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Old 1st September 2008, 11:07 PM   #1
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Default LM3886 Very Loud Buzz

Hi all - I recently aquired a few active speakers made by DAS. These use the LM3886 based amp. 2 x lm3886 for bass, 1 for top.
The problem is that when the amp is powered up, it produces a full volume buzz on the 2 x lm3886 side (bass), which I think is at mains frequency but cannot be sure.
If I connect the bass cone to the to the (1 x lm3886) side, the cone just pushes forward, lots, as if DC voltage was being sent to cone.
I've measured the output at the speaker leads with no load attached and am getting 70v on the bass side, and 32v on the other side.....

The volume controls make no difference at all....

Any help would be greatly received...

Many thanks

Colin
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Old 2nd September 2008, 06:29 AM   #2
Nuuk is offline Nuuk  United Kingdom
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It sounds like one rail is disconnected somewhere. As it is the same for both sections, I would be looking at the PSU, but without a circuit diagram or pictures, I can't be any more specific.
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Old 2nd September 2008, 06:31 AM   #3
Puffin is offline Puffin  United Kingdom
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It would seem that all is not right. Were these working when the last person used them?

If there are 2 LM3886's for each bass unit, it would seem that these may be running in bridged or parallel mode for more power.

The measurements you have taken are not what you would normally expect to see at the speaker terminals. Anything over 100mv is too high, so 70v cannot be right. Are you sure that you have measured the voltage at the right points. The readings you have are more ikely to be the rail voltages. These chips normally run at +35v,0v -35v.
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Old 2nd September 2008, 07:26 AM   #4
Mooly is offline Mooly  United Kingdom
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Bridge mode or not there has to be zero volts across the speaker points. Find the data sheet for the IC's and carefully measure the rail voltages on the IC pins. Sounds more like a zapped IC to me.
Are they hot even with no speaker attached ?
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