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Old 6th September 2006, 07:02 AM   #1
Mrpong is offline Mrpong  Thailand
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Default A strange problem with bridged LM3875

Hello,

I just built a bridged LM3875 with Burrbrown's DRV134 chip. I heard strange "zzzzzzzzzzzzzzz" noise exactly like when we listen to an AM/FM tuner without station. The noise was not much loud and it remained on a particular level no matter how much I adjusted a volume knob.

- The output signal of DRV134 is feeded to each of LM3875 board.
- The schematic of LM3875 board is based on the picture below.
- Without bridge, the LM3875 board alone works perfectly.

Need your advices please........
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File Type: gif thorsten inv_chip_amp.gif (6.2 KB, 623 views)
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Old 6th September 2006, 08:46 AM   #2
Roushon is offline Roushon  India
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I guess you made parallel of two such amp, and not bridge. If bridge, then please post the noninverted amp circuit also.

I am curious.
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Old 6th September 2006, 08:52 AM   #3
Mrpong is offline Mrpong  Thailand
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Oops.....sorry for ambiguity

According to the circuit diagram, I have two LM3875 boards without volume. Instead, the volume was installed at the input of DRV134.

The V+ signal from DRV134 was sent to 4.7uf cap at the first LM3875 board. And the V- signal of the same DRV134 was sent to the other LM3875 board.

Both LM3875 chips were set up in the inverted mode (see picture).

And no buffer circuitry is used.
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Old 6th September 2006, 08:55 AM   #4
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check the connections from the drv134 to the 3875s. Make sure the connections are tight.
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Old 6th September 2006, 09:35 AM   #5
DIAR is offline DIAR  Finland
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I have buzzing problem with my single chip LM3875 amp (chipamp.com). It's not ground loop buzzing or natural background noise. It's something else.

Try moving the speaker cables. Does it change the sound somehow.

I tried several things to get rid of the interference but I haven't found out the solution. (zobel network, inductor and ferrites in speaker cables, better power supply bypassing, caps in +/- pins in LM3875, different speakers, shielded and shorter speakercables etc. etc....). I hope to solve the problem by installing the amplifier pcb in the same enclosure with the power supply. Now the power supply is connected via braided cable which is about 0,5 metres long.

Funny thing is that the problem occurs only in one floor in my house. The interference is not coming from my house. I made sure of that by unplugging all other electricity from the house.

Do you have any potential interference sources like switched-mode power supply.
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Old 6th September 2006, 11:26 AM   #6
Roushon is offline Roushon  India
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Quote:
Originally posted by Mrpong
Oops.....sorry for ambiguity

well, i now see that there was no ambiguity, it was lack of information i had. the problem seems to be more serious than i thought. may be there is ground loop somewhere when you connect the amps.
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Old 6th September 2006, 12:21 PM   #7
DIAR is offline DIAR  Finland
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Are you familiar with this document. It's about bridge and parallel chipamps.

http://www.national.com/an/AN/AN-1192.pdf
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Old 6th September 2006, 02:24 PM   #8
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I think you have a ground/noise problem on the DRV part of the circuit which the bridged amplifiers are somewhat cancelling out.

can you use a scope to chack the DRV outputs, or if not a scope, use a small amplifier/speaker to listen to the DRV outputs.
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Old 6th September 2006, 04:16 PM   #9
Mrpong is offline Mrpong  Thailand
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Thank a lot for every comment. Let me re-check and I will get back to you guys.
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Old 6th September 2006, 10:58 PM   #10
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Do you have a scope? Check around to see where the noise is occuring then track down the problem from there.

-SL
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