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Old 24th June 2012, 06:39 PM   #1
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Default adding ground wire to reduce periodic huming noise?

I have an old Kenwood powered sub-woofer that seems to be very sensitive to "dirty power", and it makes very loud humming noise, but it's on and off. Sometimes everything sounds great, then, probably the refrigerator clicks on, and the sub-woofer starts making a loud humming noise. I notice that this device has only a two prong power chord, witch is not grounded. Most of my audio equipment does not have a three prong grounded power chord, however, I am wondering if replacing the two prong chord on my subwoofer, with a grounded three prong chord would help reduce the noise. Where should I attach the ground wire inside the amplifier? To the transformer board? Amp-board? The Plate?

I already tried just grounding the metal plate on the back of the box with a separate wire, but it had no effect on the noise.

I am already using a relatively cheap power conditioner. (Accell power squid)

Any information would be appreciated,

Thanks,
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Old 27th June 2012, 05:47 AM   #2
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It would help to narrow down the possibilities here by telling us whether the sub. always had this problem and whether it makes any difference when you disconnect the signal lead - even short the input. Is it a dual or single RCA socket? Have you checked resistance of the lead earth shield or tried another?

Since the fault comes and goes, it still could be down to an internal component/connection issue but it is better to be certain about the easier stuff first.
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Old 28th June 2012, 11:25 PM   #3
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I don't believe the noise has anything to do with the signal. I tried shorting the inputs and heard a slight humming in addition to the problem noise. There are two RCA inputs that convert to mono. I have no tools for checking resistance nor am I familiar with earth shield.

here is the schematic for the board

Click the image to open in full size.


Earlier this year I edited the signal processing board to remove the bass-boost feature (sub-sonic filter) by creating a short from pin 1 of IC 502 1/2 and, and R 522 through a 10mf capacitor.

Click the image to open in full size.

Click the image to open in full size.

It sounds great, but now after it gets warm it starts to make this sound, and sometimes it is terrible!
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Old 29th June 2012, 04:55 AM   #4
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Nice pics thanks. Though I can't see the schematic you posted, my first thought is noise susceptibility of this mod. i.e. it works like an aerial for radiated hum from the power supply and adds that to the signal. Unless this problem goes back earlier, I'd look to reinstate the original circuit, at least temporarily, and see if there is improvement.

If that is the case, look to making the mod. more compact with insulated wire and preferably on only one side of the PCB to reduce pickup of all noise.

From that point on, there are the power supply electrolytic caps to consider if this a heat-up issue. Ageing electros do introduce noise (120Hz buzz) and that is awful.
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Last edited by Ian Finch; 29th June 2012 at 04:59 AM.
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Old 2nd July 2012, 02:04 AM   #5
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I tried re-routing my mod so that it would not run around both sides of the board, between the amp and the signal processing board, or just too close to pins carrying a lot of current.

Here is a photo of the improvements!

Click the image to open in full size.

I think that the sound is improved, but there is still a humming noise when the unit warms up, which can degrade into a bad vibrating noise.

I think I would like to try to replace the caps that you mentioned, but I am unsure which ones you mean. Do you mean the two big ones on the Amp board? These are 4700uf, 35wv. Can you recommend what part I should use to replace these?
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Old 2nd July 2012, 08:33 AM   #6
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Yes, the big ones but nothing special is required. Panasonic, Rubycon, Nippon Chemicon or your favourite recognized US local brand will be fine as long as the price is fair. I nominated a 105 rather than 85 degree C part because they last longer, regardless of whether they are exposed to >85 degrees or not.' Good that there was some reduction in noise but you still need to get that "aerial" down to the PCB with shortest possible leads.

Mind that when you look at electrolytic caps to buy, you need compatible pin spacings (usually 10mm) with diameter and height that fit OK. Whilst you are fitting any new power supply parts, check the diode bridge out and that the power supply voltage (measured across the caps will do) is near the same on each power rail. If you don't have even a cheapo DMM, consider buying one.

Here is something close to what I usually keep in the parts box: ECO-S1VA472CA Panasonic Electronic Components | Mouser
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Last edited by Ian Finch; 2nd July 2012 at 08:47 AM.
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Old 2nd July 2012, 11:08 AM   #7
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Quote:
Earlier this year I edited the signal processing board to remove the bass-boost feature (sub-sonic filter) by creating a short from pin 1 of IC 502 1/2 and, and R 522 through a 10mf capacitor.
snip
It sounds great, but now after it gets warm it starts to make this sound, and sometimes it is terrible!
I can't see the attached schematic. Do you mind posting it again?
Me wouldn't blame the parts before having a closer look at the mod!
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Old 2nd July 2012, 07:52 PM   #8
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Here is a link to a PDF download of the schematic.
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Old 9th July 2012, 11:12 AM   #9
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you probably need to verify, if the problem is really related to your modifications. Undoing the mod and observing the outcome shouldn't be too difficult.
If the humming is gone then, you could try disabling the subsonic filter by disconnecting R520 from C509/C510 and shorting out C509 with a small piece of wire.
No additional parts and weird wiring needed!
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Old 10th July 2012, 09:13 PM   #10
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Quote:
Originally Posted by Juergen Knoop View Post
you probably need to verify, if the problem is really related to your modifications. Undoing the mod and observing the outcome shouldn't be too difficult.
If the humming is gone then, you could try disabling the subsonic filter by disconnecting R520 from C509/C510 and shorting out C509 with a small piece of wire.
No additional parts and weird wiring needed!

Thanks for having a look at it. I will seriously consider your solution if further attempts do not work. I am going to move kind of slowly with this. The humming noise is only present when the unit is hot, so I think it might have to do with the capacitors.
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