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View Full Version : Hmmmm . . . gotta hummmm from the sound board.



FishersOfMen
01-07-2004, 07:48 PM
Folks -

Part of our setup incorporates a soundboard that receives feeds from our sanctuary and outputs to our BOB. The camera's input is SVIDEO.

We have a hum that comes from the connection (RCA) between the BOB and the soundboard. The soundboard does not have the hum by itself, and the VT system does not have the hum by itself. However, when the soundboard is connected to the BOB, the hum exists.

We have placed all equipment on the same phase electrically, and have verified electrical grounds are sound. Has anyone else experienced this? What did you do to solve it? Could it be a defective BOB?

Thanks again- GodSpeed -

RomainR
01-07-2004, 08:10 PM
Which audio output are you using on the BOB?

Have you tried the PA output?

Jim Capillo
01-07-2004, 08:16 PM
Sounds like an impedence mismatch. What do you have for a board? Try using the XLR outs on the BOB.....

PIZAZZ
01-07-2004, 10:47 PM
My vote is for a ground loop. You can use Isolation Audio Transformers to insulate your sound board and VT system. Depending on the day of the month and temperature of the air we have that ourselves on occasion.

Iso's are about $30-45 ea. I highly recommend Shure's.

Mark Patrick
01-08-2004, 11:33 AM
I had a hum, even brought 4 new circuits into our studio and had a 8ft ground rod installed outside....Still had the hum.

Incredibly, then just unplugged a laser printer from 1 outlet and moved to another and the hum immediately disappeared completely!!!!!

Jim_C
01-08-2004, 08:11 PM
Get one of those 79cent grey ground lifters that plug into the outlet. Don't recommend really running like that for long but I'll bet $6.84 that will clear up the humm. Ground loop somewhere.

Try on soundboard first then the T3.

ted
01-09-2004, 02:03 AM
Another slight possibility is that something, i.e. cable or component is too close to a monitor.
I'm sure you tried a new cable, correct?

mgrusin
01-09-2004, 12:29 PM
I've sometimes been able to fix hums by plugging ALL involved equipment into the SAME power strip (ensuring that everything's getting the same phase power and has the same ground).

I also once fixed a really bad hum by breaking the ground entirely (by using one of those two-to-three prong adapters and leaving the ground hanging). Don't know why that solved it (miswired outlet?) but it did.

Also keep audio cables short, away from monitors and wall-warts, and don't run them parallel+close to AC cables (cross AC cables at right angles). And watch out for fluorescent lighting. And laser printers...

Welcome to the black arts. :D Good luck! -MG.

Jim_C
01-09-2004, 03:14 PM
>>I also once fixed a really bad hum by breaking the ground entirely (by using one of those two-to-three prong adapters and leaving the ground hanging). Don't know why that solved it (miswired outlet?) but it did.


yea, the magical 79 cent ground lift.
Unfortunately more times than not at Theaters and Auditoriums we go to we get Ground loop hums.
We never use any kind of house sound, we bring our own, so we don't tie into house systems, but most of the time we get a hum somewhere and 99.7% of the time the ground lifter fixes it.

I always try to make sure that little screw on the ground lift plug is wired to a ground of some kind tho. Just not the ground supplied by the outlet.

AC = :confused:
for me.

jim

Jim Capillo
01-09-2004, 06:53 PM
I've seen a guy who was almost killed by electrocution because a sound tech lifted the ground on an amp, trying to get rid of a hum. BE CAREFUL ! While 98% of the time nothing will happen, there is always that 1 or 2 % chance someone will get zapped.
:(

FishersOfMen
01-09-2004, 08:54 PM
Folks -
Thanks a million ^10 for the input. I had a feeling it was a ground loop. I installed every single item I had in the sound room on one outlet! Four completely filled power strips on one circuit! An electrician's nightmare, still had the hum tho. Our sound guys came back to finish up some wok for us, and installed two transformers inline between the soundboard and BOB.

WE HAD A SUCCESSFUL NIGHT! That was after getting an electrician in to put all of the sound equipment on the same phase loop and all.

Tomorrow night is the first test night with recording our youth church event to hard disk WITH SOUND!

10^10 Thanks for all your support. I think this issue is resolved :D

GodSpeed -

Jim_C
01-09-2004, 08:55 PM
>>I've seen a guy who was almost killed by electrocution because a sound tech lifted the ground on an amp...


Warning Recieved, Understood, and Appreciated.

:)

FishersOfMen
01-09-2004, 09:12 PM
I've seen a guy get lifted and blown across a room for cutting a live wire by accident. Talk about lifting ground!:eek:

I also used to be a mechanic - Know what 12 volts will do when shorted? Weld Gold to flesh! How about a 35,000 volt coil wire? Instant hood scoop created by your elbow!

Electricity demands more respect than fire - it spreads exponentially faster :D

Paul Lara
01-10-2004, 09:11 AM
There's something to stay alive by!