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Old 14th January 2006, 06:21 PM   #1
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Default Subwoofer Humming problem

Hey Guys,
I am a newbie to this forum, and to diy to a lesser extent.

I just purchased a Boston Acoustics digital media theater (was somewhat top of the line computer speakers many years ago) from some jerk on ebay who sent it without the proprietary interconnect cables. Luckily I have some BA stuff here and set it up just fine.

However upon powering it, there is a constant and fairly strong bass humming. The humming stays at a constant level regardless of the music volume. I opened up the box just to see if there was anything obviously and apparently loose or broken inside, but I didn't see anything obvious, and i dont know too much about circuit boards and their components.

Does anyone have the knowledge to diagnose this given the problem I have described?

I am pretty sure it lies within the powered amplification in the subwoofer, but any more than that I just have no idea. Any suggestions would be great.

I am going to post a separate post for this next question, but I am wondering If i can convert this to my home theatre sub, will I have to rip out the old circuitry and amplification and put in a subwoofer amp? or can I do it without power?
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Old 14th January 2006, 06:28 PM   #2
poobah is offline poobah  United States
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Sounds like you have a cap that has gone bad in the power supply.

Find and post a schematic and we'll tell you where to look. Sometimes a noisy cap will get hot. If you have some sort of temperature probe, you could look for bad that way. Read the thread on safety FIRST.
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Old 15th January 2006, 01:06 PM   #3
mikee55 is offline mikee55  United Kingdom
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Default Humming sub

Hi mate,do you get hum when you disconnect the signal in lead from the sub end? Do you have a Digital Multi-Meter?If a fault lies with the sub itself,put the DMM on volts DC(start at say 20volts)and put the probes on the speaker to check for any DC voltage appearing across the speaker.If you can get to the circuitboard look for 2 big round cylinder shaped componets.These are your Electrolytics for smoothing your powersupply.They could have a number in thousands printed on them,a voltage rating and there orientation by way of minus symbols printed along its body.They're either Blue,Black or grey in colour.
An example

4700uf 64v
Check to see if your getting equal voltages across both of them.
On the board they are connected so that you have

+ .......-+ ..... -
50 volts ov 50 volts
Do you see that plus on one cap is conected straight to the minus on the other?This is your o volts or ground point and all grounds lead to it.This is known as a split rail supply so check them to see if the rails are equal in voltage.Remember that you would reverse the meter leads for the negative rail.Mostly reversing leads apply to Analouge meters.My digital meter shows a negative symbol.
Check these if you can and post result back.And we go on from there.
Cheers Mikee55
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Old 15th January 2006, 08:02 PM   #4
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Hey Guys,
Thanks for the replies. On the board, I found 3 caps. 2 of them are small and next to each other, and if i read correctly, are 220uF 25v.
The third one is much larger, like the size of a D battery, and says 22000uF, 25v.

I powered the sub so it was humming, and used a multimeter first on the connection to the speaker, which gave almost nothing as a reading when the multimeter was set at 2.5v. I tried higher settings and nothing as well.

I didnt see any place to check the voltage on the circuitboard, so I placed the probes on the circuitboard on the opposite side of the caps. I did this for both the smaller ones and got no reading, the bigger one was somewhat inaccessable so I havent gotten the chance to try it.

Thanks a lot for your help so far.

I did not mention this last time in thoughts to simplify the problem, and I still dont think it is a factor, but the sub has an external switch/volume control that connects to the sub through a proprietary cable. So, when I plug the ac adaptor into the sub, the light comes on, but I believe it is not actually receiving the power until the volume switch is turned and clicks on. Humming does not occur until the volume/power is clicked on, and no matter what volume you turn it to, humming stays the same.
The volume control is for the satellites which plug directly into the subwoofer as does the external input. As I said before, everything works fine, sub sounds good, sats sound good, and everything else seems to work fine. When music is loud enough, pretty much drowns out the humming(which stays at a constant level).
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Old 15th January 2006, 08:07 PM   #5
poobah is offline poobah  United States
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Tell ya what,

Just replace all three caps and be done with it.

Or post your model number and all that so someone can come up with a schematic for you. It is almost certainly one of those power supply caps.

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Old 15th January 2006, 10:54 PM   #6
mikee55 is offline mikee55  United Kingdom
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Exclamation DON'T DO THAT !

You can't just change components at expect it to work,unless you know why it became faulty in the first place.If there was no dc across the speaker terminals then a good sign that its not a major problem.Is there a possibility of an earth connection going stray.Try to establish,with a continuity check and the amp sub turned off.Also check that no signal wire is near a mains line or vice versa.I've had a power wire with 240v in it fall near a circuit and hum was induced straight away.If you ain't too fussed about the hum,just carry on using it a just check it ain't cooking.Until a schematic is availible,not much can be done.

All the best.mikee55
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Old 20th January 2006, 06:36 AM   #7
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The sub is part of the Boston Acoustics "Digital Media Theatre" there isnt a model number or anything like some of the other boston acoustics speakers. Digital Media Theatre is the name. They dont make or sell them anymore, and I havent been able to find any documentation on the web anywhere.
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Old 11th June 2006, 09:45 PM   #8
cityman is offline cityman  United Kingdom
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Default subwoofer hum

I have the same speaker system and the same problem. The diference is that i bought them new with a gateway pc in 1999. They worked fine and sounded great but started to hum about 18 months ago. My youngest daughter now uses this pc and as she likes her music loud and this drowns out the hum it's not a big problem. However if there is a repair possible i would like to know as i would like to continue to use them.
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Old 11th June 2006, 11:49 PM   #9
SY is offline SY  United States
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Seems more likely to be a grounding issue. That might take some skill to track down...
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Old 12th June 2006, 02:03 AM   #10
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Default Re: subwoofer hum

Quote:
Originally posted by cityman
I have the same speaker system and the same problem. The diference is that i bought them new with a gateway pc in 1999. They worked fine and sounded great but started to hum about 18 months ago. My youngest daughter now uses this pc and as she likes her music loud and this drowns out the hum it's not a big problem. However if there is a repair possible i would like to know as i would like to continue to use them.

It was fine,and they slowly started humming 18 months ago?
Sounds like the caps drying out to me.
Replace that big 22000uf cap,(I'm assuming it's the filter cap?)
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