counterfeit LM1875?

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Hi

I rebuilt my gainclones a few months ago. Now I had to order some new LM1875s because I only had 1 left. So the new ones look slightly different from the older ones but I didn't think much of it.

So I rebuilt them and the right channel gainclone was giving a horrible DISASTER noise! The right channel uses the new LM1875 that looks slightly different from the older one. Eventually I added a Zobel and it started to work again. So I thought the right channel is closer to the power leads so maybe that was why it needed a Zobel network.

I know that manufacturers can make changes to the production line or maybe even subcontract out to someone else, but should the actual chips look identical?

Yesterday, the right channel gainclone has suddenly stopped working! I was not listening at high volumes. It wasn't even hot. I have isolated the problem to the right channel gainclone. The resistors are OK. Power supply voltage is OK.

I am seriously starting to think that the right channel LM1875 was a fake!? It has two slight circles indented on the front, lower left and top right corner. These two circle indents have B1 and 2 embossed inside them. On the other side, the metal part is a very slight wedge, the two sides are not parallel. Also, on the front, the plastic composite is slightly chamfered. The left channel LM1875 does not have any of these features.

On balance, do you think the right channel LM1875 was a fake???
 

tomchr

Member
Paid Member
2009-02-11 12:58 am
Calgary
www.neurochrome.com
National Semiconductor (now Texas Instruments) does not contract out chip manufacturing. A genuine LM1875 will be made in a National Semiconductor fab and tested by National Semiconductor before release.

Now, it is possible that your original circuit was marginally stable and the new LM1875 was slightly different than the old one (it could have had slightly higher gain for example) and the addition of the Zobel network brought it back into stability. The issue you describe could also be caused by a cold solder joint.

~Tom
 
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