Hum in the subs.

[mfausz]

There was a hum in the sound system last Sunday, I quess there was always some but when I added a second sub it made it 2x as loud. So after a little messing around I put an adaptor that goes from three prongs to two prongs. This was just a quick fix, what is the proper way. Would a power conditioner do the trick like Furman M-8Lx Power Conditioner

[cmchamp]

You have a ground loop, and possibly the AC your subs are on are on a different leg than the rest of the PA.
Rather than lifting the ground on the electric, try lifting the ground/shield on the connector feeding the line, or the ground-lift switch on the amplifier.
C.

[liko81]

If what you did worked, the cause of the hum was that the mixer and power amp are on two different electrical circuits, each with their own ground connection, and you were bridging these two grounds with the mic cable connection to form a big loop. Ground loops are antennas for EM interference, and the easiest solution is to "break" the loop as you have done with the adapter.

However, if I understand right, you have lifted the amplifier's actual ground pin at the wall by plugging its three-pin plug into a two-prong "cheater" and not grounding the cheater plug's ground lug. Bad, Bad, Bad; NEVER, and I mean NEVER, do this. By disconnecting the amplifier's ground, you have defeated any electrical safety the amp may have; a failure in the supply wiring or the highly-charged capacitor/transistor bank will put a high voltage into the electrical casing of the device, which will put the same charge into anything connected via signal ground to that amplifier. This can, and has, killed musicians before.

Instead, what you should do is only lift the "signal ground", basically disconnecting Pin 1 of the input XLR on the amp side; this breaks the loop created by the "signal ground" while all devices' "safety grounds" continue to function. This can usually be done by flipping a "ground lift" switch on the amplifier, plugging in a barrel adapter that breaks or buffers Pin 1 on that side of the cable, or in the extreme opening up the connector and clipping Pin 1's connection to the shielding wire. This last solution is a last resort, because lifting the ground this way can be difficult to detect, tricky to fix, and cause very frustrating problems if the cable is repurposed (for instance, a ground-lifted XLR cable used for a condenser mic will not provide phantom power, causing you to tear your hair out trying to diagnose a dead wired handheld or choir mike).

[mfausz]

Not sure if I fully understand but if it is a ground loop then all three amps are on this same electrical circuit, so what would be the easiest solution?
Option A disconnecting Pin 1 of the input XLR on the amp side on all four xlr cables.
Option B maybe a DI, with a gound lift times four?
Option C a power conditioner like an Furman M-8Lx Power Conditioner?
Option D ????

God Bless,
Mark

[liko81]

It's not so much that they're on the same electrical circuit (you'd probably blow breakers in that case); it's that each of the components has its own path to ground through its power plug, and then by connecting the shielded (and thus grounded) signal cable between them you are connecting the two grounds into a big loop called a ground loop. Ground loops basically turn that long loop of wire into an antenna for electromagnetic noise, which then bleeds into the signal path.

Here are your options in order of preference:

* Find and flip the "ground lift" switch on the amp, or on the mixer. I haven't yet seen an amp that doesn't have this. This will disconnect all four inputs' ground pins on the amp side, so the shield will be grounded only on the mixer side. While an isolation transformer would be best in terms of sound quality, this solution is free.

* Buy four "1:1 isolation transformers" and install them on the amp side of the cable runs. An isolation transformer is similar to a DI (in fact a passive DI contains an iso trans) except that its purpose is not to provide impedance buffering or line balancing; it simply takes the input and runs it through a "unity" transformer (equal turns of wire on both sides) to produce the output. Isolation transformers usually also have ground lift capability to sever the ground connection between the two sides, but on top of that they provide shock safety, better rejection of radio interference, etc. The downside is cost: four channels worth of iso transformers with ground lifts will cost you between $200-300.

* Find or make four "ground lift" XLR adapters that have Pin 1 disconnected. It is always preferable to be able to easily undo a ground lift, so a barrel adapter (like this one: http://www.jacksmusicfactory.com/def...productid=3501), or a DIY made with about 6" to a foot of cable and two ends connected on pins 2 and 3 only, will allow you to just plug it in to installed or long portable runs of cable, without modifying the cable itself.

* Disconnect Pin 1 from the amp side of the XLR cable within the connector. This has the same effect as pretty much everything else, but has the disadvantage of being the hardest to "undo"; you're no longer just flipping a switch or inserting an adapter in the middle, you're physically severing the connection inside the cable.

[pdc]

Originally Posted by liko81

If what you did worked, the cause of the hum was that the mixer and power amp are on two different electrical circuits, each with their own ground connection, and you were bridging these two grounds with the mic cable connection to form a big loop. Ground loops are antennas for EM interference, and the easiest solution is to "break" the loop as you have done with the adapter.

However, if I understand right, you have lifted the amplifier's actual ground pin at the wall by plugging its three-pin plug into a two-prong "cheater" and not grounding the cheater plug's ground lug. Bad, Bad, Bad; NEVER, and I mean NEVER, do this. By disconnecting the amplifier's ground, you have defeated any electrical safety the amp may have; a failure in the supply wiring or the highly-charged capacitor/transistor bank will put a high voltage into the electrical casing of the device, which will put the same charge into anything connected via signal ground to that amplifier. This can, and has, killed musicians before.

Instead, what you should do is only lift the "signal ground", basically disconnecting Pin 1 of the input XLR on the amp side; this breaks the loop created by the "signal ground" while all devices' "safety grounds" continue to function. This can usually be done by flipping a "ground lift" switch on the amplifier, plugging in a barrel adapter that breaks or buffers Pin 1 on that side of the cable, or in the extreme opening up the connector and clipping Pin 1's connection to the shielding wire. This last solution is a last resort, because lifting the ground this way can be difficult to detect, tricky to fix, and cause very frustrating problems if the cable is repurposed (for instance, a ground-lifted XLR cable used for a condenser mic will not provide phantom power, causing you to tear your hair out trying to diagnose a dead wired handheld or choir mike).

Ditto and just to be clear, you can be on two different circuits all day long. Being on a different circuit, with a different breaker, will not solve the problem. ALL of your stage receptacles, sound system, and anything connected to your sound system, needs to be on the same phase/leg. Everything else, lighting, electromechanical (coffee makers, kitchen equipment HVAC, etc) must be on a different leg.

You also want to isolate your video from lighting with ballasts, analog dimmers, Variacs, etc. As those things age, they get noisy, contaminate the power, and in some cases emit all kinds of EMF, which causes you and your guitar players batty.

Lifting the ground is dangerous and violates the National Electric Code. If you have three phase power, an electrician can often fix this with a screw driver at the breaker panel. If you ever have a fire, and you have those in place, your insurance company can refuse to pay up. Those nifty things are for grounding when all you have is a two conductor receptacle.

We see allot of people grounding to plumbing. This is unacceptable, especially in modern construction, where everything terminates to PVC. In older buildings, we have seen people ground to a metal pipe of some kind, but not without removing 50 years of paint first. We have also seen churches old enough to have everything terminate to clay. Not good. You need a substantial grounding rod, properly installed.

There are many articles over at ProSoundWeb about grounding schemes. Technically, if you are going to lift the ground on the male (output) side of the cables, you need to do it everywhere from the mixer output on. You do not need to snippy snippy on the input mic cables. You might also look at your equipment rack. There are many audio products that cause ground loop issues, being in the same metal rack as everything else. There are companies that make plastic isolation rings, keeping the metal rack from touching the equipment. You could very well have everything wired correctly and have clean power, with this chassis grounding issues causing your problems.

[mfausz]

We do have an older building with three phase power coming into the building but only using single phase, this is what the electrician told us. I know that there is four different circuits going into the sound booth from panel A, and the power going to the rack with the three amps comes from a single circuit on panel B. The building use to be an old elementary school and we converted the gym into the sanitary and by code all wires have to be in conduit which serves as a ground if I understand right!!
Thanks for all of the info this site rocks!!
God Bless,
Mark

[Brad Weber]

Originally Posted by mfausz

The building use to be an old elementary school and we converted the gym into the sanitary and by code all wires have to be in conduit which serves as a ground if I understand right!!

Not unless the conversion occurred many years ago. There was a time when some electrical systems, especially in residences, used metallic boxes and conduit for the safety ground, however for many years a dedicated ground conductor has instead been used. That helps insure a low resistance path back to the earth ground at the service entrance.

What ends up happening is with your booth electronics served from circuits on one panel and the amps served from a different panel is that because the grounds take different paths, there can be some voltage potential between the different ground references. When you connect them with the audio cable and electronics then that voltage difference can generate ground loop hum in an audio system, especially in systems with electronics that do not properly address the signal ground, which includes many products.

When a groundf loop occurs the options are to try to eliminate the voltage potential difference or to break the loop path. Eliminating the voltage potential difference by doing what you can to get everything at the same potential is the preferred approach but not always practical. Breaking the power safety ground path is unsafe and should never be used as a solution to ground loops. However, breaking the audio signal ground is safe to do and can still break the loop path.

[Gracetech]

What Brad said..

crt

[wogster]

Originally Posted by pdc

Lifting the ground is dangerous and violates the National Electric Code. If you have three phase power, an electrician can often fix this with a screw driver at the breaker panel. If you ever have a fire, and you have those in place, your insurance company can refuse to pay up. Those nifty things are for grounding when all you have is a two conductor receptacle.

We see allot of people grounding to plumbing. This is unacceptable, especially in modern construction, where everything terminates to PVC. In older buildings, we have seen people ground to a metal pipe of some kind, but not without removing 50 years of paint first. We have also seen churches old enough to have everything terminate to clay. Not good. You need a substantial grounding rod, properly installed.

Any electrician that does either, should lose their licence.