Lm 1875 problem? :(

[fr1s]

Hi. I'm new here.
I just started building one of these amps in school for my project, I got the schematics of this amp from of the National site from their .pdf data sheet on page #8, it uses single power supply for two amps.

Here is!

Original:

(parts are displaced, move them to the right and up)

And mineslightly redesigned)




Also I used the same parts that are used in the National version in .pdf file

So the problem is that it plays for 6 seconds then it stops, it's on 30V with low current. I cant hear it at 25V at all, for power supply I used school power supplies, I don't have one made yet.

But at home I used my home printers power adapter(31v 2.5A) it give me really buzzing sound mixed with music, also my GND is not connected anywhere.

Also can you help me out with the power supply?
is this one any good?
Power Supply

Thank you

[pacificblue]

Originally posted by fr1s

It looks, as if the big electrolytic capacitor for the negative rail was the wrong way round.

Originally posted by fr1s
But at home I used my home printers power adapter(31v 2.5A) it give me really buzzing sound mixed with music, also my GND is not connected anywhere.

The printers power supply is probably a single supply. You need a split supply.

Originally posted by fr1s
Also can you help me out with the power supply?
is this one any good?
Power Supply

Pretty standard with little capacity. Try to make it bigger and use bigger capacitors.

[fr1s]

It looks, as if the big electrolytic capacitor for the negative rail was the wrong way round.

Look at the schematic, you can see better in there.
I have two Big Caps, so which one is the wrong way round?

The printers power supply is probably a single supply. You need a split supply.

That's what I thought :/

For the power supply what Cap's should I use??

[pacificblue]

In your photo the capacitor next to the IC has the negative connection on the right side. In the layout that side is marked with a +. Turn the capacitor around and hope it is not broken.

[johnnyx]

There is no 0v terminal for the PSU. The terminal marked + is positive speaker out, the one marked - is the negative speaker out, and this could be used as the 0v connection for the PSU. V+ is positive supply, V- is negative supply. I suspect that your problems are due to using a single rail supply for a split rail amp, and the caps are shown the correct way round. I found that a small 100n 100v ceramic capacitor soldered directly between pins 3 & 5 prevent hf instability.

[fr1s]

Originally posted by pacificblue
In your photo the capacitor next to the IC has the negative connection on the right side. In the layout that side is marked with a +. Turn the capacitor around and hope it is not broken.

oh ****
but on the original design it doesnt say which side is - or +.
but which way is it supposed to be?

[pacificblue]

There are several clues.
Clue one is the schematic in the datasheet. Those capacitors are C6 and C7, and you have C6 reversed.
Clue two is your own layout. The yellow plus symbols belong to the capacitors. Both capacitors should be the same way round with the minus markings to the left.
Clue three are the potentials. Vee or V- has a lower potential than GND, therefore the capacitor's negative pin goes to Vee or V-. Vcc or V+ has a higher potential than GND, therefore the capacitor's negative pin goes to GND.

@johnnyx:
Compare the photo from post #1 (3rd link) to the layout in post #3. The capacitors are drawn correctly in the layout, but in the photo one is reversed.

[mjf]

hello.
it looks like that the elcap in opposite the opamp is built in reverse...........
greetings........

[mjf]

hello.
perhaps problem caused by missing +position of the elcap in the schematic and so on..................
greetings.............

[fr1s]

Originally posted by johnnyx
There is no 0v terminal for the PSU. The terminal marked + is positive speaker out, the one marked - is the negative speaker out, and this could be used as the 0v connection for the PSU. V+ is positive supply, V- is negative supply. I suspect that your problems are due to using a single rail supply for a split rail amp, and the caps are shown the correct way round. I found that a small 100n 100v ceramic capacitor soldered directly between pins 3 & 5 prevent hf instability.

there is 0v terminal, it's just not labeled. It's below the lower 1000uF cap

So it my redesigned schematic alright?
are the polarities right way on the big caps?