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Old 25th October 2004, 03:28 PM   #1
kmj is offline kmj  Sweden
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Default help from pro's wanted.

Hi, I have a little problem which is non-diy related but since you guys know these things this seemed lika a good place to ask.

The thing is that i bought an Amplifier from China and had it delivered to me in sweden. When i got everything wired up i noticed a kind of thumping in the loudspeakers....Totally random and it sounds like somone taps a finger on an Acoustic guitar. It doesn't need to be a source (cd etc.) connected. The only manipulations made are the switching of the powercords plug. Theres the same eletricspecs in sweden and China.

Does anyone have an idéa of what it can be and what can be done about it?
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Old 25th October 2004, 03:44 PM   #2
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Is this one of the Chinese valve amplifiers that are being sold on ebay, (among other places)?

If so then I suspect LF instability. Is the noise a variant of the old 'motorboating' sound?
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Old 25th October 2004, 04:07 PM   #3
kmj is offline kmj  Sweden
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No tubeamp, rather this one:
Xindak A100

it's not like motorboating just lika a random thumping...somtimes its not present just to appera minutes later. Tryed to chande powercord by the way...didn't help.
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Old 25th October 2004, 04:16 PM   #4
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If the "taps" comes randomly, I wouldn't suspect the power supply....

Random taps and noises are most common with bad solderings or connections.....

If you do not want to send it all the way back to China, you may consider to "lifting the hat" of your new amp, and apply some cooling/freezing spay to try to find the bad spot......
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Old 25th October 2004, 04:25 PM   #5
kmj is offline kmj  Sweden
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that could be something to do....but why the coolingspray?
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Old 25th October 2004, 05:14 PM   #6
kmj is offline kmj  Sweden
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update....
it seems like the thumping is present at a greater degree at startup. In the first few minutes and later it lessens, not dissapearing thou.
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Old 25th October 2004, 06:12 PM   #7
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You are closing in

When the "taps" are more presant when amp is cold, it's seems even more likely that the problem are caused by bad soldering...

When using cooling spray on a wormed up amp, you cool down a specific small part of the PCB (or other connection) at a time... If the "taps" then starts again, you have hit the right spot....
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Old 25th October 2004, 06:59 PM   #8
kmj is offline kmj  Sweden
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"and there was light"
I will investigate this after a visit to the local hardwarestore and come back with the result...or lack of
thanks!
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Old 25th October 2004, 08:19 PM   #9
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Otherwise I live in Copenhagen
Not fare away
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Old 26th October 2004, 06:51 AM   #10
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thanks, i will save this thread so i dont loose it
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