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Church Sound and Video Advice > Audio Tips > Radio Interference


We are picking up a radio station on our sound system. It seems to be worse at night. Can I get rid of it somehow?

 

RFI Troubleshooting Part 1Watch an 8 minute video session on RFI Troubleshooting Part 1

RFI Troubleshooting Part 2If Part 1 didn't solve your problem, watch this 5 minute Part 2 - "Beyond the Mic Lines". Watch Part 1 first.

 

A Common Problem:

This is a fairly common problem. When we install a sound system, we sometimes have to fight to get rid of radio stations. Unfortunately there can be many causes for this and often it requires a professional with experience in this area to solve the problem.

If you read through the write-up below, you will find some tips which may help you. If this works, that is great. Due to the complex nature of RFI and AC Hum troubleshooting however you might need more help.

If your problem is AC Hum, click here.

Worse at Night:

As you may have noticed, the interference usually appears or becomes worse at night time. This is because the radio wave transmission characteristics actually change at night. Due to changes in the earth's atmosphere, the radio waves are "pushed' closer to the ground, making them stronger than usual. Radio stations usually reduce their transmitter power to compensate for this, but if you are located near a transmitter, the signal strength is still much higher than normal. If your sound system is prone to radio interference pickup, this is the worst time for you.

Causes of Radio interference:

Sound systems are made up of several pieces of electronic equipment as well as a lot of wiring. Each run of wire is potentially an antenna. The microphone wires tend to be the most sensitive because they are longer than the wiring which interconnects the equipment. The signal level on microphone wires is also much lower than the other wiring. This means that you need more gain on the mixer, which also increases the volume of the radio interference.

Microphone wires are not the only source of radio pickup however. Sometimes ground loops between interconnected equipment is the cause. Other times, the quality of the equipment itself might be a factor. Building AC wiring can also influence the problem.

Eliminating the Radio Interference:

If you would like to try to find the problem yourself, I recommend the following.

Since there can be many causes of Radio Interference, you need to take a systematic approach to troubleshooting it. Here are the initial things I usually look for and try. This list doesn't include all the possibilities, but it is a good starting point which covers most simple cases.

1) Make sure that your microphones, mixer, and microphone wiring are all "balanced".

If you are using a 3 wire system, it is balanced. If it is a 2 wire system, it is unbalanced. You can usually tell this by checking the microphone connectors at the mixer end. A balanced system uses a 3 pin XLR type of connector (see below) while an unbalanced system usually uses a 2 conductor 1/4 inch phone connector. If you have the XLR connector, you can go a step further by opening the connector to make sure that none of the 3 wires are connected together.

XLR Type
XLR Type
1/4 inch phone plug
1/4 inch Phone Type

If you find that the system is unbalanced, you will need to replace it with a proper balanced system. An unbalanced system is only useful in home type applications, where the microphone wires are less that 15 feet long. Even here it is still prone to interference and noise problems. There are no professional unbalanced sound system. All professional systems are balanced. If your system is unbalanced, you should replace the equipment.

2) Find out if the interference is coming through the microphone lines.

Assuming that you have a balanced sound system, the next step is to find out if the interference is coming from the microphone lines. This is often the case.

First, turn on the system and get it to the point that you can hear radio interference. Once you have done this, turn OFF all of the microphone channel faders (or volume controls) and see if the interference disappears. If it does, try one channel at a time to find out if it is coming from all the channels, or just some of them.

If it is only some of them, try unplugging the microphones from the affected channels to find out if it is a wiring problem or a microphone problem. Also try different microphone cables to see if you have a bad cable. This is often the cause when radio interference suddenly appears on a system which was working fine previously. It could also be a bad microphone of course.

If you still have the problem even with the microphone cables unplugged from the jacks, you can open the connectors at the mixer end and platform jack end. You should find the cable shield connected to pin 1 only of the connectors. The two insulated wires connect to pins 2 and 3, only one wire on each. There should be no wire connected to the case of the connectors (fourth pin).

If all of the problem is coming from the microphone lines and the microphones, cables and wiring check out OK, you can probably fix the problem using Radio Frequency Interference filters. Alectro Systems makes a MLRFIF for this purpose.

The MLRFIF Mic Line RFI Filter is manufactured by Alectro Systems Inc. It eliminates annoying radio interference on low impedance balanced microphone lines. The unit consists of a 12 inch cable with a standard (XLR type) male 3-pin mic connector on one end, and a female 3-pin mic connector on the other. All components are top professional quality. The RFI Filter is built into the male connector so that it is as close to the mixer input as possible. To install it, you simply unplug the existing microphone cable from your mixer, plug the RFI Filter into the mixer in its place, and plug your mic cable into the other end of the Filter cable.

The Mic Line Filter has no effect on the audio signal since it has a flat response from DC (below audio frequency) to 20KHz (above audio frequency). It almost totally annihilates Radio Frequencies from 500KHz up. This covers all radio bands from AM through Microwave. At the bottom end of the AM radio band (600KHz) the interference is reduced by more than 98.5% from its original level. As the frequency of the interference increases, the attenuation of the filter also increases, reducing the interference to even lower levels.

Radio Interference Filter

Alectro Systems MLRFIF Radio Interference Filter Cable

Price and Data Sheet




3) If it is not coming from the microphone inputs, you need to continue your search.

If you find that turning off all the input channels of the mixer doesn't eliminate the problem, you need to continue your process of elimination.

Again turn off all the input controls and confirm that the interference is still there. If it is, unplug all the inputs to the mixer, one at a time, starting with the microphone channels. If this doesn't eliminate it, continue to disconnect the other inputs such as Tape, CD, VCR, Computer, etc. If at any point, you find that one of these inputs is causing the problem, you will need to deal with that device.

If disconnecting one, or some, of the inputs eliminates the interference, there are three likely causes for this. The easiest one to check is the cable which connects the device to the mixer. Try changing it for another similar cable to see if this fixes the problem.

The second thing to check is for ground loops. This is often the cause. Assuming that the interconnection is balanced, try disconnecting the shield from the source (where the signal is coming from) end of the interconnecting cable. If it is an XLR type connector, this is pin 1. If the problem is not with the interconnecting cable or grounding, there might be something wrong with the device itself. If this is the case, it probably needs service or replacing.

4) If the interference is still there after all inputs have been disconnected from the mixer, you will need a set of headphones to continue.

Again disconnect all the inputs from the mixer (only the inputs, not the outputs) and confirm that the problem is still there. If it is, plug a set of headphones into the headphone jack in your mixer. If you don't have a headphone jack, plug it into an output jack such as Left out, Right out, Mono out etc. If the interference can be heard on the headphones, unplug your connection from the mixer to the equalizer (or amplifier if you don't have an equalizer). If this fixes the problem, you probably have a ground loop between the mixer and the equalizer or amplifier. You can check this by disconnecting the AC ground from the amplifier and/or mixer as described in (3) above.

5) If none of the above fixes your problem, it is probably more complex, and you will need help. You should contact Alectro Systems for help.

 


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