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Subwoofer Hum problem

10K views 13 replies 12 participants last post by  DonH50 
#1 ·
First let me say that this is my first post, so if I forget to include specific details or something else, forgive me.



Background info:


I've had a Mirage Nanosat 5.1 Surround Sound System for a couple of years now and it's worked beautifully up until a few days ago. I was watching netflix through my PS3 when suddenly my Subwoofer (began humming, very loudly I might add. I've done some trouble shooting and found it's something with the sub itself as it makes the hum with and without the audio cable (will hum if the only cord attached is the power cord). At first I thought it was a ground loop problem; however, after letting it discharge and opening it up I can't find anything wrong with it. I also tried another outlet in another room (in case another device was interfering, as I read could be an issue), still hums.


Here's a picture of the inside (if it helps), not sure what that white stuff on the side is.





If anybody knows anything, that's be great. Otherwise it'll be off to the repair shop.



Thanks
 
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#2 ·
A hum like this is most often associated with a failure in the power supply. Unfortunately, troubleshooting that is something a novice would find difficult at best. From the photo it looks like a bipolar analog supply, which are the easiest to work with, but I can't give you much more than that. You might see if there's a service manual online, or I would suggest having the thing serviced. I use a particular guy for amp servicing, but he's in California, perhaps that's not good for you. I ship stuff to him, he fixes and sends back. Doesn't seem to matter what it is. PM me if you need his contact.
 
#6 ·
Hi,


I'm just responding to the thread about the hum in Mirage subwoofers. I have a customer who brought in a Mirage Omni 8 with major hum. The speaker is at full excursion when the woofer is plugged in and either on or in auto. I suspected the power supply. Upon examination I found the main caps to be swelled. I replaced them and still same issue. I tested every component in the main amp board due to the fact that this company does not supply a schematic or any support other than replacement parts that are expensive and sometimes not available. I found no failed parts in this unit. The only thing I have not tested is the transformer. I suspect that there is a bad winding and possible straight AC is getting through. I cannot find the rating of this transformer and short of replacing this, I feel this unit is not worth fixing.


mike
 
#10 ·
Hi,

I'm just responding to the thread about the hum in Mirage subwoofers. I have a customer who brought in a Mirage Omni 8 with major hum. The speaker is at full excursion when the woofer is plugged in and either on or in auto. I suspected the power supply. Upon examination I found the main caps to be swelled. I replaced them and still same issue. I tested every component in the main amp board due to the fact that this company does not supply a schematic or any support other than replacement parts that are expensive and sometimes not available. I found no failed parts in this unit. The only thing I have not tested is the transformer. I suspect that there is a bad winding and possible straight AC is getting through. I cannot find the rating of this transformer and short of replacing this, I feel this unit is not worth fixing.

mike
IME a driver at full excursion means DC to the voice coil, which in turn is usually shorted output transistors. Could be bad diodes in the power supply, or a broken feedback loop (which may cause oscillation but I have seen the normal internal offsets rail the output).
 
#7 ·

Quote:
Originally Posted by Solid State Amp  /t/1433257/subwoofer-hum-problem#post_22919407


Hi,


I'm just responding to the thread about the hum in Mirage subwoofers. I have a customer who brought in a Mirage Omni 8 with major hum. The speaker is at full excursion when the woofer is plugged in and either on or in auto. I suspected the power supply. Upon examination I found the main caps to be swelled. I replaced them and still same issue. I tested every component in the main amp board due to the fact that this company does not supply a schematic or any support other than replacement parts that are expensive and sometimes not available. I found no failed parts in this unit. The only thing I have not tested is the transformer. I suspect that there is a bad winding and possible straight AC is getting through. I cannot find the rating of this transformer and short of replacing this, I feel this unit is not worth fixing.


mike

You can figure out the rating of the transformer by the Amp the Mirage uses, for example a STK or STA etc. the look for the data sheets on the amp and it should tell you what the

Range of voltage the amp will accept. Also the transformer should only step down the voltage from 120 or 230 to just above what the Amp chip will accept in DC. The transformer would

have to go through a rectifier to get to that DC voltage. So you can test the AC voltage before it gets to the Rectifier and then after in DC.


Athanasios
 
#8 ·
I've the same system, Mirage nanosat, and recently the same problem. I've had no problems with the subwoofer or small satellite speakers, until recently. When powered on the unit buzzez very loudly, whether RCA plugged in or not, nor when plugged in electrically in another room. Do I really need a replacement ?
 
#9 ·
I had the same problem, I read it tends to happen to them so I didn't bother trying to fix it. I bought a used one on eBay to replace it, and now it doesn't work either. It just doesn't make any sound. I'm debating picking up an s-10 next time they come on Woot, but I might have to go with another brand of the woofers aren't reliable. I love their speakers though.
 
#13 ·
Sorry to revive a dead thread, but I have acquired one of these humming subs from Kijiji our popular classified site here in Ontario and did some probing on the scope. I assume they will all fail the same way so I hope to share my success/fail fixing it.

There is a bridge rectifier I decided to probe both +/- outputs simultaneously and the output lead to the speaker and the (+) output of the bridge rectifier while feeding it a 30hz signal.

The rectifier makes an obvious 120hz wave on the (+) but there is no discernible wave on (-)
(-) has a very steady -56v DC compared to ground.
on (+) the 120hz wave is 53v peak to 0v ground.
The output noise is very consistent on the downward wave with the biggest spike co-coinciding with 0v to ground on (+)...
There is some higher frequency noise before and after - but does it matter? maybe not...

So my plan of attack is to:
1) solder out the bridge rectifier and test it
4) provide DC power at rectifier (+) (-) from lab power supply
2) replace 2 4700uf capacitors
3) measure at each transistor when the noise is cleaned
 

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#14 ·
You have a lab supply powerful enough to run a sub? I'm impressed, and jealous...

Interesting results to be sure. Hopefully it is a shorted diode and/or open/bad capacitor and not a blown output stage in the amp messing with the power supply. The HF noise could be a circuit oscillating briefly due to the rail modulation.
 
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