Newsgroups: rec.audio.tech,;,alt.audio.pro.live-sound,;,rec.audio.pro
Subject: Re: Hum with Dimmerpack
References: <[email protected]>
Lars Oeschey  writes:

> since we have our new (used) dimmerpack, we have quite a big hum in
> the audio. Usually this is from frequencies injected into the
> audio-cables, but here we have the hum even if the mixer is completely
> muted, so it seems to be directly connected from the dimmerpack to the
> power-amps. The powerams are powered with standard 220V connectors,
> while the dimmerpack has a 380V three-phase connection. Anny ideas
> what to do to correct that?

Wellcome to the hell of the dimmer noise in audio systems.
Light dimmers are known to generate all kind of noise which
gets easily to audio systems (and sometimes to video systems)
througn various means. It needs to e quite careful practices
and good equipments to get the audio system to work with
dimmer systems. It cna be done but it won't be easy.

The audio dimmers are non-linear loads which are known to
cause noise and distortion to mains voltage. This noise
can get to the audio system through the power section of
the not very well designed audio devices. To keep this
at minimum level it is recommended that audio and lighting
systems have as separate power feeds as possible
(lighting system has heavy 3 phase feed from main electrical
distribution panel and the audio system has it's pwn feed
from there).

Dimmer systems are also known to cause quite much noise
current on neutral wire. In installations where neutral and 
ground share a same conductor this can be a serious noise
problems. New installation where 5 wire 3 phase wiring is
used the situation is easier. To keep the noise minimum
heavy 3 phase feed from main electrical distribution panel
separate from other electrical feeds is good practice.

Dimmer systems noise filters and other electronics can
easily lead part of the electrical noise from live
wires and neutral wires to the groundign conductor
of the mains. If the grounding conductor where
it gets happens to be in audio system ground loop
a noise problem is practically guaranteed. A separate
power feed for lighting keeps the dimmers pretty much
out of the audio system ground loop. If your lighting
controllers or lighting board is connected to your audio
system (for example music sync is used in light controlling)
it is essential to have an audio isolation transformer
between you mixing desk and your lighting equipment.
This transformer is essential to keep lighting dimmer
system ground noise out of your audio system, because
typical lightingting controllers have their audio input
ground connected to their case (if you connect those
to your mixed directly you will create a ground loop
which gets dimmer noise to your system).

Noise can also get to your audio equipment and wiring
through capacitive and inductive coupling. To keep
those minimum all the connections and equipments 
should be well shielded and not too near the noise sources.
For equipments this means that the equipments that are
anywhere near the dimmer system should be in metal cases,
preferably in steel cases. Dimmers are known to cause
some magnetic noise around them (they have large
filtering coils etc. inside them which can leak some
magnetic noise). Keeping the audio equipments like mixers
and amplifiers one meter or more away from dimmers instead
of stackign them makes wonders. 

The power feed entering the dimmer rack and especilly the
output cables from dimmer can be serious noise sources. 
You should keep your audio cabling as away from them as
possible and always used balanced cabling in anything
which goes anyway near those cables. It s a good idea
to route the power feed to audio system and audio cables
some distance away from any mains cabling connected to the
dimmer system. Even 20 cm of separation makes wonders
compared to the situation where the lighting cables and
audio cables are side to side.

Hopefully my tips are of any help in reducing the dimmer
noise in audio systems. Besides those tips I gave it is
good thing to remeber that not all light dimmer packs
are the same. Some dimmer packs are better filtered 
(less noisy) than some other.

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/