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Old 22nd March 2011, 12:58 AM   #1
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Default LM4780 oscilation with reference PCB

Hello,

I just build a LM4780 amplifier, using the reference PCB from national.com. The sad thing is that it goes into oscillation after 30-60 seconds after powering it on. Sometimes even faster with the source in. The oscillation occurs with source in or without it.
The only thing I changed from the difference layout were the Ci1 and Ci1, because I was unable to find 68uF, I used 47uF, but I don't think this is the problem.
Any advices are appreciated.
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Old 22nd March 2011, 01:27 AM   #2
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How do you know it's oscillating? Is it low frequency (motorboating) or high?

The 47uF shouldn't make a significant difference. If it's a reference design (is it the one in the datasheet?) the chances are you've built it wrong. If this is the case it can be very hard to find your error, you check it and check it but you don't see it.

w
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Old 22nd March 2011, 08:18 AM   #3
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here's the sound:
http://www.swooms.com/doc/oscillation.wma

So you are telling me the reference design is wrong ? To be honest, I used press and peel, but it wasn't that good, so I have to aid it with a marker. If it's the PCB to blame, I will post pictures soon.
This makes me very sad, first time I tried using a strip-board, and it was a failure. This is the second attempt...
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Old 22nd March 2011, 09:42 AM   #4
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He is saying you have probably assembled it incorrectly. Not that the design is wrong.
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regards Andrew T.
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Old 22nd March 2011, 09:52 AM   #5
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meaning that I put the wrong parts in the wrong places, I will check, but I really don't think so.
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Old 22nd March 2011, 12:32 PM   #6
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yeah, my bad. Rg and Rsn 2.7k not 2.7ohm. Going to the store
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Old 22nd March 2011, 12:56 PM   #7
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Dou You have a picture of the built panel?
Did you electricaly isolate the IC from heatsink ?
(my TDA amp did funy things without it)

and how does Your PSU look like?

The bad resistor value should solve Your problem, if not then the above information might help us a fair bit.
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Old 22nd March 2011, 01:16 PM   #8
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I will post pictures, as soon as I replace the resistors
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Old 24th March 2011, 05:37 PM   #9
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so, after I changed the resistor, the amp works ok, .... )) maybe wonderful is the right word, I just love how it sounds. And I am using some not so good Sony speakers (can't wait to change the tweeters) but still it sounds wonderful.
Yesterday I spent many hours with making the case. It lacks the upper part and left, but I have some perforated metal, and I will install it soon. Here are the pictures:

LM4780_AMPLIFIER

I also have to paint it, or cover with some textile don't know
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Old 25th March 2011, 02:44 PM   #10
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Glad to see this this report as I still have five of them that I haven't done anything with yet. jer
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