Author Topic: Help: Radio Frequency Interference (Read 223 times)

s_wood · « **on:** October 06, 2003, 06:24:01 AM »

I had a gig nearly ruined over the weekend when radio transmission from the local police department somehow made their way into my amplifier. Not good.

My bass was not the problem...it was running through my muted tuner at the time, and the volume was off, too. My signal chain was Boss tuner>Trace Elliot compressor>SansAmp DI. The amp was an older SWR Bassic Black combo. I wasn't using a power conditioner (it's in the rack with my F1-X and Superfilter).

Any ideas as to what happened, and as to how I can stop it from happening again?

Thanks.

adriaan · « **Reply #1 on:** October 06, 2003, 07:25:25 AM »

Even if you use the best sort of shielding on all parts of your rig, a radio transmitter at short distance (say within a few hundred yards) can work its way past the shielding. Nothing much you can do about it. If the signal gets into the chain before the compressor, then the compressor may well boost the signal.

Location and orientation are the keywords, so you might try moving the rig or just turning it a bit to see if the signal drops.

jet_powers · « **Reply #2 on:** October 06, 2003, 03:14:55 PM »

I was always under the impression that a bad or frayed cable could act as an antenna.

Can any of the more technically oriented members speak to this?

JP

811952 · « **Reply #3 on:** October 28, 2003, 07:14:58 AM »

I grew up with a 500 foot 50kw FM tower just outside my bedroom, so I've got some experience with this. A lot of times you can simply re-orient your rig and kill most of it. Another thing which often helps is putting a choke on your audio lines (the ferrite barrel you loop the line through, like you see on a lot of usb cables these days). Of course there is shielding, shielding and shielding (it HAS to be grounded or it's just a bigger antennae). My brother is an RF guru and a fellow bassophonist, so I'll ask him and post his response here.. As far as that goes, maybe Mica can ask her dad about it...
John

811952 · « **Reply #4 on:** October 28, 2003, 08:04:25 AM »

Here is what my brother has to offer:
It kind of depends on the frequency and power level of the interfering signal... although there are some general tricks that will be effective:

1. Proper shielding and grounding is the most important technique

2. RF chokes or ferrite beads can be very effective for broadcast frequencies

3. Bypass capacitors from signal and power leads to the shield ground at the point where they enter the shielded area. The capacitor values should provide a very high impedance to the audio signal, but a very low impedance to the offending RF energy.

4. Use balanced lines with shielded, twisted-pair cable when possible.

5. Use a noise filter type of AC surge suppressor.

Most RF interference problems are caused by carrier detection by rectification of the RF signal by non-linear components (transistors, op-amps, tubes, diodes, poor connections, etc.). This can happen in the local active electronics in the instrument and/or in any of the amplifier stages. RF can enter through any external wiring including power line and speaker cables.

Hope this helps...