Getting Rid of Buzzing

Hi,

Our AV Technician at the school I work at recently left and I have been left with taking over maintaining the AV in our school. The problem is I'm the IT Manager and know next to nothing about AV.

I recently had to make a change to the projector in the main hall and this necessitated a change to the way the projector connects to the speakers.

The sound comes out of the projector's 3.5mm audio out connector into a adaptor that connects to an ethernet cable for the run to the mixer box. The ethernet then connects to an ethernet to RCA L+R (red and White) adaptor. The RCAs connect to a pair of mono 1/4" jacks that go into the mixer box that has inputs from this and other sources including 3.5mm aux and wireless microphones.

Since installing the setup the speakers now hum continuously when the projector is turned off, but not when it is turned on. I have been told I need a "DI" box but I have no idea what one is, what I am looking for or even one that supports an ethernet connection.

Most of the setup was installed by the previous AV Tech so I don't understand it. I've just been lumbered with it.

Can anyone help please?

 

A DI (Direct Injection) box is usually used to connect instruments like an electric guitar directly into a mixing desk rather than to an amplifier. Depending on the type, it can also isolate two circuits to break hum loops, and convert between balanced and unbalanced.

DI boxes will have a mixture of unbalanced jacks for instruments and/or XLRs for balanced circuits. I don't know of any that will have ethernet cable connections, although an ethernet cable makes a perfectly good multipair balanced audio circuit, they're just not used that way.

Without knowing the exact details of what connects to what, especially the grounding of the various components, I think it impossible to advise in detail, but I would check that the signal grounds are continuous from source to destination as a starting point. In particular, I'm also suspicious of the ethernet to RCA adaptor, as ethernet cables are normally UTP so how the adapter treats the earths makes me suspicious. If the cable is STP, there's a better chance it'll work, again depending on how earths are arranged.

S.

 

Thank you for your reply Sergeauckland.

The ethernet cable is Cat6 STP. I chose this over Cat5e as generally that is UTP and using STP was, ironically, an effort to eliminate any interference that would manifest as buzzing.

 

DI (Direct Injection) box serves multiple purposes in audio setups, especially in live sound and recording environments.

 

As it's buzzing when the projector is off and not when it's on it's likely the projector is presenting a high impedance to the line when off and hence allowing hum pick up.
You'll need to mute the channels the projector connects to on the mixer to prevent this.

 

Ah, I thought it was humming, but instead it is buzzing.