Wiring Disaster...Please Help!

[ghetto3jon]

hello. i just built xlr patchbays in the walls of my control room and performance room in order to get xlr cable though the wall. i took one of these: http://www.milestek.com/shop/produc... &cid=574&kwd=MAVPBSM&l=550&p=Audio+&+Video, snipped the ends of some everyday microphone cable (3 wire cable), and connected it to one of these: http://www.milestek.com/shop/produc... &cid=574&kwd=MAVPBSM&l=550&p=Audio+&+Video. i made eight connections like this...mounted flush in the wall. i'm almost sure that everything was soldered correctly...but there is an incredible hum on every channel. the same exact hum on every channel. am i missing something? i would greatly appreciate some help if anyone knows what i'm doing wrong...

 

[fitZ2]

It sounds as if you have wired the shields from the mixer wired to the wrong pin on one set or maybe through both. Probably reversed a pin from male to female jacks though. This creates an open ground from the mixer I believe. Either that, or you have wired all your jacks "chassi grounds" to the ground pins too, which I believe can create ground "loops". Hmmmmm, better let someone else answer this afterall......the grounding thing gets REAL complicated sometimes and it's been a Loooooooooong time since the last good thread on this stuff was posted....sorry.
BTW, what type of grounding scheme have you set up in your studio? That actually has a lot to do with this too.

fitZ

 

[mshilarious]

What kind of cable did you use, 2-wire plus shield or just 3 wire? Do the wires run near any electrical wiring?

Also you might shop around for a cheaper source of connectors.

 

[Track Rat]

Recheck how you wired your connectors. There should be NO hum if you did it right. Pin #1 of ALL XLR's should go to shield, pin #2 is + and pin #3 is-. As long as 1 goes to 1, 2 goes to 2 and 3 goes to 3, it'll work.

 

[lectrond]

Ground Loop

It sounds like a ground loop. Using proper shielded cable would cancel any outside noise from inducting a hum on the sound conductors. A ground loop is formed when current travels through the shield or ground wire. Your house is grounded at the water pipe feed and through a copper rod that is driven in to the earth near your house. The earth is negetive potential and will draw any potential voltage above 0 volts to it. This ground circuit is the 3rd pin on all the outlets in your home (after the 1950s) When equipment is connected, this ground can be extended to the metal shell of the equipment to protect a person from shock should a wire break loose and electrifiy the shell. In audio, this ground is extended as a foil shield over the wires that carry the audio wave. When radio waves generated from transmitters or vacuum cleaner motors pass through the cable the small electrical energy is attracted to the foil shield because it is negetive ground potential is sent through the house circuit to the water pipe or to the copper rod. Most of these signals do not reach the conductors. (So what) (wheres the friggin humm coming from)
Audio equipment uses lower voltages to operate and so all audio equipment has voltage transformers which isolate the power it uses from the line voltage coming in from the street. Once the power is isolated, it no longer recognizes the earth as a Negetive ground potential. It now looks to the device from which it is generated so that each device now has its own ground potential. Such as your Amp chassy has ground potental to your guitar cord. Your mixer has ground potental to your line plug. Your pre Amp has its own ground potental as well as your studio speakers. Plug them all in and HMMMMMMMMMMMMMMMMMMMMMMMMM. The sheilds on all the interconnected lines are now connecting all these individual ground sources togeather and current is now flowing through your audio cords shileds. At this point you can start by unplugging everything and putting back togeather until the humm returns. When it does, use a ground loop isolator like this one from radio jap I have linked below. I have two of these and I use them to connect audio lines between devices like (keyboard and mixer) or (Tape machine and studio amp). Let me tell you, it is easy to just use this isolator then it is to go hunting ground loops. They are very hard to control.
Question: Has anyone tried using Cat-5 cable. I hear it works perfect without sheilding

Isolator below
http://www.radioshack.com/product.asp?catalog_name=CTLG&product_id=270-054