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Old 17th November 2005, 12:08 PM   #1
beanbag is offline beanbag  United States
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Join Date: Dec 2004
Location: california
Default Want to bypass power supply, but...


I recently rebuilt the power supply in my LM3886 amp. Before it was a single bridge, and now I changed it to the double bridge design. Before, there was only a 1 Ohm, 100nF snubber directly across the terminals of the elctrolytic cap. There was a huge amount of HF noise (MHz), but the amp sounded fine. After the rebuild, I put one terminal of the snubber on the terminal of the elctrolytic cap, and the other terminal connected to the star ground. I made changes in other parts of the circuit, but anyway, the noise on the power supply is much less now.

However, the amp sounds different, I think. I am not sure, but it seems like female vocals sound more blurred or "stuffy". Again, I am not sure since I didn't listen to my amp for a few days while it was being upgraded. The reason I connected the other terminal of the snubber to ground was coz I thought you wanted to reduce high frequencies with respect to the star ground, not necessarily the negative terminal of the electrolytic cap. First question is, was this the right thing to do?

I'm also interested in adding some smaller caps near the actual LM3886. The problem is that due to the point-to-point wiring, the amp is about 4 inches away from the star ground. That means that I will have 100nH worth of inductance. At very high frequencies, it comes out to a reasonable fraction of an ohm. Any suggestions on what to do?

Finally, I don't really understand the idea of putting a cap between the + and - pins of the amp. I can do this locally, but doesn't it just couple the noise and signal from one supply to the other? What is the point?
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Old 17th November 2005, 04:47 PM   #2
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Maybe you can make a star ground next to the chip, and wire the two chip's star grounds together on the PSU ground. I did this and it allows 1/2 inch ground path !
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