Question Subwoofer amp mechanical buizz

NdG1977

Novice Member
Hi Guys,
I just put back at work my old surround system (nothing impressive, but that's available):

JBL 5.1 SCS175
H/K AVR1550.

It still works, and that's already something, but I have a problem with the active subwoofer that keeps buzzing, it's not loud, but in the evening is noticiable and drives me crazy.
As soon as I turn it on it starts buzzing and doesn't matter what's connected to or what's plugged in, volume or cut freq don't have any effect. constant and stady noise even with woofer disconnected.
Putting the ear up close it's clearly mechanical.
It looks like it comes from the transistors and not from the transformer, but could be a transfer from the plate...
The Caps are not bulged and everything seems ok; it works perfectly apart from the buzz....
It's kind of complicated to get inside the circuit board as it is mounted upside down and all glued down, but I could easily get to the transistors and caps.
What do you suggest to try next? Got a multimeter, but no oscilloscope.
Change Caps could solve it? My competence in electronics is limited, but enough to fiddle around with a soldering iron and change a couple of components. It's not worth to service it professionally as in the worst case I can put a brand new amp in and might even be cheaper.
I attached a few photos.

Thanks!
 

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dobrykamil

Well-known Member
What If you detach transformer form base plate?
 

drstuf08

Active Member
you could try removing the earth connection from the mains cable

it helped the hum on my power amp, there are many subs out there with no earth pin in the power connector i assume to stop ground loop hum.

or try another power socket or try turning off the lights in the room

if all of that doesnt work its probably a bad cap
 

Conrad

Moderator
Don't you need to be really careful with that?
I thought only double insulated stuff didn't need an earth.
 

NdG1977

Novice Member
Thanks guys for all the answers!
I Loosened the transformer and turned on the device hum was there... then screwed it back very tigh, turned it on...no change...if it's on it is there, no matter what...
I haven't tried to detach it because it's glued down with a kind of bituminous glue and it's not going to be easy...can surelly try...
No Earth pin in the connector already.
Tried different sockets in other areas of the house and no change. Can actually try to bring it to a friend house and try it there, but not having a earth pin I suppose it cannot be a earth loop right? Maybe his power is less polluted, who knows...
I wanted to avoid buying decoupling stuff because it would be probably worth more than the sub!
 

NdG1977

Novice Member
I did a couple more tests:
Removed transformer, when loose it makes some noise, but less than when on the back plate.
Put it back and now is a little better; worse than when free, but better than before.
When I mount it back on the box it gets noisier, surprise, it a reflex...kind of expected....
Found a temporary fix let the screws in the rear a little loose, now is better. Still need to find if the cause is the line, transformer or caps.
I suppose there is no way to know if not changing the components...
 

NdG1977

Novice Member
PS, I measured the noise with an extremely precise device, an unknown app on my phone :smashin:, and it looks like is 300hz, when on goes from -90 to -75/80 db peak.
 

noiseboy72

Distinguished Member
you could try removing the earth connection from the mains cable

it helped the hum on my power amp, there are many subs out there with no earth pin in the power connector i assume to stop ground loop hum.

or try another power socket or try turning off the lights in the room

if all of that doesnt work its probably a bad cap
Please, please don't remove safety earth's from equipment. Not only do you risk giving yourself or others an electric shock if the sub goes faulty, you also risk a fire, as high earth currents could flow down the audio ground and that cabling is not designed for that level of current flow.

Isolators and good electrical practice such as star earthing, keeping unbalanced cables short and away from mains cables etc. are much safer and will cure the problem in a much safer way.

Just because "it works" doesn't make it safe now or in the future....
 

NdG1977

Novice Member
I found a post of a guy that claimed that grounding the chassis to the earth pin in the wall socket solved the hum for him. Tried that, no change.
I also have a similar issue on the receiver, transformer humming but in this case also the speakers do some noise... Starting to think it might be some DC offset? I definitely have to bring the 2 to a friend house and check if it does it there too...
 

noiseboy72

Distinguished Member
Mechanical buzz from the transformer basically comes from poor construction of the transformer in the first place. Stray magnetic fields cause the transformer to vibrate and the mounting amplifies this. Thick neoprene washers can help with this and with toroidal transformers, you will usually see them fitted. Conventional transformers tend not to be fitted with any noise absorption, but a 3mm slice of rubber between the chassis and the transformer will quite often cure the issue of mechanical transmission of the noise and reduce it to an acceptable level.

If the mains voltage to your house is a little high, this makes things worse, but there's not often a lot you can do about that!!
 

NdG1977

Novice Member
Thanks! Just got some physical capacitors (AKA neoprene strips) to put under the mounting of the back plate... It should make it bearable.
Just a couple more questions to understand how it works!
The amp. doesn't have any ground so from my understanding there couldn't be any ground loop, but from what I read my neutral could be slightly off... Can this be measured with a normal multimeter? I have AC (end of scale 200v) that thankfully from ground to N shows 0. Would it make sense to do the same measurement in DC?
 

noiseboy72

Distinguished Member
Ground and Neutral are tied together at the household inlet in most cases, so should be zero. DC offset is not unknown, but is uncommon. You can get weird things happening if you have lots of LED lamps or other devices with small switched mode power supplies. These create high frequency harmonic distortion and that can manifest itself as buzzes and audible noise from transformers.

Try putting the neoprene under the transformer and experiment with what tightness the bolt likes to be to reduce noise to a minimum.
 

NdG1977

Novice Member
Thanks for the replay!
Unfortunately I have quite a lot of LEDs almost all my lights...and being smart they are always on, just dimmed to 0.
I’ll bring the sub and avr to a friend house and try it there to see if they behave the same, in the meanwhile I’ll turn off all the electricity but 1 section at a time to see if I have it in every circuit.
Thanks again!
 

NdG1977

Novice Member
Tried it on the 1st floor with the whole ground floor turned off and downstairs in different rooms with the rest of the house turned off at the main, and no improvement, still have to try it at the house of a friend, but with lockdown in place is kind of tricky.
It looks that it could be an internal issue or a main power issue, and I'm pretty sure is not a ground loop (No ground pin and grounding the chassis doesn't help.

I also have the AVR humming, but in this case you can hear it from both the transformer and the speakers, especially the front right one.
I was planning to tackle this issue later, but could it be related?
t was unplugged for 6/7 years and plugged in just a few weeks ago and showing this behaviour ever since, eventually also before.

Getting back to the SUB should I change the Transformer? Try a mains conditioner? Recap (even if they look fine)?
I measured the DC unloaded at the output of the caps and it reads 42V.

What other tests could I perform?
 

Racquel Darrian

Distinguished Member
When I had a constant buzz on my Mordaunt Short Sub I sent the control panel to A&J Audio who did a cheap but excellent repair and sent it back to me.

Worth just talking to them.

A&J AUDIO VISUAL
 

Ultrasonic

Well-known Member
you could try removing the earth connection from the mains cable

it helped the hum on my power amp, there are many subs out there with no earth pin in the power connector i assume to stop ground loop hum.

This is dangerous advice that on some forums would have been removed because of this. Nobody should do this as it could result in death, either directly or by causing a fire (and if the cause was found could also invalidate insurance cover). I would strongly encourage you to reinstate the earth connections that your equipment was designed to have (and is only legally sold with this in place).
 

NdG1977

Novice Member
Hi, I found the electrical scheme of the Amp.
I was planning to change a few caps and check diodes to see if things improve.
I'd replace the 4 caps on the mains C23, C24, C25 and C26. would you say it's a good starting point?
As said they look good from a visual inspection, but I'm kind of out of options...
 

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