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Topic: Hum in the line
Message: Posted by: sethb (Nov 10, 2009 07:31AM)
I volunteer at a public library that has an Anchor AN-130 PA. When I plug the audio line from the DVD/CD player into the mic input, I get what sounds like 60-cycle hum. The more I turn up the volume, the louder the hum gets. But with no connection, no hum (so the unit itself is fine).

I'm using a grounded three-prong power cord, so I can't reverse the plug to see if that solves the problem. And because it's a three-prong plug, I probably shouldn't have this problem in the first place. My only other thought is that perhaps the outlet itself is improperly wired and not grounded correctly?

Any ideas would be appreciated. SETH
Message: Posted by: ClintonMagus (Nov 10, 2009 08:59AM)
Are you plugging the DVD/CD player into the "Mic" input or the "Line" input?
Message: Posted by: sethb (Nov 10, 2009 10:09AM)
It's going into the Mic input. I tried the "line" input, but the signal from the player wasn't strong enough, so I had to use the "mic" input.

I also got the same hum (but not as loud) when I did use the "line" input.

Now you've got me thinking -- I wonder if the problem is actually with the DVD player, and not the PA?? I hadn't considered that possibility. SETH
Message: Posted by: ClintonMagus (Nov 10, 2009 11:43AM)
Sounds like a bad ground in either the player or the cable.
Message: Posted by: sethb (Nov 10, 2009 01:10PM)
OK, thanks VERY much for the lead!

I will try a different outlet, and also try a different DVD player. The unit the Library uses works OK but is about 7 years old (combo VHS/DVD unit). I had never considered the player as a possible source of the problem.

As I always say, I'm a magician, not an electrician! :) SETH
Message: Posted by: Kline (Nov 10, 2009 02:18PM)
Seth,
There are a couple things to try -
1. Do not plug the Anchor system and the DVD player into the same outlet.
2. What is ther mic inout ? ( 1/4" or xlr ? )
- since it is a mic input, you might want to try a DI box with a ground lift
3. What type of cable are you using ? if it s a 3 wire ( red, white and yellow plugs ? ) try to by pass that option and use "single" lines for the audio

This should help - hopefully - iut is hard to determine the cause without seeing the set up
Please don't hesitate to email me with any questions
S
Message: Posted by: Fitz (Nov 10, 2009 03:21PM)
You may also want to try a ground lift, it will take the 3 prong Edison plug and convert it into a 2 prong with no ground. They will usually cost you about a buck at Wal-Mart or Home Depot. It's always good to have these when your doing sound.

Fitz
Message: Posted by: g0thike (Nov 10, 2009 04:25PM)
Um, Direct Box. Problem solved.
Message: Posted by: Kline (Nov 10, 2009 07:12PM)
That's what I said ! :)
ha ha
Message: Posted by: Dan McLean Jr aka, Magic Roadie (Nov 10, 2009 10:21PM)
[quote]
On 2009-11-10 16:21, wizard_of_oddities wrote:
You may also want to try a ground lift, it will take the 3 prong Edison plug and convert it into a 2 prong with no ground. They will usually cost you about a buck at Wal-Mart or Home Depot. It's always good to have these when your doing sound.

Fitz
[/quote]
Do NOT try an AC (power) ground lift. It's unsafe, and, in many places, illegal. The AC ground is there for safety. The grond lift on a direct box lifts the audio ground, and not the AC ground. A signal ground lift, like what's found on a direct box, is 100% safe. Try what Steve Kline has posted, and please tell us how you've made out.
Message: Posted by: sethb (Nov 11, 2009 06:48AM)
Thanks to all for your ideas. To answer your question, the mic input is a standard 1/4" jack.

Now I have a question -- what is a direct box? SETH
Message: Posted by: James_Lee (Nov 11, 2009 06:57AM)
Here is what your looking for.

http://pro-audio.musiciansfriend.com/product/Behringer-ULTRADI-DI600P-Passive-Direct-Box?sku=182499
Message: Posted by: silverking (Nov 11, 2009 09:18AM)
Have you tried different connection cables from the DVD player to the PA?

I'm with Dan, lifting grounds is deadly, don't do it.
There have been many bodies in the morgue over the years as a result of an uniformed sound man sticking a ground lift on a piece of gear, rather than troubleshooting the problem.

Rather than keeping illegal ground lifts in your "fix-it" kit, better to keep spare cables (duplicates of the ones you actually use) and a couple of D.I.'s. A simple plug-in AC analyzer (with the three lights on it, showing that the wall plug is wired properly) is also a good item to have.
Finally, a multi-plug cable tester is priceless in your kit.

Finally, have a trouble shooting plan:

In your case that plan would assume (based on what you've told us) that the Anchor itself (when plugged in) doesn't hum.
Then connect the audio cables from the DVD player to the Anchor,
If you then get hum, the hum is likely a result of either the DVD player, or the connecting cable.
Replace the connecting cable, if the hum is gone it was the cable, if it's still there then it's not the cable, and likely focused on the DVD player.

Connect a different device (DVD player) to the Anchor with the original cables, if the hum is gone it was the DVD player, if the hum is still there it indicates a potential problem with the actual building AC, or more likely, a problem with how you've interfaced your equipment with the building AC.

Make sure the DVD player and the Anchor are on the same AC circuit (this often requires an extension cord).
(For testing purposes, you can just place them side by side)

The key to troubleshooting audio is to have a linear plan, and stick to it. That usually (not always, but usually) will get you to a point where you find the culprit with 100% confidence. That linear process usually attempts to start at one end of the signal chain (work from one end to the other), and works it's way in a logical manner through the entire chain.
Message: Posted by: Fitz (Nov 11, 2009 12:30PM)
I didn't realize ground lifts were so bad. What should I do when I'm in a venue I have no control over and it has "dirty power" the ground lift works but I guess is not an appropriate solution? My situation is not a permanent install but a one-nighter.

As far as the Anchor AN-130, from what I remember they have RCA inputs on the back. You may want to try different RCA cables from the DVD player to the RCA inputs on the speaker. (As suggested above)

Thanks,
Fitz
Message: Posted by: silverking (Nov 11, 2009 01:54PM)
Fitz, the term "dirty power" isn't really an accurate one for the kinds of hook ups we're talking about in this forum.

Although it's not impossible to get onto the same circuit as a washing machine, or a big fan with bad brushes (which could create some noise in your PA) it's very unlikely it would cause a 60 cycle hum.
In a case like the above, I'm afraid the only solution is to seek out a different, and "clean" circuit (this is where a few extension cords come in handy).

60 cycles is far more likely to be a grounding issue as opposed to a "dirty power" issue.
"Dirty power" is more often something to be worried about when hooking up a dedicated distribution system via a 3-phase tie-in and Cam-Loks, which is essentially creating a complete temporary power service.
Because it's not a permanent install, and because there are so many variables and temporary cables involved, it's easy to create major issues that can be a huge pain to solve...........but pros don't lift AC grounds.

But [i]most importantly[/i], an AC ground lift on a one nighter is just as likely to kill you as an AC ground lift on a permanent install......all it needs is a millisecond where it decides that your body is the ground.

If you lift the AC ground with a "cheater" plug, then your ground-lifted gear begins seeking a new ground......and if it picks you, you're treading very close to the emergency ward, or the morgue.
Message: Posted by: Fitz (Nov 11, 2009 04:36PM)
Thanks for the info! In addition to your suggestion I think I may have found another solution to my problem...

Sincerely,
Fitz
Message: Posted by: rattman (Nov 13, 2009 06:41PM)
I agree you would not want to lift the AC ground but using a DI box with a ground lift is completely different and is commenly used by the "pros" to resolve hum in the lines. The problem now days is a lot of new equipment like laptops do not use a groud plug so there is a difference between the ground level of the laptop and the mixer and therfore a hum can develop using a direct box with ground lift resolves the problem.
Message: Posted by: sethb (Nov 20, 2009 04:01PM)
OK, I finally had a chance to connect the Anchor PA to a different DVD player at a different AC power outlet. I also used a different patch cord to connect the DVD audio to the PA. I figured this would certainly solve the problem -- but surprisingly, it didn't.

When I turned on the PA alone, with no external connections, I got a faint hum with the volume up about 1/3. Plugging the DVD audio line into the MIC input made a higher-pitched and louder hum. Plugging the same DVD line into the LINE input gave me a slightly louder hum, at the original lower pitch.

So now I am completely confused. However, if I can solve the hum problem with a $30 direct box, that will be cheaper than dragging the whole thing to a tech. So that's my next step, unless anyone has a better idea. SETH
Message: Posted by: Dan McLean Jr aka, Magic Roadie (Nov 20, 2009 06:08PM)
[quote]
On 2009-11-20 17:01, sethb wrote:
... if I can solve the hum problem with a $30 direct box, that will be cheaper than dragging the whole thing to a tech. So that's my next step, unless anyone has a better idea.
[/quote]
A direct box is not a guarantee. Be sure you can get a refund if it doesn't solve your problem.
Message: Posted by: Kline (Nov 20, 2009 06:25PM)
Seth,
Are you using a grounded AC cord with this unit ?
I do have one of these set up in my studio for quick sound checks for my media star and I have experienced a slight hum from time to time.
Is the DVD player next to the unit ? try to move it so that the audio cable does not touch anyting other than each unit at the ends of the cables ( at the female junction )
The other thing to try with this is to make sure the treble level is to high.
that's about all I have for you .
Hope this helps
Message: Posted by: Bill Hegbli (Nov 20, 2009 06:45PM)
Where do you put the CD/DVD player in relation to the PA. Most PA are not shielded. Is your player shielded. Move the player several feet from the PA. Turn down the Volume on the CD player if it has a sound control, and use the PA sound control.

If the wires are old, replace them with shielded cables.
Message: Posted by: g0thike (Nov 21, 2009 12:52AM)
SethB,

Before others share their ideas, so far it sounds like you don't need a direct box. What Anchor model are you working with?

Is it a all in one solution, a powered/speaker/mixer with builtin microphone receiver?

G0THIKE
Message: Posted by: sethb (Nov 21, 2009 05:45PM)
To answer your questions, it's an AN-130, which is an AC powered speaker, and there is a built-in wireless receiver. There are also rear mic and line inputs.

The DVD player doesn't have a grounded plug, but it is polarized (so I can't try the old "flip-the-plug-over" trick).

The PA is about 40 feet from the player, connected by a 16-gauge mono speaker line with RCA plugs on either end. I don't believe the speaker line is shielded. The speaker line is not near any AC lines that I know of. SETH
Message: Posted by: g0thike (Nov 22, 2009 01:10AM)
SethB,

You got a 20 watt PA system. The fact that you get a hum when turning on the system 1/3 volume and nothing connected, means that most likely it is either the wireless receiver or the unit is at fault.

The best advice I can give you is to contact Anchor Audio.

Though my last guess is to change the frequency (channel) of the wireless receiver. If that is not possible, try testing the unit somewhere else. The receiver might be picking up something, interference.

G0THIKE
Message: Posted by: Dan McLean Jr aka, Magic Roadie (Nov 22, 2009 07:28AM)
[quote]
On 2009-11-21 18:45, sethb wrote:
The PA is about 40 feet from the player, connected by a 16-gauge mono speaker line with RCA plugs on either end. I don't believe the speaker line is shielded.
[/quote]
If the player-to-PA lines are, in fact, unshielded, that could very well be your problem, or a least a very significant contributing factor. Every cable that is before the amplifier portion of the circuit must be shielded.

Even if the player-to-PA cables ARE shielded, they are "high-impedence & unbalanced" (don't worry about what that means), and 40' is far to long for such a signal.

Both factors (unshielded & 40') could very well cause such symptoms.

Try putting the player near the PA, and using short cables (less that 6'). Don't get the player TOO close to the PA because it will cause a different kind of interference for the wireless receiver. If you still hear a noise, pick up the player and move it around to see if the noise goes away. "Unbalanced, high-impedence" signals are capable of short cable runs.

If the shortened-cable test works, then here is your solution. Run short cables from the player to two direct boxes. Then, run long mic cables from the direct boxes to the PA. The reason for this, is that a direct box's purpose is to convert an "unbalanced, high-impedence" signal into a "balanced, low-impedence" signal. The latter is capable of long cable runs, as long as the PA has "balanced, low-impedence" input connections. In your case, these PA inputs would probably be XLR mic inputs.

If your PA doesn't have XLR mic inputs, then I don't see a solution, other than putting direct boxes at BOTH ends of a long mic cables, therefore going through the second set backwards.

Good luck!
Message: Posted by: sethb (Nov 23, 2009 03:34PM)
Thanks for all the input. I realize that we did this sort of backwards -- we should have amped the signal before the 40' speaker cable run.

However, being a public library, funds are limited and we had to cobble a sound system together from what was on hand.

Even with a short player-to-PA cable, I still had hum. So I think what we will do is just try to boost the signal to the PA and use as little of the PA volume as possible. Then I'll put an amp and a speaker into next year's budget and hope for the best. There is a 3-watt amp in the digital projector, but I imagine that is basically worthless for this purpose. SETH