zac850 said:
Yea,
DMX next to audio lines shouldn't be a problem, its
DMX next to
power lines that dim that give the hum. I'm not sure of the physics of it, but if the powerlines are at full, there won't be a hum, and if there at 0 there (clearly) will not be a hum. Anything from 1-99% on the dimmers will give a hum.
I'm sure DMXtools or Ship can tell us why, but its been explained to me and I still don't understand it, so I just accept it as fact.
Actually, it's audio (your
snake) next to
power, especially the
power to a
dimmer, that's nasty.
Start with the relationship between magnetism and electricity. When you move electricity through a
wire, it generates a magnetic field. We use this concept in motors and speakers, working the magnetic field of a coil of
wire against the magnetic field of a permanent magnet to produce physical movement.
When you move a magnetic field through a
wire (or a
wire through a magnetic field), it generates electricity. We use this principle in microphones, where sound vibrates a coil of
wire in a permanent magnet's field. The amount of electricity generated depends on many things, including the speed of the movement.
When you put an audio cable next to a
power cable, you're creating a
transformer of sorts. The
power is AC:
alternating current. The electricity starts from zero, builds up to a maximum, falls back to zero, then switches directions and does the same thing. Well, the magnetic field caused by the electricity moving through the
wire does the same thing. This magnetic field, as it builds up and collapses, moves through the audio cable. This can generate a small
voltage in the audio cable. Fortunately, with 50 or 60 Hz
power, the movement is so slow that the
voltage generated is almost negligible - until you add a
dimmer.
Solid-state dimmers use an electronic
switch to turn the
power on part-way through the AC cycle. The
current flowing through the
wire doesn't follow the
voltage during it's gentle rise, but snaps on suddenly. This sudden turn-on means that the magnetic field doesn't grow gradually, it grows very quickly, moving through the audio cable at a much higher speed and, as a result, generating a much higher
voltage - one that can create a noticeable buzz. Because it occurs as a pulse, it contains a lot of high frequency components - you're more likely to hear it in your mids or even your tweeters than in your subs.
The size of this sudden change also contributes to determining the
voltage. When the
dimmer is at zero, it never turns on, or if there is a keep-warm setting, waits until the
voltage has fallen back to near zero before turning on, so the pulse is very small. Similarly, when the
dimmer is at 100%, the electronic
switch turns on just as the
voltage is beginning its gradual rise, so the
current follows the
voltage smoothly and slowly. When a
dimmer is at about 50%, the electronic
switch is turning on just as the
voltage hits maximum, resulting in the largest sudden change... and the loudest buzz.
Most dimmers have filters to try to eliminate this problem, but they're not always successful. The filter components are some of the most expensive parts of a good
dimmer... and one of the areas where certain manufacturers cut costs to deliver unbelievably low-priced dimmers.
John