Violin Dan
Member
- Joined
- Dec 28, 2002
- Location
- Kansas City, Mo, USA
I've been fighting RF interference in a PA system at our church. The offending station frequency is 1510 KHz (which corresponds to a 1/4 wave distance of 162.91 ft).
The system is controlled from the rear of the sanctuary and shielded balanced cables run to mic jacks mounted on the floor various distances from the mixer. The cables terminate in XLR connectors on both ends, with the shield attached at each end. There is no earth or AC main ground at the mic end. The speaker setup is a distributed system mounted at the ceiling in 6 positions, three on each side. The building is about 10 years old with wood frame construction. The sanctuary is on the second floor.
The RFI occurs with certain combinations of mic cables plugged into the mixer with three or four of them being the worst offenders. This can occur with or without a mic attached to the cable. Some introduce no RFI at all. At its worst the station can be heard loud and clear with no inputs turned up. At best, with all the offending cables unplugged, the output is clean. Often the RFI is only heard when the podium mic is up, and is worse with a high gain condenser like the AT mini gooseneck, and not so bad with a medium gain condenser like the Shure SM94, and non existant with a dynamic. It is not always present and is better some times than others. It seems to be the longer runs that offer the worst RFI, but none of them approach being a 1/4 wave on the offending frequency. I'm sure that they are all under 100 feet, but could easily be 50 to 75. The RFI can be introduced by simply touching the shell of the XLR to the mixer ground without plugging in the signal cables themselves. Shorting the signal cables to the shield at the mic end makes no difference.
The system is powered off AC mains using the main ground as system ground. The breaker box is about 100 feet from the control booth, and could actually approach the 1/4 wave distance taking into account the routing of the cable run. I have no idea what else is on the circuit, if anything.
I have tried the following solutions:
1. Lifting the third wire ground from various combinations of power amp, mixer, effects etc. This only made it worse.
2. Put ferrite beads on the ac to the power amp, on the speaker leads and the offending mic cables. Only slight improvement, the most helpful being on the power amp.
3. Replaced mixer and power amps with new units. No difference.
4. Unplug the offending cable runs. This works but defeats the purpose of having microphones in the first place ;>}
I am considering lifting all the equipment from the AC main ground and running an earth ground which should be closer than the breaker box. It could still possibly be 30-50 feet. Other than that, I'm about out of ideas.
Sorry this post is so long, but I'm about at my wit's end. I have important services coming up next weekend, so I hope that someone has tackled this one and emerged victorious and can share with me the solution.
Tired of "Talk Radio 1510",
Dan
The system is controlled from the rear of the sanctuary and shielded balanced cables run to mic jacks mounted on the floor various distances from the mixer. The cables terminate in XLR connectors on both ends, with the shield attached at each end. There is no earth or AC main ground at the mic end. The speaker setup is a distributed system mounted at the ceiling in 6 positions, three on each side. The building is about 10 years old with wood frame construction. The sanctuary is on the second floor.
The RFI occurs with certain combinations of mic cables plugged into the mixer with three or four of them being the worst offenders. This can occur with or without a mic attached to the cable. Some introduce no RFI at all. At its worst the station can be heard loud and clear with no inputs turned up. At best, with all the offending cables unplugged, the output is clean. Often the RFI is only heard when the podium mic is up, and is worse with a high gain condenser like the AT mini gooseneck, and not so bad with a medium gain condenser like the Shure SM94, and non existant with a dynamic. It is not always present and is better some times than others. It seems to be the longer runs that offer the worst RFI, but none of them approach being a 1/4 wave on the offending frequency. I'm sure that they are all under 100 feet, but could easily be 50 to 75. The RFI can be introduced by simply touching the shell of the XLR to the mixer ground without plugging in the signal cables themselves. Shorting the signal cables to the shield at the mic end makes no difference.
The system is powered off AC mains using the main ground as system ground. The breaker box is about 100 feet from the control booth, and could actually approach the 1/4 wave distance taking into account the routing of the cable run. I have no idea what else is on the circuit, if anything.
I have tried the following solutions:
1. Lifting the third wire ground from various combinations of power amp, mixer, effects etc. This only made it worse.
2. Put ferrite beads on the ac to the power amp, on the speaker leads and the offending mic cables. Only slight improvement, the most helpful being on the power amp.
3. Replaced mixer and power amps with new units. No difference.
4. Unplug the offending cable runs. This works but defeats the purpose of having microphones in the first place ;>}
I am considering lifting all the equipment from the AC main ground and running an earth ground which should be closer than the breaker box. It could still possibly be 30-50 feet. Other than that, I'm about out of ideas.
Sorry this post is so long, but I'm about at my wit's end. I have important services coming up next weekend, so I hope that someone has tackled this one and emerged victorious and can share with me the solution.
Tired of "Talk Radio 1510",
Dan
