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The Magic Cafe Forum Index Ľ Ľ F/X Ľ Ľ Generators & RF interference (1 Likes) Printer Friendly Version

Owen Anderson
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Last summer I rented a tiny Honda 350 Watt generator to do an outdoor event. It had power to spare when running my sound system.
I ended up using a wired mic for the show because my wireless, an older Samson lav, produced a very distracting continuous static/hum.
I'm guessing this is because it wasn't a 'clean' power source. Is there a way/device to filter such static?
Owen Anderson
Kevin Ridgeway
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The best answer would not to use that type of setup. However, your question was asking if there is a way to fix that situation.

Go to your local hardware store and get one of those three prong to two prong A/C adaptors. Plug your wireless into that. Basically you have a created a poor man's ground lift.

But be aware that like you said it isn't good clean power and it is an older mic.

Hope that helps.

Kevin
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OCanada
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I had a similar problem when using a Lav. A clever sound guy pluged me into a "DI" box with a 'Ground Lift' and voila no hum, no fuss, no muss. Generators are tricky because they run at different cycles. This trick may work for some but not others. Good Luck
Brian Glow
silverking
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Please don't lift grounds with a "cheater"...it's opening the door to potential shock hazards that can kill you.
Electrical codes everywhere have banned these three prong to two prong plugs.
Don't cut off the ground either.

Humm's and buzz's are always caused by something, and they are usually fixable.

A D.I. has a switch marked "ground lift", but unlike cheaters, this is lifting the audio ground, not the electrical ground.

Please don't lift AC grounds, it's a killer.
Dan McLean Jr aka, Magic Roadie
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Quote:
On 2005-12-14 13:44, silverking wrote:
Please don't lift AC grounds, it's a killer.

Usually, I don't bother typing "I agree" posts, because they usually just seem like a waste of bandwidth, but in this case, "Please don't lift AC grounds!!!"
Dan McLean Jr
Kevin Ridgeway
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My bad if I posted some possibly dangerous info.

Kevin
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Kristen Johnson aka Lady Houdini
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g0thike
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Being a sound novice what does a "DI" box do, should I buy one for my show.

G0THIKE
Owen Anderson
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My thanks to those who have joined this thread. The 'hum' I experienced was, I assume, because I was using a wee 350watt generator and not a 'clean' power source.
It was an outdoor picnic event far away from all plugs but hair plugs...

I'll also ask what a DI box is/does? A 17 word essay on lifting a ground would be useful too. Are you attaching a wire to something that goes into the ground? Like when a movie crew clamps on to a fire hydrant?
Owen Anderson
Dan McLean Jr aka, Magic Roadie
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Quote:
On 2005-12-14 20:25, Owen Anderson wrote:
I'll also ask what a DI box is/does?

A DI (direct injection) box, also just called a "DI" (without the word "box") usually has two 1/4" connectors and an XLR. Its function is to turn an unbalanced, high-impedance signal into a balanced, low-impedance signal. In other words, it makes it possible to plug something like a guitar (or a keyboard or an inexpensive wireless receiver) into a the XLR mic input of a mixer. Usually, this is done because the former (unbalanced, high-impedance) has serious restrictions in cable length, while the latter (balanced, low-impedance) does not. DI's also have a "signal ground lift" that can sometimes be helpful in eliminating or minimizing hum. This is absolutely different from an "AC ground lift" which can be very unsafe.

Quote:
On 2005-12-14 20:25, Owen Anderson wrote:
A 17 word essay on lifting a ground would be useful too.

"The only sure sign of intelligence is the tendency to ask intelligent questions."
Many essays & even several books have been written on "grounding" in an audio system. if one bokk would have sufficed, or given universally-satisfactory answers, then most of the rest would never have been written.

Here are a few basic facts, though. (For those in the know, please bear in mind that this is put in the most basic, easily-understandable way.) Let's say you have two components, and that each has a 3-pronged AC cable. Each component's "ground pin" (the round one) leads to something that electronics folks call "ground" or "earth", and for all intents & purposes, that's exactly what it is! Well, if you use an audio cable to connect those two components, then you connect the two AC grounds, thus creating a loop that audio folks call a "ground loop". That's not necessarily a problem, unless in intoduces a hum.

"Lifting ground" refers to disconnecting it at one of its ends. This can help to emiliniate or minimize the hum, but there ain't no guarantee.

There are differences between AC ground and signal ground, and not all powered components have AC ground pins, but what I've said covers the basics of ground loops and lifing ground. I may be wrong on this next statement, but I think that any more detail will only serve to make any explaination too weighty. As I said, this is the stuff of essays & books.

Cheers from Toronto!
Dan McLean Jr
silverking
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A D.I. is a device which (depending on who you talk to) stands for “direct injection” or “direct interface”. Its primary function is to interface unbalanced devices to balanced devices. A very common use would be for a bass guitar player in a band to plug his bass “directly” into the D.I. box, and then the D.I. box would (via a balanced XLR cable) be plugged directly into the main P.A. The same holds true for electronic keyboards.
On most D.I.’s is a small switch labeled “ground lift” which, when engaged, lifts (or breaks) the audio ground. The audio ground is part of the low impedance audio signal on an XLR cable.

There are two kinds of D.I.’s, “active” and “passive” ……the active D.I. has a transformer in it, and is generally suitable to getting rid of hums and buzzes in a sound system.

A D.I. will be a very wise investment to make for most magicians using small P.A.’s that they regularly interface with a larger “house” sound system.

Generators open up a completely different set of potential problems, but a D.I. is still useful in troubleshooting a hum or buzz.
When a movie crew hammers a 10 foot copper rod into the ground, or clamps onto a fire hydrant, they are actually creating the ground for their generator. The word “ground” means literally that, in large or complex buildings there’s an actual rod hammered into the ground, or large disc buried under the building that forms the building ground.

“Lifting a ground” with AC power implies somehow cutting off the ground wire found in a typical three prong American type U-Ground plug. The most common method was using the “cheaters” mentioned above. The problem with lifting an AC ground is that, by removing the ground wire that has traveled the exact same distance as the “Hot” and the “Neutral” wires (the other two wires in an extension cord) you will now cause the electrical device you’re plugging in to seek it’s ground in some other location. The safety concern is that it will usually attempt to seek its ground through your body when you touch it. Because you are not the “real” ground in the extension cord, and because you’re a lot closer to the device you’re plugging in than the actual plug in the wall, you’ve introduced “electrical potential”, which is difficult to explain in a paragraph, but it basically says you’ve opened the door to electrocuting yourself.

Sorry if I’ve rambled on longer than you were asking for.

(Dan, your site is brilliant, a much needed resource for magicians)

Cheers
Majiloon
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Hi Dan, Just popped in for a peek while in yet another airportÖ obviously bored.

I think- (and I may be wrong) but the initial question was for a filter- and perhaps what Owen experienced is a generator with a bad Sine Wave filtration circuit- or worse- did not have one, thus leaving him with Square Wave instead. This would account for the RF noise. But the noise is generally a high winding or snow sound accompanied by hum.
Nothing to do with the cable- or the need for a Direct box.

I would recommend that the next time you use a generator- make sure that it is newer and has a ďPure sine wave filterĒ installed. They usually do, but sometimes they (especially if you are renting) go bad. This affects anything that operates RF, as these devices (wireless microphone receivers) are particularly susceptible or prone to picking up on these unbridled frequencies that are also siphoned into the power socket, atmospheric, or both.

I could be wrong, but it was an interesting topicóand now my plane is ready- Merry Christmas!

Cheers,
Kelly Duro
No longer taking Private messages , thank you.
magiccollector69
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I see that this is a very old thread, but ground loop hum is an annoyance near and hated to my heart, so I just wanted to add a couple points that people may find useful.

Most performers have little control of venue power because they're not performing in a fixed magic performance venue. With that in mind:

- If you're hooked into wall power at a venue and are getting hum, see if there are other nearby wall plugs to try. The reason is this; different wall sockets can be on different circuits. It's not uncommon to get hum on a circuit which is being injected by some other device on the same circuit. Because a given circuit can involve an entire room, something you're not even considering could be the cause of the hum and it's coming down to the wire to your device. If you're on stage where there may be a local electrician, just ask him straight up if there's another circuit you can try. You may have to use an extension cord to reach a distant plug, which brings it's own issues, but.... I have reduced or eliminated ground loop hum from home theater systems using this trick.

- If you're operating on a power strip with other devices, hum can sometimes be reduced by changing the order of where your device is on the power strip, relative to the end of the strip that goes to wall power.

Good luck, and be careful Smile
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