Amplifier plays loud humm

Thread Starter

kaae

Joined Oct 2, 2013
13
Hi, i Got a subwoofer amplifier that i leant out for some friends. I guess after playing loud on it for some days it just suddenly began to play a really loud huming noise 60 hz i Would guess,its so loud that you cant hear the Music Being played. And already if i turn up the attenuater to like 10 a clocl. it Will make the bass driver play really loud and almost hit it's x-max, like if the amplifier gets a really loud signal.
Any ideas what could be Wong. I have already opened it but couldn't see any defect or suspecies components.?
Thanks
 

MrChips

Joined Oct 2, 2009
23,054
What the heck! You must think we have supernatural powers.

To begin, what is the make and model of the amplifier?

Do you have a schematic or can you lead us to the link for a schematic?

So you've already opened it and expect us to imagine what you saw. Show us some photos, clear and well focused, please.
 

DerStrom8

Joined Feb 20, 2011
2,390
If I had to guess I'd say it's some bad filter caps in the power supply. That's usually the cause of mains hum in amplifiers. Check the power supply for bulging or leaking capacitors. But keep in mind, even if they aren't visibly bulging, they could still be dried up and damaged on the inside.

Matt
 

Alec_t

Joined Sep 17, 2013
11,972
Also check input sockets for damage, which might have resulted in an open circuit ground as mentioned in post #3.
 

tubeguy

Joined Nov 3, 2012
1,157
Thanks for the video.
It appears to be a problem with the input section, (prior to the volume control), because it appears to stop when you turn down the volume,
It sounds like a motorboating type of oscillation, not exactly a power supply hum.

Do you have any test equipment ?
 
Last edited:

MrChips

Joined Oct 2, 2009
23,054
I agree with tubeguy.

That is called motorboating, not supply hum.

The noise is from the input stage, not the power amp output stage.

Most likely coming from the section under that metal shield.
 

Thread Starter

kaae

Joined Oct 2, 2013
13
okay, so how should i approach it, you say it might be the input section, so is there any way i can measure my way towards the fault ?

i only got an multimeter, no osciliscope.
thanks.
 

MrChips

Joined Oct 2, 2009
23,054
You will have to reverse engineer the input section, i.e. try to draw the circuit schematic.

Start with the input jack and try to visually trace where the signal goes and draw all the connections. If you can provide clear photos of the top and bottom side of the board of the input section I'm sure someone here can help you.
 

MrChips

Joined Oct 2, 2009
23,054
You could also start at the power supply into the input board.

Look at the four capacitors at the left hand side of the board and replace those.

 

Thread Starter

kaae

Joined Oct 2, 2013
13
earlier iv'e already suspected the two largest capasitors but one one of them looked suspicies, so i replaced that one but it didn't change anything. so i guess i have to rule those ones out, i might try and replace the smal ones though as you said.
 

MrChips

Joined Oct 2, 2009
23,054
There are two dominant frequencies in your sound clip, one at 30Hz and another at 83Hz.

That could suggest that this is not 60Hz AC hum pickup.

Motorboating can be caused by feedback in the amplifier circuitry where a large current draw causes the power supply to drop and is fed back through the input stages via the power supply or via the preamp inputs.

The fact that the hum goes away when the volume control is turned down should provide some kind of clue.

Having no circuit diagram and no oscilloscope presents quite a challenge to fixing this one.
 

Thread Starter

kaae

Joined Oct 2, 2013
13
Thanks after replacing the Four smaller capasitors the motorboating disapeared !
Havnet tried it out with input signal yet but everything seems fine for now.
 

crutschow

Joined Mar 14, 2008
26,721
Motorboating can be caused by bad filter capacitors. I would check the electrolytic filter capacitors connected to the power rail(s) on the signal board. One quick way to check is just to temporarily connect an equal or larger capacitor across each of the ones on the board, one at a time, and see if it changes the motorboating.
 
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