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Tamp ta1400, mains hum problem

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Tonskulus View Drop Down
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    Posted: 17 September 2018 at 5:16pm
Ok, maybe I should ask this one! 
One of my T-amps is making strange and annoying 50Hz hum from speaker.  Its quite "sharp" hum, as can be heard from the video above. 
Notice as I put my hand on the transformer, hum disappears almost completely.  Same thing, if I put my hand over ch2 filtering capactors, hum decreases noticeably (not so at ch1 caps) so the problem might be in ch2 circuitry. 
I measured +/-15V power supply caps and they were okay.  There IS NO input connected, so the problem is inside the amp itself. Maybe something to do with groundings?  Just couldnt find any.. checked the connectors etc.
Also if input is connected and any (bass) input signal is coming at very low volume, hum disappears.  I'm using this amp bridged to my subwoofers. 

Turn up some volume and listen:



Edited by Tonskulus - 17 September 2018 at 5:19pm
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mini-mad View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mini-mad Quote  Post ReplyReply Direct Link To This Post Posted: 17 September 2018 at 6:43pm
Let me be the 1st to say "it's humming becuase it doesn't know the words!" Big smile


Couldn't help myself... how competent are you with electronics? 

It does SOUND like it could be an earthing problem but how to go looking for it involves a "good" knowledge of electronics... have you tried switching out the power lead to start?... tried a DIFFERENT socket in the house to make sure something outside of the amp isn't causing the earth him?

Try the simple stuff 1st..
If it sounds like a gorilla is trying to escape, turn it down.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 17 September 2018 at 10:12pm
Check the bias. And stop using it in bridge mode until its fixed
Kevin

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Tonskulus View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tonskulus Quote  Post ReplyReply Direct Link To This Post Posted: 18 September 2018 at 4:38pm
Can't be faulty mains plug/power lead because problem only occurs @ channel 2.   Ch1 is totally fine!

I'll measure the bias next.. no idea what it should be tho, but for sure I can compare ch1 to ch2.  
I'm ok with electronics (have designed and built many amps etc..), but this thing is tricky. Just asking if someone has same problem with t-amp + solution, not to waste too much time for this one :)





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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 18 September 2018 at 5:38pm
That sure is a strange one. Putting pressure on the transformer cutting the noise does seem to point to some sort of mechanical fault in the transformer. Maybe a loose coil, or laminations? I have an old toroidal that I use for a car/van battery charger I built that occasionally hums as well. That is definitely mechanical.  

Is the PSU common for both channels, or are there separate secondary coils? That could be a clue.

Also, have you considered contacting the service departmaent at Thomann? They may be able to help.


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Tonskulus View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tonskulus Quote  Post ReplyReply Direct Link To This Post Posted: 18 September 2018 at 7:33pm
Well, it does not look like transformer failure. Hum decreases even if I put my hand close to it (leaving lets say 3mm gap). And also same thing if I put hand on those filtering caps.. hum decreases notceably. 

PSU is half common:  single transformer but separate secondary windings for both channels. 

One thing I just noticed: hum disappears completely if I disconnect signal connectors beetween pre-amp board from power amp boards. So it looks like the problem is in preamp board. There are also two +/-12V regulators, I just changed all the electrolytics there. Let's see if it helps.. if not, i need to change everything including regulators itself and propably bridge rectifiers. They rarely fail, but I have seen that happen in this very same amplifier. One channel was missing high voltage rail, caused by broken main rectifier. 



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Tonskulus View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tonskulus Quote  Post ReplyReply Direct Link To This Post Posted: 18 September 2018 at 7:51pm
Ok, it was silent for about one minute and then.. hum started again :/

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 19 September 2018 at 8:56am
Ok, so possible earthing problem on the preamp board? Maybe try run another earth cable to the board from the main earth point of the PSU. If not, change the +/-12V regulators and see if that helps.


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