Subwoofer Humming problem

[jdvernarr]

Humming BA Digital Media Theater

Using DC power was a great idea. Has the fix remained effective? Is the hum completely gone?

I am asking since I recently purchased a Boston Acoustics Digital Media Theater on ebay and it also has the hum problem. (Hums at a noticeable, constant volume level as soon as it is turned on without any input signal. Not a problem at high volume levels but not OK at low volume levels.)

I couldn't find a suitable 12V DC power supply laying around the house so I am planning to buy one that I found on line at the below link.

http://www.skycraftsurplus.com/index.asp?PageAction=VIEWPROD&ProdID=1183

I also have the analog version of this system for about 9 years(which looks identical minus the digital input in the back) and it is very quiet. No hum at all.

Thanks for the help,

John V.

 

[svenyonson]

Hi John,

Yes, I am listening to music right now! Absolutely no hum. Makes me wonder why they even bothered to design the unit to use AC input when DC supplies are so commonly available and this would have saved them the engineering effort to build an internal rectifier circuit - which they apparently designed very badly.

So, go buy the DC power supply. I didn't check your link yet, but make sure the DC supply you buy has adequate power ( amperage ). I forget the requirements ( look at your existing ac power supply ), but mine is 2.0 amps which I am sure is enough.

Steve

P.S. The symptoms you describe match the problem described in this thread - there is a hum present even with no input signal to the amp.

 

[jdvernarr]

Sub Woofer Humming Problem

Hi Steve,

My 12V DC adapter arrived today. With the new 12V DC adapter, my hum problem went completely away, just like you told me. Yeah!!!! A very happy ending since I use mine to listen mostly to DVD movies which have alot of quiet moments.

I ended up buying the adapter here :

http://www.12vadapters.com/adapter/power-supply/12v/3-5-amp.html for $17 through ebay stores.

The original 12V AC adapter was 3.0Amp so I went up a notch to a 3.5Amp to play it extra safe. The 12 V DC connector plug fit perfectly into the back of the subwoofer.

Thanks greatly for your help Steve.

John V.

P.S. I hope this post helps others that are annoyed with their humming Boston Acoustic Digital Media Theater.

 

[bobkn111]

Works for Me!

I had the same problem as OP- constant HUMM when powered on with remote switch, regardless of line-in connected or not.

Thank you so much for the suggestion of using a DC 12V power supply- I found one lying around and plugged it in and the humm was gone and the speaks work fine.

One quesiton-- the 12V DC power supply I had is 1.25 Amp, and the old BA power supply was 3000ma - the unit operates fine with the 1.25 amp supply- do you think that is sufficient/safe to operate or should I buy one the supplies 3 amps?

Please advise!

Thanks.

 

[jdvernarr]

I expect that you could have a problem if you play the speakers loud since your power supply is much less than 3A. So, I would recommend buying one that has at least 3 A rating. The speaker system is well worth spending the $20 on.

Larger is better. I have not had any problems with my 3.5A power supply unit. The hum has never returned in more than one year and I play it almost every day. Good luck.

 

[arence]

WoW, DC PSU fixed my humming problem too....
Mine is Altec Lansing ATP3.
Many thanks.

 

[weltersys]

A lot of old computers are thrown out with fairly high powered DC supplies, often with several different voltage levels.

They can be recycled for a variety of projects.

 

[pheonix358]

Hi all, has any one considered that the transformer may be the cause of the hum. I would look at bolts, nuts etc or if it can be safely done, press against the transformer whilst it hums. Our friend is new to all this and may not have checked for the precise location of the sound.

Terry

 

[LivinEZ]

Hi Steve,

My 12V DC adapter arrived today. With the new 12V DC adapter, my hum problem went completely away, just like you told me. Yeah!!!! A very happy ending since I use mine to listen mostly to DVD movies which have alot of quiet moments.


P.S. I hope this post helps others that are annoyed with their humming Boston Acoustic Digital Media Theater.

Thanks VERY much!!! Used an old IBM laptop power adapter and presto...no freaking hum.

 

[jbdaven]

Ok I accidently discovered the same solution substituting a 14.5 DC volt source with 4.5 amps. The original input source is 12 AC volts with 3 amps. Therefore, the conversion from AC to DC is done in the subwoofer box. With the new source being DC, the conversion is no longer required in the subwoofer box and somehow accepts DC current. This same solution may work with other devices with similar AC to DC conversion issues. In addition, it may also point out possible hum resulting from defective DC sources on other devices.

 

[Ramjets]

Hi Steve,

My 12V DC adapter arrived today. With the new 12V DC adapter, my hum problem went completely away, just like you told me. Yeah!!!! A very happy ending since I use mine to listen mostly to DVD movies which have alot of quiet moments.

I ended up buying the adapter here :

12 Volt DC - 3.5 Amp Power Supply for $17 through ebay stores.

The original 12V AC adapter was 3.0Amp so I went up a notch to a 3.5Amp to play it extra safe. The 12 V DC connector plug fit perfectly into the back of the subwoofer.

Thanks greatly for your help Steve.

John V.

P.S. I hope this post helps others that are annoyed with their humming Boston Acoustic Digital Media Theater.

First, I would like to give a big hand of appreciation to Steve and John V. Can you believe its been 3 years since this thread was last visited and responded to? It’s a testament to the value of the iNet and to the diyAudio community!

My Boston Acoustics Digital Media Theater Speaker System was part of a Gateway desktop I purchased way back in 1998. The Gateway has long since been replaced, several times over, but I refused to give up my B/As. But, then about 2 years ago, my little gems became afflicted with the dreaded “hum” and I was forced to unplug them. But, I never gave up on them and lovely stowed them away until such time as I could afford to get them repaired. Because I’m not savy enough to do work, I always had it in the back of my head that I’d most likely have to ship the subwoofer/PSU to B/A for repairs (can anyone say “big bucks” here?).

Before I went to that expense, though, I thought I’d gleen the iNet one more time and, lo’ ‘n behold, I ran across this sweet little forum . . . AND, THE SOLUTION.

Using the link John V. gave, I ordered and received my “brick” in less than 3 days. Plugged it in and NO MO’ HUMMMMMmmmm. Thanks to you guys, I can expect to get another 15 years of enjoyment out of my B/As!

Thanks again . . . cheers,
Craig

Oh, and PS: Just in case some of some might be interested (or, not) in what my new desktop consists of:

OS: Windows 8 64bit
CPU: i7 3820 @ 3.6GHz Quad Core-Closed Loop Liquid Cooled w/Push-Pull & Dedicated VRM Fans
RAM: 32GB DDR3-1600MHz w/HyperX Spreaders & Dedicated HyperX Fans
GRAFX: nVidia GTX-660 1.5GB GDDR5
AUDIO: 7.1 Beats Audio; Polk Audio and Boston Acoustics w/Powered Sub-Woofer; Creative Fatal1ty Phones w/Mic
OPTICAL: Blu-Ray / SuperMulti DVD Burrner
HDD: 3TB SATA
PSU: 600 Watts
LAN: LAN: 1000 Base-T Ethernet & 802.11 a/b/g/n Dual-Band Wireless
PIPE: 6mips Down / 1.5mips Up
27" HD HP Monitor
15-1 Mem Card Reader; 4x 3.0 USB, 4x 2.0 USB
Logitech MoMo (Big Red) Formula Wheel w/Pedals
Thrustmaster Cougar FLCS & HOTAS Throttle Quadrant

 

[sentner]

I have the same speakers, with the same problem.

If you look at the back of the subwoofer, you will see the input labeled:

12v in AC/DC.

I simply plugged in a power supply from an old laptop ( 12vDC, 2.0amp ).

Presto. Hum gone.

I noticed the same marking on the back, and came to the same "solution". But, I'm not happy with not knowing what component(s) are causing the power to not properly rectify to DC within the unit. I've tried changing out the small caps, and the bridge rectifier, but still have the hum when using the AC power supply. I don't think the large cap has to do with power filtering, but rather has to do with the amp circuitry, so doubt that would produce the hum (and from others' posts it seems replacing it hasn't helped them)... Anyone tried anything else? Perhaps a circuit mod to add additional or larger caps on the power inputs?

 

[Greebster]

I noticed the same marking on the back, and came to the same "solution". But, I'm not happy with not knowing what component(s) are causing the power to not properly rectify to DC within the unit. I've tried changing out the small caps, and the bridge rectifier, but still have the hum when using the AC power supply. I don't think the large cap has to do with power filtering, but rather has to do with the amp circuitry, so doubt that would produce the hum (and from others' posts it seems replacing it hasn't helped them)... Anyone tried anything else? Perhaps a circuit mod to add additional or larger caps on the power inputs?

The cap does all the hum reduction. replace

Adding more would not hurt.

 

[sentner]

The cap does all the hum reduction. replace

Adding more would not hurt.

Caps that large are less likely to fail in a relatively low power system like this, and the small ones have been replaced already. And the hum does not exist when using a DC power supply, which makes this seem unlikely to be the problem. Since others have had the same exact issue and replaced all the caps with no effect, this tells me another component is failing, and that there is likely a design flaw.

 

[richie00boy]

Maybe it's an issue of wire routing or bad PCB layout, rather than component problem.

 

[Greebster]

Caps that large are less likely to fail in a relatively low power system like this, and the small ones have been replaced already. And the hum does not exist when using a DC power supply, which makes this seem unlikely to be the problem. Since others have had the same exact issue and replaced all the caps with no effect, this tells me another component is failing, and that there is likely a design flaw.

From what I have seen this is nothing more than a mental exercise. Meaning no measurements and thus blindly replacing parts guessing what does what.

With a meter this would be very easy to diagnose and repair. I'd most likely have the issue solved in 30min or less blindfolded and with one arm tied behind my back.

Buy a DMM

Here's mine

, bought this one in '97 when my Fluke 87 was stolen. Prior to that was an 8060, 77, Simpson 260 and a hand full of noname bobo meters.

 

[sentner]

From what I have seen this is nothing more than a mental exercise. Meaning no measurements and thus blindly replacing parts guessing what does what.

Which is why I don't want to just replace the large cap without diagnosing. The small caps are cheap and since you need to remove them to test it's no more work to simply replace than to test.

With a meter this would be very easy to diagnose and repair. I'd most likely have the issue solved in 30min or less blindfolded and with one arm tied behind my back.

Buy a DMM

I have one... Though it only goes up to 400uf (most DMMs don't go much higher for caps). Yours goes up to 10,000uf, which is still too low to test the large cap directly (need multiple caps connected together to check for capacitance).

That said, I'm sure it would be easy to diagnose to an expert in this type of circuit troubleshooting. If you'd like me to test anything at some particular points (I assume you're familiar with this circuit since you're making the suggestion that it's an easy one to diagnose), please let me know what measurements you'd like to see in order to help.

 

[Greebster]

Which is why I don't want to just replace the large cap without diagnosing. The small caps are cheap and since you need to remove them to test it's no more work to simply replace than to test.



I have one... Though it only goes up to 400uf (most DMMs don't go much higher for caps). Yours goes up to 10,000uf, which is still too low to test the large cap directly (need multiple caps connected together to check for capacitance).

That said, I'm sure it would be easy to diagnose to an expert in this type of circuit troubleshooting. If you'd like me to test anything at some particular points (I assume you're familiar with this circuit since you're making the suggestion that it's an easy one to diagnose), please let me know what measurements you'd like to see in order to help.

Even if the value is above 10k µF, under low current draw situation there would be little hum. As you turn up the level the signal is modulated by this "hum". The problem is likely to be large leaking current from the big cap causing this issue. Measure out of ckt with an AC milliammeter or in ckt as described below. The large (and small caps) change the time constant of the filtering and help to maintain this level, until either too much current is drawn or too much current is leaking, placing an unnecessarily high load on the transformer.

To test diodes if one is faulty forward bias one and check conductance. Normally done out of circuit. If necessary I'll resort to cutting ckt traces to isolate, desolder a leg, etc. In ckt testing you should be measuring the DC power rail with an AC setting on the meter. On a full wave bridge rectifier two opposing connections of the four are AC (in) (anode to cathode connections), the other two (cathode to cathode and anode to anode) are pulsed DC out, modulated by the AC frequency.

Not seeing the schematic, the single large cap indicates a single ended cap coupled amp. The smaller two indicate a split power rail handling the low level signal ckts. A guess by the blind

If you have access to a dual channel Oscope you can make a component tester, simple and cheap (google it), that will bias most parts and you can visually see if a device is shorted/open/or turning on, complete with at what voltage the turn on occurs at. My meter has this functionality built in. The only one that I am aware of that has ever had this feature. Limitations is frequency and little capactance to work effectively. Large caps like yours would display a straight vertical line indicating a short at low frequencies, which is what it should do. In these cases isolating the + cap terminal is necessary. This is a power off test, as in pull the plug NOTHING connected to tested ckt for safety reasons. NEVER FORGET THIS.

 

[Greebster]

Not knowing where you live, probably have a cap/diode/rectifier that would have solved this yesterday

Have collected parts for far too many years, have one 82,000 µF cap that is rated for 10v, computer grade spade screw terminals, lil beast. What todo with it is another thing altogether

Have some 20-30 4700µF 50v short 'n fat nichicon's. A pair of GE computer grade, screw terminal, 23,400µF 75vdc 90vdc max BIG BERTHA caps, a bloody assortment of other "pairs" and one of a kinds.

Once upon a time these parts were pristinely organized, but after several moves and damage to bins it's a mess, in a alot of boxes, hither and yonder.
Wife said, so you need a garage (we're in an apt atm), how big, two car? Wish I could get away with that dear, remember the garage at the house in Kernersville? About that size... three car garage with enough room to park two and put a 30' boat in. Except I would need more storage for all your stuff (for her two storage units full).

My uncles garage grew over the decades. Garage (three car), to shop addition, to storage addition, to massive awning on the east side (leeward) to park his RV. An ex rock bands 1962 Greyhound bus he had customized.

 

[sentner]

Even if the value is above 10k µF, under low current draw situation there would be little hum. As you turn up the level the signal is modulated by this "hum". The problem is likely to be large leaking current from the big cap causing this issue. Measure out of ckt with an AC milliammeter or in ckt as described below. The large (and small caps) change the time constant of the filtering and help to maintain this level, until either too much current is drawn or too much current is leaking, placing an unnecessarily high load on the transformer.

The hum is fairly loud and is the same regardless of volume setting. However, when connected to a 12VDC power supply instead of the 12VAC one, the hum disappears. Other than the hum, the sub works fine.

To test diodes if one is faulty forward bias one and check conductance. Normally done out of circuit. If necessary I'll resort to cutting ckt traces to isolate, desolder a leg, etc. In ckt testing you should be measuring the DC power rail with an AC setting on the meter. On a full wave bridge rectifier two opposing connections of the four are AC (in) (anode to cathode connections), the other two (cathode to cathode and anode to anode) are pulsed DC out, modulated by the AC frequency.

My first thought was leakage on the BR, so I swapped that out (I'd been putting in a small order from Digikey so I grabbed one of these just in case). Had no effect though.

Not seeing the schematic, the single large cap indicates a single ended cap coupled amp. The smaller two indicate a split power rail handling the low level signal ckts. A guess by the blind

I had traced the circuit back from the subwoofer terminals to see if the large cap was directly attached. However, one lead for the sub goes through one of the small caps, and the other goes to an amplifier chip. If it helps, the amp chip is a TDA8561Q, the datasheet for which calls it a "2 × 24 W BTL or 4 × 12 W single-ended car radio power amplifier"... The other small cap is also connected to a different pin on that chip.

The large cap looks like it goes to the other amp chip (TDA8563Q, 2 × 40 W/2 Ω stereo BTL car radio power amplifier with diagnostic facility), which appears to power the main L&R speakers. However, testing continuity makes it appear that it may also be directly connected across the DC rails, so perhaps it is a problem. I may have to pull it out and test.

If you have access to a dual channel Oscope you can make a component tester, simple and cheap (google it), that will bias most parts and you can visually see if a device is shorted/open/or turning on, complete with at what voltage the turn on occurs at. My meter has this functionality built in. The only one that I am aware of that has ever had this feature. Limitations is frequency and little capactance to work effectively. Large caps like yours would display a straight vertical line indicating a short at low frequencies, which is what it should do. In these cases isolating the + cap terminal is necessary. This is a power off test, as in pull the plug NOTHING connected to tested ckt for safety reasons. NEVER FORGET THIS.

Unfortunately I don't have an oscilloscope. Well, I do, but it's a very old Tektronix that I picked up for $5 at a hamfest and it doesn't work properly..