Grounding?

Alexbt

Alexbt

New member
I just bought a used Crate KBA-150 keyboard amp.

It's very old looking but sounds pretty good.

I have one major problem so far.

It has an XLR output. I connected it to my mixer and I get a hum. Adjusting the volume, eq, basically any controls on either the amp or the mixer and I can't make the hum louder or softer. Disconnecting the sources doesn't change anything. It's not the cable, it's just when it's connected from the XLR that I get the hum.

Is this likely a grounding problem? I don't know how possible it would be to fix in all situations.
 
Plug the amp into the same power strip that the board is plugged into. If that illiminates the hum, it is a ground loop.
A direct box is a good way to isolate grounds.
 
Farview said:
Plug the amp into the same power strip that the board is plugged into. If that illiminates the hum, it is a ground loop.
A direct box is a good way to isolate grounds.
Click to expand...


The amp is plugged into the power strip that the power strip the board is plugged into.

The XLR out from the amp was going to be my way to eliminate the need for a direct box, at least for stage performances. Here in the studio I usually plug right into the board.
 
You may be able to get away with lifting the shield on the cable you use to connect the keyboard to the mixer. Try it at both ends to see which gives you the best results.
 
Shield lifting your XLR cable may help your problem. Direct outputs buzzing are not uncommon at all in amplifiers. I personally think that this is because many manufactureres don't pay enough attention to that feature and just blindly wire up a jack for direct outputs with no type of transformers or buffers. If you do decide to shield lift a cable, try desoldering pin 1 on the MALE end of the xlr cable. This way you are keeping the shield in tact throughout the whole cable until the very last second. In general application, which end you shield lift doesn't make a difference, but the more proper way (the way most people choose to do it) is by leaving the shiled in line until as close till the end of the signal chain as possible.
 
Let me just be clear--there are no grounding issues when going from the keyboard/module into the board. Just the amp to the board.

I'm not understanding this shield lifting thing at all.
It also sounds virtually useless in live applications I come across where sound techs hook up their own XLR cables to the snakes.
 
Alexbt said:
Let me just be clear--there are no grounding issues when going from the keyboard/module into the board. Just the amp to the board.

I'm not understanding this shield lifting thing at all.
It also sounds virtually useless in live applications I come across where sound techs hook up their own XLR cables to the snakes.
Click to expand...

Whoops, then try lifting the shield from the amp to the snake :p. Sorry.
One question, though, do you get the hum when only the amp is hooked up, or does the keyboard need to be hooked up as well? This will tell you where the problem really is. In any case, you would have to bring your own cable to gigs, and explain it has a lifted shield.

One of the benefits of a DI is being able to lift the shield with a switch, hopefully breaking any ground loop present, without having to remember your special cable.

Unfortunately, like xstatic said, crappy noisy XLR outputs on gear like keyboards and amps are very common. Don't be surprised if wherever you play the soundguy wants to use a DI anyway. At the club where I work, I almost never use the XLR outs on gear. If they are noisy, I have to use a DI anyway, and I've had some issues with phantom power applied to these shoddy outputs. The console has phantom switches in groups of eight channels only, and I often need to have phantom on every channel to get it where I need it. So I basically insist that I be allowed to use a DI on keyboards, bass amps, etc.

ps- both the amp and keyboard have three-prong grounded plugs, I assume? Also, if the amp is old, the outputs may just need some work.
 
If it is just your amp, you may even have a bad transformer that needs repairs inside it. However, you may still have grounding issues. Grounding problems do not only refer to the power source or AC circuit. They can be introduced through equipment as well. If you do need to build a shield lift cable, you could try building one that is just 6 inches long and always leave it plugged into your amp. That way even club guys could use it. Basically, don't completely write off grounding problems yet.
 
The amp makes noise from the XLR DI out to the mixer with nothing plugged into it.

The keyboards and mixer do not have grounding pins on them.

How do I make a sheild lifted cable?
 
Alexbt said:
The keyboards and mixer do not have grounding pins on them.

How do I make a sheild lifted cable?
Click to expand...
xstatic said:
If you do decide to shield lift a cable, try desoldering pin 1 on the MALE end of the xlr cable.
Click to expand...

How old are these keyboards and mixers that they don't have ground pins? This rig isn't very safe.
 
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