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Old 27th February 2009, 07:27 AM   #1
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Default First chipamp build need advice lm4780

I'm building my first chipamp from an audiosector m4780 kit and a 300va 22v+22v antek transformer. For my first build i decided to assemble one stereo unit. I'm now ready to put everything into a test chassis but I cant figure out a few things. I was hoping someone could clarify.

The first thing i cant figure out is what should be grounded and where? I see from the schematic that there are 3 grounds coming from the amplifier board, zero from the rectifier board and the ground from ac power. I've read through a number of builds but can't seem to find what I'm looking for. I can see three extra connections points on the amp board two near the cz/rz networks and one labeled chg. Are these the ground points for the amp board? Should i connect all four mentioned grounds to the chassis?

http://www.audiosector.com/lm4780%20amp.pdf

The antek transformer has two pairs of red and black primary wires. Can i tie the like colored pairs together or should i run one pair to power and cap of the others? Here's some info on the transformer.

http://www.antekinc.com/AN-3222.pdf

Since this is my first build i was wondering if a fuse on the ac power before the transformer is good enough security for a beginner?

If i can find out how to post some pics i will. Thanks.
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Old 27th February 2009, 11:59 AM   #2
Nuuk is offline Nuuk  United Kingdom
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Start off here . I should answer a lot of your questions.
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Old 27th February 2009, 03:03 PM   #3
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chevy, use nuuk's bulb tester, don't even think of skipping it...
yrs with newbie experience, ben
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Old 28th February 2009, 07:04 AM   #4
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What a great article Nuuk, thanks for taking your time and putting that together. It helps to have things explained step by step when your just starting out.

Now that i know what needs to be grounded can someone help find the location where I should ground the power supply? From what I gathered from the article I need to take the 0v lines from the rectifier and attach them to earth ground. The only problem i see now is it looks like the lines i need to ground also need to go the the amp board. If you look at the schematic for the rectifier board and the schematic for the amp board you can see that the four lines coming from the rectifier board go into the amp board (PG+ V+ PG- V-). Maybe someone that's assembled a kit like this could share what they did for the power supply ground?

Amp
http://www.audiosector.com/lm4780%20amp.pdf

Rectifier
http://www.audiosector.com/lm4780%20psu.pdf

Click the image to open in full size.

I've also decided to add fuses after the rectifier board for at least a short time until i know things are working correctly. Is there a formula to calculate the correct amperage fuse to use?

Thanks for the help.
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Old 28th February 2009, 08:28 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
the power supply has two sets of flow and return.
It doesn't have, nor need another return (ground).

The amplifier has a Power Ground and I think it has a Signal Ground.
The speaker has a ground that comes back as the black amplifier terminal.
Each of the input RCA has a return. These are the barrels of the RCA sockets.
All these returns (grounds) must be connected together at the central star ground (I call it the Audio Ground).

To make the amplifier safe to use, we must connect the Audio Ground to Safety Earth.

The Safety Earth MUST be a permanent connection from the third protective earth wire to chassis. This MUST be a continuous cable from the mains input terminal to the permanent fixture to the chassis. Preferably very short and close to the input socket/terminal.
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Old 28th February 2009, 08:31 AM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by chevy1965usa
I've also decided to add fuses after the rectifier board for at least a short time until i know things are working correctly. Is there a formula to calculate the correct amperage fuse to use?
It's a waste of time and effort to put in supply rail fuses before the main smoothing capacitors.
Instead use a light bulb tester from your wall socket outlet.
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Old 28th February 2009, 02:01 PM   #7
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It's not only waste of time but it's dangerous for your speakers; in case one rail fails, you may get substantial DC offset at the otput and damage the speakers. That's the main reason I'm always against using fuses after rectifiers. Properly chosen fuse on transformer primary should be sufficient for protection.

Check this thread: http://www.diyaudio.com/forums/showt...56#post1508956 Although it's about LM3875, most of the info will apply to LM4780 as well.
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Old 28th February 2009, 05:08 PM   #8
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Quote:
Originally posted by Peter Daniel
in case one rail fails, you may get substantial DC offset at the otput and damage the speakers.
Does the undervoltage protection not work as described in the datasheets? With one rail missing it should switch the output off.
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Old 2nd March 2009, 03:03 PM   #9
sasmit is offline sasmit  India
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with the-ve rail negligible 8v or so..4780 will motorboat..it shuts off at arnd 1v on negative rail. With +ve rail near 1V it continues to work ..but the output too distorted . I have two separate boards for +Ve and -Ve rails ..one of them was failing ..while switching it to different rails for checking ...noticed this.
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Old 2nd March 2009, 06:44 PM   #10
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So it does not work according to specs?

According to specs the undervoltage protection should set in, if the negative rail is less than 8 V from ground and if the positive rail is less than 14 V from the negative.
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