mtv (04-06-20)
Great 400watt Sub... Seen one pass my bench many years ago...
Start by replacing that OLD crappy RCA lead with a New one and I bet the hum goes away..
Or move the Power board Adapter 1 meter away.. RCA cable is stuffed.
Last edited by Johnno; 04-06-20 at 03:44 PM.
mtv (04-06-20)
I see this fairly often.
Poorly-shielded cables picking up hum and all sorts of crap from modern switchmode power supplies, compters, TV's etc.
I'd be fitting a good-quality shielded RCA cable as a bare minimum.... wrapping several turns trough a torroidal ferrite core as a filter will also reduce interference.
Do the same with the power cable... even a clamp-on core will help.
This is of course assuming the hum is entering the amp from outside.
Faulty electro caps in the internal power supply are common in older gear.
Hi all,
Problem solved! - Wait for it..!!
With no humming, I decided to change the RCA leads as suggested.
I slid out the audio (Tv, DVD Player, AMP), bench on the tiled floor so I could get access to the rear of the Amp - and the humming started!
WTF I thought?
I placed my hand onto the double adaptor (ready to ditch it as per MTV's comment), and the humming stopped.
On wiggling the double adaptor the humming starts and stops... Then I noticed that if I put upwards pressure on the RCA socket, the humming stops and if I press down the humming starts...!
I have basically wrapped the RCA lead upwards on the amp.
As of into the second day no humming. Does this mean I have a faulty 'earth' on the RCA socket inside the Amp?
In all fairness, the number of times I have moved the amp with all cables, I must have stretched and pulled them a fair bit over time and three flat/house moves...
The question begs (well outside my pay scale!), why would the humming stop when I would switch the ceiling light on?
Many thank you's for all the help and advice people.
Keep well.
GT250.
2 days no hum does not mean problem solved.
First try a different cable. Forget the adaptor and plug one cable from the amp to one of the sub's inputs. Wiggle it. Try the other input with one cable and wiggle.
If it hums when you apply pressure on both sockets you need to remove those Philipps screws and get dirty.
I am amazed you haven't done that yet. Surely you would want to have a look inside
Re-solder the jacks and check for cracks on the copper tracks that surround them.
If one socket stays quiet despite the wiggle/pressure test just use that. There is no need to split the mono sub out from the amp into two inputs of the sub.
I have never heard of a stereo sub woofer, would make no sense.
The two inputs are for amps that don't have a dedicated sub out like a classic stereo amp. Bass is not always centred.
Last edited by Uncle Fester; 05-06-20 at 07:08 PM.
Update: A deletion of features that work well and ain't broke but are deemed outdated in order to add things that are up to date and broken.
Compatibility: A word soon to be deleted from our dictionaries as it is outdated.
Humans: Entities that are not only outdated but broken... AI-self-learning-update-error...terminate...terminate...
I agree wholeheartedly with all that is expressed in Uncle Fester's post above.
Assumptions are always dangerous.
Go the step further and do the job properly.
Surely some one has a usb type scope that could be lent to johnno then we can see what the problem is ? If its 50 hz its broken/dirty /coroded earthing if its 300hz then its switch mode supply caps or broken ferrites if its there all the time but varying in level (but not heard ) then its in the amp circuitry chasing guesses on a forum is great that every one get to put their two bob in but it dont solve it . Even a fluke scope meter will show something
Hi Guys,
I might have not explained myself properly...
The double adaptor is/was in the Amplifier - not the Sub.
The Amp has a single RCA SUB Out socket. So in my naivety, I used a double adaptor to take the Twin RCA leads from the AMP to the SUB (which has dual RCA inputs).
I have removed the double adaptor and using a new single RCA to RCA lead as per MTV's suggestion.
When the RCA socket on the AMP is pressured down it humms and when I apply pressure upwards the humming stops.
I must have broken/cracked the internals of the RCA socket in one of the many times it has been moved over many years.
As it is now, there has been no hummng for three days. Whereas it was humming every day before.
Alas, it still amazes me why flicking the light switch on stopped the humming..
Cheers,
GT250.
It actually quite explainable. Flicking light switch most likely creates a potential change in the sub woofer power circuit resulting in the voltage potential change of the RCA connector ground and PCB ground which creates a micro spark. That micro spark creates a thin conductor improving the connection. As it is very thin conductor it can easily break again due to temperature expansion causing the humming noise again.
This also might mean your light switch has a problem or the house earthing is not perfect.
loopyloo (08-06-20)
That's what I was getting at earlier.
It's pretty obvious the amplifier had a fault, even if only that it's failing to filter line noise properly, but the fact that the light was triggering the noise could have meant the light is also faulty. I don't know what's in the light fitting circuitry but if the light is faulty and earthed and it's wired active to the light (neutral to the switch) then when it's off, maybe it could inject noise into the earth line. I've seen stranger things happen.
Anyway if it were me I'd check the light too if only for peace of mind.
Last edited by loopyloo; 08-06-20 at 12:54 PM.
I have one where the RCA connector had cracked solder on the ground side. It was damaged from being pushed against a wall. It would crackle and hum. Easy fix.
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