Problem with Altec Lansing MX5021 Subwoofer

I have an Altec Lansing 2.1 speaker system. (MX5021 90W RMS
output, 30Hz - 22KHz audio range). It has been fine for more
than 3 years, but presently I am having some problems with it.

After power up, the speaker is almost okay. But very low volume
growling noise (i.e. random sound of low frequency in the range
30-50Hz ) comes out of the subwoofer. But noise is audible only
if someone attaches the ear to it. After some 1 or 2 hours
later the growling noise increases in volume and it becomes
audible even from 6 feets (But still low volume). To make the
things worse : this higher amplitude noise interferes with the
bass portions of the music and creates bits of bass which is
very disturbing. When I am hearing music, it perceives as if
the bass is becoming high and low periodically.

However, if I power off the speaker at that time and keep it
off for next 7-8 minues, it seems to become okay again.
However, the problem repeats again after 2-1 hours. I use
windows 7 with X-Fi XtremeMusic sound card, but when the noise
starts, I have experimented detaching the signal cables from
the subwoofer and the noise still continues. So definitely,
this is not a problem related to OS or soundcard drivers.

I've found one more way to make the speaker output "clear" when
it starts the growling noise. This solution does not work always.
I have some single tone wave files which I use to test the speaker
output. Astonishingly, if I play the 30Hz tone and 50 Hz tone for
8-10 seconds, the speakers seem to become "noise-free" again for
some time.

Btw could it be a power line problem ? I power the speaker from
home AC 230V 50~60Hz power supply. Recently we changed
residence and the noise problem started shortly after that. Is
it due to any power factor problem in AC power supply ? I have
tried changing the AC outlet (but at same residence) with same
results. However, I haven't tried powering it directly from an
UPS, so I am not sure if the problem is still there when the
speaker system is powered by generated supply of UPS battery.
But even if there is problem in the AC supply at my new
residence, should it really have any effect on speaker
performance ? After all they are stepped down inside the
speaker system and also there should be filter capacitors and
regulator IC.

I have also tried moving the satellites and the subwoofer, far
from any magnetic device, keeping the signal and power lines
physically distant. I have also tried keeping those lines as
less inducing as I could, i.e. so that no local magnetic field
grows up along the power line which could create
noisy/extraneous electric signal in the signal lines. In fact,
I have been using the speaker system for more than 3 years, at
that time I didn't have to be careful about these things and
the speaker was fine.

I am not so acquainted with AC power devices and audio
amplifiers. It will be fine if someone could explain me where
the problem actually is, and what should be my measures.

Hi,

This sounds to me like an electrical problem with the subwoofer's amplifier, from your description I'm guessing that one of the components has developed a problem. It could be a capacitor losing integrity and leaking DC current into the electrical path, or possibly a preamp integrated circuit going bad.

If you're lucky, it could even be something like a cold solder joint.

If it's a capacitor, a tech could easily spot it, usually, when they're that bad they've leaked some material.

Either way, you will have to have someone look at it, don't try it yourself.

Ap

Thanks for your reply..... I will be back when I get some update :) ... meanwhile we have guessed the reason for the noise...yes, most probably its a damaged capacitor or inductor (may be there is a buck/boost type converter inside it) which is causing kinda forced undamped oscillation due to the bad reactive device...when I try the low frequency single tones sometimes the noise dies out, most probably due to out of phase interference (i never know when the noise is at what phase) ......whatever, measure is to change the bad component :)

Hi Dwarf, Thanx a lot :). You guys at windows 7 forum are really doing great

I had a confusion that the power supply at the new residence might be unclean, and that might be the root cause. But today I checked it powering everything up from an UPS, and the noise starts again.......now a days the noisy oscillation grows really quick.....within 15 minutes it increases amplitude now....seems the problem is increasing, and I have decided to use headphones (got a BOSE one ) keeping the speaker unplugged until its repaired.

I have also tried shorting the inputs, which apparently does nothing. I mean noise neither increases nor decreases. I guess the noise is additive. Shorting the input means giving 0 voltage, which also means 0 output plus additive noise.

I haven't seen the Altec's power supply circuitry yet. But guess is : there might be some sort of feedback system in it, may be some sort of inductor-capacitor boost regulator, which has 2 poles. If the capacitor is somehow out of order, it may oscillate in the dominant pole low frequency. As the power supply itself is the cause, this results in forced oscillation, at last clipped by the power supply value.....or may be just things are kinda loose inside it. In any case the box needs to be opened and checked ...