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Old 22nd June 2012, 02:53 AM   #1
tsiros is offline tsiros  Greece
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Default Basic LM4780 amp

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?
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Old 22nd June 2012, 04:03 AM   #2
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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
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Old 22nd June 2012, 12:43 PM   #3
tsiros is offline tsiros  Greece
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thanks for your post, but you didn't really answer any of my questions
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Old 22nd June 2012, 01:33 PM   #4
AndrewT is offline AndrewT  Scotland
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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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Last edited by AndrewT; 22nd June 2012 at 01:52 PM.
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Old 22nd June 2012, 01:37 PM   #5
tsiros is offline tsiros  Greece
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i thought the lm has thermal protection circuitry? at least that's what it says in the datasheet

i understand what you say about the rest.
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Old 22nd June 2012, 01:45 PM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by tsiros View Post
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.
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Old 22nd June 2012, 01:50 PM   #7
tsiros is offline tsiros  Greece
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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
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Old 22nd June 2012, 01:55 PM   #8
AndrewT is offline AndrewT  Scotland
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The protection circuits trigger long before Tjmax is reached. Read the datasheet.
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Old 22nd June 2012, 01:58 PM   #9
tsiros is offline tsiros  Greece
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Quote:
THERMAL PROTECTION
The LM4780 has a sophisticated thermal protection scheme
to prevent long-term thermal stress of the device. When the
temperature on the die exceeds 150C, the LM4780 shuts
down.
Quote:
TJMAX = 150C
??

edit: i'm sorry i am not good with analog
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Old 22nd June 2012, 02:01 PM   #10
AndrewT is offline AndrewT  Scotland
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Quote:
Note 9: The maximum operating junction temperature is 150C. However, the instantaneous Safe Operating Area temperature is 250C.
I interpreted this to mean something quite different.
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