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tekkas 8th January 2009 11:19 AM

broken sub, loud hum
Hi guys,

I just got my sub back off a mate, and it's come home busted...

It's a 12" active sub for home theatre etc....

I plug it into the power and a loud hum is instant, even with volume all way down etc, and nothing on imputs etc...

I can't see any damage inside it, but i measured .5v across the speaker terminal which would be the cause... but unsure what to do or check from here...

Any help would be appreciated, as really want to save this sub....


tekkas 9th January 2009 12:25 PM

bump? sureley someone can help...

AndrewT 9th January 2009 12:38 PM

measured 0.5Vdc or 0.5Vac?

musimedia 9th January 2009 02:05 PM

broken sub, loud hum

I have the same issue with a Misson M2S active sub. There was a power spike in my neighborhood and alot of electronics gave up.

If the sub is plugged in and turned on, the power light turns on and there is a loud hum in the amp and low vibration from the sub.

I haven't looked at it properly yet, but I know that the fuse did not blow. The issue is probaly a fauly power transformer and maybe the rectifier got hit too. I made a quick visual inspection and the internal amp shows no signs of fault.

trying to locate a schematic for it...


tekkas 10th January 2009 07:28 AM

Yeah exactly the same as my prob...

And was DC not AC across the speaker terminal...

AndrewT 10th January 2009 09:31 AM

0.5Vdc on the speaker terminals.
Check the PSU voltages.
And check they are the same when they arrive on the amplifier PCBs.

Take care not to short anything out, nor to touch anything directly connected to the mains.

Add insulation sleeves to your DMM probes.

amptech 23rd January 2009 12:05 AM

sub amp fault
Hi there,

remove your speaker from it's amplifier as any large dc will burn it's coil out and you'll need an new driver :eek:

Now if you can get a 10 ohm 10 watt resistor or higher watt type all the better because (1) if you have a meter set it to say 200vdc and connect the resistor and the meter in place of it's speaker load and repower up what reading are you getting now? it does no matter if the resistor get warm it saves you burning your speaker out.

You might have large dc the o/p thats why your hearing humm.
carry out some voltage checks, a balanced working amp will o dc on it's o/p.

Lumba Ogir 24th January 2009 06:08 AM

and he should not forget about the lamp bulb too.

amptech 25th January 2009 12:02 AM

faulty sub with hum
what is the dc voltage supply rails with in the circuit?

yes, you can use the old lamp trick a long with ac mains power in and all shows is there's a heavy load being pulled ie, a short.

You need to trace the fault ie, supply voltage or o/p short.............

What type of o/p device is in the o/p stage ic or transistors?

If there's transistors use your meter and read the base pin voltages if there's large voltage there the transistor is being hard switch on.

Faulty power o/p ic's need replacement as they can be damaged all to easy.

longthrow 25th January 2009 02:59 AM

can you post schema or pic/board pic so others can "visualize" your trouble..:smash:

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