Audio/Visual Systems Power Supply

[brian john]

Do not, ever, lift the equipment's power cord/line ground!!

But I have seen this done so many times, it is wrong?????? I was told that it resolved their issues. I explained it only hide the issue in some cases but does not resolve the problem.

 

[fdew]

This is good advice. I am not an electrician. I am a sound guy. so I see it from a different direction but I agree.

All of these things will help quite a bit. Especially the Telescoping shields. We are trying to avoid two things. A ground loop that will introduce 60 Hz hum into the audio and noise from other equipment coupled into the audio equipment over the power lines. Things like light ballasts, some dimmers, motor controls, ETC. The single phase is an attempt to keep the amount of wire running between our equipment short. If we have a dedicated neutral and in some cases a dedicated hot, and in extreme cases a Telescoping safety ground (or Star Ground) we feel we will have less likelihood of noise getting in. In real life the same phase is the easiest to get. It is amazing the number of times moving the plug to a outlet on the same phase helps cut hum.

I completely agree that removing the safety ground is never the way to solve a hum problem. In an existing building the best way is to install AUDIO isolation transformers in the audio line between AC powered components (Typically guitar amps) and the mixer board. The transformers need to be high quality with Faraday shields. The best are made by Jensen and are in the form of a isolation transformer (1:1) or a DI box (Direct box) to convert from the high imp output of most Guitar amps and keyboards and the low impedance needed by most mixers.

At my church if you cut the ground pin to your equipment you can't plug it in. If you do it to church equipment I cut the plug off and we do with out until I can fix it. (I only had to do that once.)

Frank

I have always run power from more than one phase or leg if you prefer and never had any problems, as long as a few conditions were observed:

• Run dedicated neutrals for each circuit used for the audio equipment;
• Connect ALL of the "source" (before the power amps) equipment to the same phase/leg;
• Try to distribute the amplifier loads equally amongst the phases/legs; and
• Observe good audio wiring techniques, especially use of "telescoping" shields.

Isolated grounds are not necessary in most applications as long as you use good audio wiring practice and the EGC wiring is properly sized and bonded as per Code.

The dedicated neutrals are a must. I know that some will say they're not necessary, but 20+ years of experience and over 700 screens wired tell me otherwise. I will not try to argue the physics of why, other than to state that it avoids any possibility, no matter how slight, of harmonics getting into the rack.

Telescoping shields means that the shield of the audio line is connected at one end only. In my specific application, that means from the Cinema Processor (source) to the power amps, the shields are connected at the amps end only. Most of the lines into the Cinema processor are connected shields at the Cinema processor end only.

For most other audio installations, try connecting the shields at the switcher or processor end only first. You may have to reverse and connect at the source equipment end only, or sometimes even at both ends.

I think it was already mentioned here, but bears repeating:

Do not, ever, lift the equipment's power cord/line ground!!

You're not solving the hum problem that way, just creating a potentially fatal hazard!