What causes power supply hum

[mark1101]

I changed where I plug my system into when I took down the x-mas tree. Actually, I put everything back the way it used to be. However now, I get a power supply hum in my EQ. It is a brand new DBX-1215. It didn't hum where it was plugged in last week. I get no hum through the speakers, the sound is fine. However, you can just stand near it and hear a med/high pich hum when it is turned on. The other components are just a CD player, and Scott 299. Both are fine. I checked all connections, reversed AC plugs where not 3-prong. Had no effect on hum.

I am suspicious of the CATV outlet. The system was not near it when it sounded fine. Now it is right near the CATV outlet and cable runs behind the audio system. I checked those connections and they seem fine.

OK, besides telling me to lose the EQ, what could be the source of this hum?

Thanks

[MarkBK]

Power Supply transformer? - if it's an acoustical type hum - meaning you hear it coming from the EQ itself - maybe the transformer mounting is loose and vibrating.

[mark1101]

OK, maybe I should reword. I here a med / high pitch hum (or whine) definitely coming from inside the EQ. I can here it from 3 feet away. This occurs immediately when it is turned on whether any other components are powered or not. In other words, it has nothing to do with the music. The box hums when it is turned on. It didn't before I moved it. The music sounds fine and the speakers have no audible degradation due to this that I can here.

Thanks again

[MarkBK]

I'd still suspect a loose componenet oscillating - if comfortable - open it up (power off and unplugged of course) and check all the mounting hardware. Now plug back in with cover off and hands off too and listen - worst case you should determine what is generating the noise.

If it is caused by interference (Stray RF from Cable etc.)- look at the power cable etc of all the gear - try re-arranging them - or try moving the unit around while powered on to see if the sound varies in pitch or intensity - if it does it is probably RF interence - them you need to work on arranging the equipment to minimize the RF.

[Wardsweb]

Mechanical ocillation in the power transformer can also be caused by DC on the line. If you have a two prong power cord, try turning it over in the receptical to reverse the polarity.

[D-MAN]

here's some possible issues worth chasing down:

1) grounding issues - if you are using a different outlet, it could have the ground wired incorrectly providing that your EQ has a 3-pronged plug. In such a case a 75 cent two-prong adapter can eleviate a questionably grounded outlet. Ground the chassis independent of the outlet in such a case (or just let it "float"). Except for surviving a lightning strike or other such electrical mayhem, it will work. I have an amp that is very touchy about grounding, but the rest of my components don't seem to care (some only have 2 prong AC plugs anyway). I have one more question about the outlet in question: is it nearer to the refridgerator or other source if AC noise in the house circuit than the previous one? This could be a source of noise, but not usually humming. Just a thought.

2) cabling problem - perhaps a susceptable interconnect cable is placed too near a source (usually a power transformer, wall wiring, ect.). A worn or broken cable can become "suddenly" susceptable to outside interference. Try different cables and placements thereof.

3) apply a ground wire to the chassis of the humming component if it doesn't have one already. Ground it to real ground such as a drain pipe if your outlet ground is in question. This need not be permanent but can serve to indicate where the problem lies.

Good luck.

DM

[WMcD]

I get a bit pedantic and technical. But the following may help.

Real power supply hum arises because the capacitors are not working. Ususally because they are old. I think this is not your problem.

Many "hum" problems arise because of ground problems. Behind all this is that all the chassis or big metal boxes should be a some reference zero volts.

If you look at the RCA type input to a pre amp, you see the center connection and the chassis "ground". All the amp will do is amplify the difference in voltage between the chassis or sleeve and the center pin. Naturally the center pin carries the music signal.

The problem arises when the "chassis" or ground has a small a.c. signal imposed on it because some outside source. For example, the CD player may be be sending zero volts to the center pin. However, if the chassis "ground" is not at zero volts, and instead has some signal; then the chassis voltage is going plus and minus relative to the no signal condition on the center pin.

That is an important concept. The center pin connection from the CD player, VCR, DVD, etc. is quiet. But if the chassis voltage is not really zero, the amp is amplifying the voltage, even if slight, on the chassis.

What is the outside source? Hard to say. Cable TV connections are notorious for not being at real zero potential. Hence you see suggestions for the use of blocking transformers.

The overall problem is to keep all the chassis components of the system at some zero voltage. When all the RCA connectors are firmly in place, this usually works. In fact, the gross "blap" you hear when connecting the RCAs with the power on is just because the ground sleeve connects well after the pin. The more radical solution is to make a hard wire connection between all the chassis.

But getting back to your problem. Did something change during Christmas? Was the system reassembled? I think I'm very much a perfectionist. And time and time again, evidence shows I'm a dummy and have overlooked the obvious. Check all the connections to make sure they're clean and tight.

Gil

[mark1101]

Thanks for the info guys. This weekend I will have some time to spend on sorting this out.

[Andy W]

Here is one of the best explanations I have found.

http://www.dself.dsl.pipex.com/ampins/groundloops/grndloop.htm

[Steve Phillips]

Problem usually comes from a half-wave device.

Something that may be plugged into the same outlet as the amp in question.

Starting point;

Turn off the breaker that will shut off the outlet the amp is plugged into. Check to see what else is now off.

Start with unplugging everything else that was off. Turn the breaker back on. Then plug items back in one at a time checking for the hum.

Most common devices.

Clock radios, Glade plugins, lighting dimmers, coffee pots( automatic types with plate heaters).

Power conditioners won't help.

[Colin]

One of the best little nuggets of advice I mined from this Klipsch stream of information is to use a cheater plug on the offending piece of equipment. I went around plugging in and turning on each unit of front-end equipment until the source of the hum was identified (the Klipsch KSW200 sub). Then I used a two-prong plug on the end of the KSW200 three-prong power cord. Using a two-prong adapter in a chain of three-prong equipment reduces grounding hum. Insulting the speaker cables from 60Hz power line hum is also said to be a cheap and effective tweak (was with my walnut-oiled Cornwall 1s, but is less effective with my classic Klipsch corner Khorns, since they reveal too much line noise).

[Andy W]

A "cheater plug" can work to eliminate hum in some cases, but doing so eliminates the safety from shock hazard that will exist by not having the equipment "earth grounded." A failure in the insulation can leave the chassis electrically live. This is called Class I insulation. In this type of equipment, the ground pin ensures that the fuse will clear or the AC panel circuit breaker will open in the case of insulation failure.

Equipment that comes from the factory with a two-prong cord, must use double or reinforced insulation to prevent a shock hazard. This is called Class II insulation.

The so-called "cheater plugs" are only approved to be used when connecting Class I equipment to obsolete and outdated AC outlets (those having no third hole for earth ground), and the ring connector on the "cheater plug" must use the center ground screw on the outlet box so a safe connection can be made.

-Andy W

[jdm56]

I don't know if everyone is on the same page. I believe the power transformer hum problem is a mechanical noise from the transformer itself, not something that is in circuit and being amplified through the speakers. My receiver had the same thing happening. The fix was a new power transformer. No more hum.

[mark1101]

James is right. In fact I contacted DBX and they confirmed it was a batch issue. They have offered to replace the defective transformers for free.

It was simply mechanical noise.