Re: 50 HZ hum in UK


"Lloydj" <ljeffords@xxxxxxxxxxxxxxxxxxxx> wrote in message
news:1194559452.441751.97860@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Guys,
We're installing our audio gear in London and on first power up the
gear is humming like a demon. We were able to test our signal in the
US before shipping only as far as the amplifiers because they came
hard-wired for 240V, and it was clean up to that point. We are
connected to the building through rack power strips with appropriate
UK plugs. The building is undergoing renovation so anything could be
at work.
It sounds like a typical bad ground condition, so I'm asking for those
with experience in this realm to lend ideas on how to overcome this
situation without lifting grounds, or a suitable testing means that
would say the problem is the building ground itself. We've got
computers involved and don't want to lose ground (earth).
Thanks ever so,
LJ


If possible run the FOH gear (mixer and accessories) from the same circuit
as the amplifiers. This works for me 99.9% of the time.

If this is not possible or practical install a good quality isolation
transformer in each 'send' (audio circuit to amps).
The transformer should have a 1:1 turns ratio but a DI (direct box) can work
in a pinch DI boxes often use a 4:1 turns ratio however so your gain staging
will be affected.

You may have to cut the audio shields at one end to resolve the issue. This
in conjunction with the transformers may brake the ground loop.

Do a check with a cable tester or ohmmeter on each send. A miswired
connector can cause much grief.

Check the system with all stage lighting off. Dimmers can cause a lot of
hash on the power lines.

I'm assuming analog sends.

One other thing to watch out for: Hearing assist inductive loops can cause
hum in dynamic mics and pickups. Check to see if the venue has one of these
and see if turning it off makes a difference.


.

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