Hi everyone,
I'm part of a non-profit org in L.A. and we have a pair of Mackie Thump 15 powered speakers. Usually we use them in our hall, but sometimes for outdoor events. When in our hall we're getting a buzz. Sometimes loud, sometimes not, sometimes it goes away. But it's behavior isn't connected to any particular action. In other words, we can switch mixers, turn the fluorescent lights off, plug them into the same circuit or separate circuits, but still can't figure it out. The AC wiring to our electrical outlets are in a conduit that runs along the wall, and again - fluorescent lights. I know those might be the problem. But here's something I'm curious about after reading snippets of info by googling. Is it actually that the mic is picking up vibration from the AC current or from the ballasts in the lights? Some answers from the google searches said that the equipment was very sensitive and wiring and also fluorescent lights could create vibration that the equipment picks up. Does that mean that the mic is picking it up? Could I test it by standing with the mic outside the hall with the mic to see if it goes away? Or maybe try just unplugging the mic?
And, if that's the case, whether its the mic or electrical noise through the AC source, is it possible that I could build a little box around all of the conduit with some type of shielding inside? In other words I could just use some 1 x 2 and dry wall to box the conduit in, but is there some kind of shielding material I could stuff in there that would help mitigate the buzzing? It would be a lot of work but I would do it if it was a sure thing. Already I'm thinking of switching out the fluorescent lights for LEDs or incandescent. Big undertakings but if I were confident that either or both jobs would solve the buzzing, I'd do it.
Any words of wisdom would be really appreciated.
Thanks,
Scott
I'm part of a non-profit org in L.A. and we have a pair of Mackie Thump 15 powered speakers. Usually we use them in our hall, but sometimes for outdoor events. When in our hall we're getting a buzz. Sometimes loud, sometimes not, sometimes it goes away. But it's behavior isn't connected to any particular action. In other words, we can switch mixers, turn the fluorescent lights off, plug them into the same circuit or separate circuits, but still can't figure it out. The AC wiring to our electrical outlets are in a conduit that runs along the wall, and again - fluorescent lights. I know those might be the problem. But here's something I'm curious about after reading snippets of info by googling. Is it actually that the mic is picking up vibration from the AC current or from the ballasts in the lights? Some answers from the google searches said that the equipment was very sensitive and wiring and also fluorescent lights could create vibration that the equipment picks up. Does that mean that the mic is picking it up? Could I test it by standing with the mic outside the hall with the mic to see if it goes away? Or maybe try just unplugging the mic?
And, if that's the case, whether its the mic or electrical noise through the AC source, is it possible that I could build a little box around all of the conduit with some type of shielding inside? In other words I could just use some 1 x 2 and dry wall to box the conduit in, but is there some kind of shielding material I could stuff in there that would help mitigate the buzzing? It would be a lot of work but I would do it if it was a sure thing. Already I'm thinking of switching out the fluorescent lights for LEDs or incandescent. Big undertakings but if I were confident that either or both jobs would solve the buzzing, I'd do it.
Any words of wisdom would be really appreciated.
Thanks,
Scott
Just 1/4. I have two different mixers and neither has xlr outputs for a main signal. Maybe before anything else I should shop for a new mixer?
with just 1/4's linking things your likely getting hum and buzz due to unbalanced lines....
it only takes an opamp or two to make a balanced line driver.
so just what are you using currently for a mixer?
it only takes an opamp or two to make a balanced line driver.
so just what are you using currently for a mixer?
Mackie 402vlz4. Any recommendations? I've seen that there are some lower end yamahas that have xlr for main outputs.
Many mixers with 1/4 output jacks, they are TRS. Tip ring shaft. So if you hook up twisted pair to the tip and ring, you have achieved the same as pins 2 and 3 of XLR. Pin 1 of XLR or shaft of TRS is shield, which you only hook up at one end of the mixer to thumper cable.
Mikes with XLR output (most of the pro ones) 2 and 3 go to the twisted pair of the mike cable. Twisted pair mike cables are much more resistant to electrical interferance than wire shield coax cable. The input jacks of the mixer should have TRS 1/4 phone jacks. Tip+ring wire to the 2 & 3 of the mike cable. Unfortunately 1/4 phone input of most mixers is high level, and will not have enough gain for a dynamic microphone. I have bought a good sounding lapel condenser mike with XLR output as low as $50. Condenser mikes have an internal transistor to boost the signal, and require phantom power (DC volts) from the mixer. Condenser mikes are much more resistant to electrical interference than dynamic mikes.
You can check the interference of the thumper by plugging a shorted plug to the input. Short tip to ring. If the thumper never buzzes, the sound is being picked up by the cables. Still buzzes, the thumper has a grounding or internal problem. Be sure the 3 pins of the electrical outlets have 3 wires, one a green one that runs all the way back to the electrical distribution panel. Many venues have sleazeball wiring, with 3 pin outlets and two wire romex or 2 wires in the conduit. No green wire to the 3rd pin, call an electrician to straighten out the grounding. Some setups use the conduit as the safety ground, which may work or it may not. Conduit nuts can oxidize and lose contact over the years. Not as secure as a dedicated green ground wire back to the panel. Wire under screw at proper torque, much less risk of copper oxide blocking the current.
Safety grounds can be okay at mains voltage, but block low voltage microphone current. Copper oxide is highly non-linear resistor.
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the VLZ has balanced output....sez so right on the mixer.
so adapters or full TRS 1/4's should go a long way to resolving your noise issues.
i do agree that the mixer needs an upgrade...i'd want a 4 band EQ with mid sweeps for a start.
so adapters or full TRS 1/4's should go a long way to resolving your noise issues.
i do agree that the mixer needs an upgrade...i'd want a 4 band EQ with mid sweeps for a start.
Not vibration, but RF (radio frequencies) and subharmonics from the ballasts can be broadcast directly or through AC lines.
If the noise does not stop when the fluorescent lights are off, they are not the problem.
Most likely you have a gain stage issue, easily fixed, see below.
Unbalanced lines are more problematic for picking up radiated noise, but it would be unusual for radiated noise to be putting out enough to be the problem in a mic line, unless the mic is very low gain (or miswired), the unbalanced line is more than 15 feet long.
You could listen to the headphone output of the mixer to determine if the noise is being picked up on the mic line, and whether it follows the gain setting. If you unplug the mic, and hear noise other than hiss when the input is advanced, the hall may have some RF issues. Crap computer power supplies, old LED or compact fluorescent lights, could be a problem.
If the noise you hear on the headphones is not present when the output of the mixer is not plugged in to the Thump, but starts when plugged in (with the mixer's gain turned down) you may have a ground loop issue.
With your relatively modern equipment, a ground loop could mean the AC is miswired- hot/neutral swaps could be the issue, and can be very dangerous.
No.
The conduit is an electrical shield, and should be connected to the earth ground.
If you have a ground loop issue, you should check the AC circuit is wired properly- 115 to 120volts from the "Hot" (smaller slot outlet) to the neutral (larger slot) and "U" ground, very little voltage (under 1 volt) should be present between the neutral and ground and conduit.
There should be almost no voltage between the ground and conduit. The resistance between neutral and conduit should be near "0" ohms.
That said, the Thumps have a mic to line level input range, which will amplify any noise from the mic/mixer if set too high.
Gain staging:
With the Thump input set to "infinity", fully counterclockwise, (off..), you should set the 402vlz4's channel and mixer output to "U" (unity gain) and adjust the input gain to see peaks near +8dB on the output meters with a loud input on the mic.
Adjust the gain of the Thump for the sound level you need, which could be as low as 7 to 9 "o'clock", barely cracked open.
Good luck!
Art