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I just had a quick question about a buzz coming from our system. I recently hooked up subwoofers thru the auxiliary output of the sound board (A & H GL2400) when I first noticed it. The mains and monitors output cables are run though a sort of rack-mounted direct box, each channel with a ground lift switch, each XLR, before sent to the amps. This seems to eliminate hum. However, there are no more open channels on this device for the subs. What should I do? The owner's manual for the soundboard recommended "... Disconnect the cable screen at the destination end only. This breaks the offending loop while keeping the signal shielding down theLength of cable." I wasn't sure what that meant. I would greatly appreciate your input.
Thanks, Greg,
Greg, here is my answer:
Hum is usually the result of different grounds at the electrical outlets. To verify this just run an extension cord from the ac source that the amps are plugged into to the mixer. Remove all the DI boxes and connect the mixer to the amps with balanced cables. Make sure the all the cables are correctly wired:
Pin 1 shield
Pin 2 + white/red
Pin 3 - black
Do not lift the grounds on any of the AC inputs to the amps or the mixer as this could result in electrocution. OUCH!
If the hum is gone then you will need the have your electrician run an isolated ground to the mixer and amp AC sources. This is a common ground from one point running to the amps and mixer using a green wire not just conduit.
Also removing the DI boxes/hum eliminators may improve the sound quality...unless the boxes use very expensive line level transformers (Jensen are about $75 for just the raw transformer) the signal is being squished thru a $5 instrument level DI box.
If you still have hum check each channel by removing the input connections. There may be a DI box on stage that is also interacting and causing a ground loop. If that is the case try a different DI or make sure that any instrument or amp on stage is also on the same dedicated ground as the mixer and amps.
As far as the Subs If you have self powered units they should have a x-over built in that has a low pass filter.
On the speaker input panel there is a line in and a pass thru connection.
From the mixer you would plug in to the subs then out to the mid/high amp.
If they are not self powered and you are using a sub amp, it must have a low pass filter or you will need an electronic crossover to divide the signal into highs and lows to feed the amps.
Note the full range speakers should have a high pass filter to cut off the lows; if they don't then they will be reproducing the lows along with the subs. This will degrade the sound quality.
Do not run the subs from an aux output. The hp filter on ea chan controls what the subs reproduce...typically all but the kick, bass & keys channels should have the hp filter engaged. This will keep the rumble from vocal mics from ever getting to the subs!
Simple! Call me if you have questions!
Blessings
Jeff
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