...my problem is when i put an external board through the
mixer i get a buzzing kind of humm that sounds like to much
power running through the speakers, but the handhelds run through the digital fine...
So, let me see if I've deciphered this correctly.
You have a PA in your
auditorium. Your
mixer, which I shall refer to as a
console, sounds like a
Mackie TT24. Now, your
system is fine when run normally, i.e., just the handhelds running through the
Mackie.
You're getting a buzz (which sounds to me like a
ground hum, around 60 Hz, 50 Hz if you're overseas) when you
plug in another
console to yours, correct? That's what I get by reading your original post. Or are you referring to an outboard
processor (typically referred to as "outboard"), like an
equalizer,
compressor,
gate,
effects processor,
etc?
Is this "mic 3" input in the
auditorium getting the output of another
console or a
microphone? I'm under the impression that another
console is plugged into it. If that's the case, (well, regardless if it's a
console or not) start unplugging things until the buzz goes away. What is likely happening is that there is a piece of equipment that is on a different
circuit than the rest of your PA.
Voltage is leaking into the
ground on the different
circuit, which creates a
voltage potential between the two pieces of equipment, as the
shield wire (think of it like a
ground wire) in the mic cable connects the two, whether it be two consoles, or a
console and outboard gear. That
shield wire lets the offending gear
send voltage down it to the other equipment, which is what creates the buzz.
Now, you just have to determine whether you're using two circuits or not, and isolate the offending gear. If you're just a student, I would suggest having an adult help you with that part (my apologies if you are an adult). If you find the piece of gear creating the buzz, and put it on the same
circuit as
FOH (the
Mackie and its outboard), your buzz should go away. If it doesn't, then there's likely a different problem. But let's
tackle this one first.