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Old 8th February 2005, 01:03 PM   #1
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Default Bridged LM4780 Amp

Finally got around to testing this amplifier -- THD + N averages 0.01% across the audio spectrum -- the 3dB down point is about 10Hz -- the longest sinewave test I did was 3 hours at 120 watts into an 8ohm resistive load -- and it didn't blow up! As I have said before, the device must be firmly affixed to the heat sink -- and I am using a 12V fan to cool it -- the fan runs at 7V which is quieter than the hard-drive on the TIVO:

Click the image to open in full size.

it's a proto -- the big polypropylene caps have been moved for the boards that were sent out.
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Old 8th February 2005, 08:46 PM   #2
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output graphs -- into a resistive load:
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Old 8th February 2005, 08:57 PM   #3
jleaman is offline jleaman  Belgium
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look's good i should try one of these now : O )

How do you like it .. ? What are you comparing it to or what could or would you compare it to ?
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Old 9th February 2005, 05:33 PM   #4
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what are the ratings of your transformer?

Im planning to use toroidal transformer with the LM4780 ic and tell me what is the capacitor value that you are using per rail...
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Old 9th February 2005, 05:34 PM   #5
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hey hey can u tell me how to bridge it ive seen in the datasheet but i couldnt understand it well, if u have schematics then please send me some... and if u have pcb design then send it please....
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Old 9th February 2005, 05:47 PM   #6
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Quote:
Originally posted by diysmartdeep21
what are the ratings of your transformer?

Im planning to use toroidal transformer with the LM4780 ic and tell me what is the capacitor value that you are using per rail...

in the bridged configuration one half of the LM4780 is inverting, the other is non-inverting -- the difficulty with the bridged configuration is that the power dissipation of the chip as heat
(not sound) is much greater than a "non-bridged" -- National Semi says it is 4 times as much, I actually measured it as being somewhat greater !

my transformer is 400VA -- I think that this is about right for two bridged devices (if you are using it to drive music through speakers), if you were using it as a laboratory amplifier I would probably go to 600VA.

Distortion is comparable to that outlined in the Nat Semi website for a single, non-inverting amplifier (edit) (note that I cut out of the graph the 78kHz and 100kHz distortion from the graph, it is 7.4% at 78kHz and I shut it down before going to 100kHz):
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Old 19th March 2011, 05:25 PM   #7
Cobra2 is offline Cobra2  Norway
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Please help with noise... A 120 Watt Bridged
I have this bridged LM4780, built & double checked the bom, but have some problems:
When music somes on low, it sounds ok. As soon as I turn up the volume a tad, I hear "scratching", and somthing similar to white noise starts adding more and more to the music. This noise stays there when turning off signal. But dissapears when muting on/off. And it is silent until it gets a signal above a few mV again...
I tried adding 22pF caps over the ~46.4-51.1k feedback resistors, and 22pF on input.
No change. ( PSU gives +/-36V DC, and heatsinks are big...)
What next?

Arne K
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Last edited by Cobra2; 19th March 2011 at 05:28 PM.
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Old 19th March 2011, 06:03 PM   #8
AndrewT is offline AndrewT  Scotland
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convert it back to single chipamps.
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Old 19th March 2011, 06:06 PM   #9
Cobra2 is offline Cobra2  Norway
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Meaning that the LM4780 is "difficult to cure"? Or just in this (bridged) config?

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Old 19th March 2011, 06:10 PM   #10
AndrewT is offline AndrewT  Scotland
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It will be simpler to prove that each chip is operating correctly when used as singles.
Having done that you may decide to leave it as a pair of 60+60W channels, Having the same power as the bridged 120W channel and so much simpler.
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