thermal protection circuitry? at least that's what it says in the datasheet
The datasheet certainly admits to having internal protection/limiting circuits. The chipamps still fail disastrously. And can destroy speakers. Try removing just one supply rail fuse, now replace that fuse and remove the other fuse. Make sure your speaker is NOT CONNECTED.
 
i wonder, if there has been any experiments on how the amp behaves when reaching Tjmax does it lower Pout? does it savagely cut off supply?

normally i would trust the manufacturer before i trusted a forum member, but i think it is in my best interests to look into it a bit more
 
Don't waste time designing a thermal protection that the chip already has. They are designed to last something like 1000 hours when in continuous thermal protection. I abuse chip amps just to see what they can take. Dead shorts, low impedance on the output and inadequate heatsinks to test the protections. The authentic chips haven't failed. The counterfeit chips (often found on ebay) will often fail instantly or early.

I tend to oversize the heatsinks, use ones with a thick backplane and no chip to sink insulators for the best thermal path.
 
ok then, how do i differentiate between original and counterfeit?

second, the backplane of the LM is not electricaly insulated... so the heatsink will be at V-, how do i take care of that?

It is best to buy your parts from an established vendor. Many of which have been around before the WWW. Ebay, and a lot of the companies who ship from Hong Kong are probably sending you fake pieces. It can be hard to tell fakes from real parts. There may or may not be visual differences in the parts. The fake parts may sound just fine, but can let you down when you are counting on them.

I used heavy nylon bolts/spacers to tie my heatsinks down. You can also use a copper heat spreader and electrically isolate it from the heatsink. The electrical insulated surface would be large and a much smaller thermal resistance. These modern high power audio ICs need to dissipate quite a bit of heat though their small thermal pads. make it easy for them.
 
but won't the sink still be at V- and thus dangerous?

Dangerous to whom? Is the amp for yourself or a consumer product? With the heat spreader method, the heatsink is isolated.

With a sufficiently large heatsink, you may be able to use the insulator pad, but these cut into your thermal safety margin and tests should be done to determine the temperature of the amp in the worst case operating environment.
 
i would like the sink to be outside the case, so that it gets all the air it wants.

perhaps with the added convection, i can get away with using an insulating pad?

i think it is better than having the sink choking inside a case.

now that we are taking cases, i am considering a wooden box, spray painted glossy black (3 coats) and with brushed aluminum front, no screws visible. power switch on the back, mute switch on the front and a volume knob.
 
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