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Stuck under an FM broadcast radio antenna

R

randy_the_hack

Oct 29, 2008
35
0
Georgetown, KY
#1
Guys, my sole venue is my church... where I've been playing for years (>12 years). When we first started using guitars, bass, drums, etc. the sanctuary was very new and we were all trying to figure out how to get decent sound out of the system. Then the college sold the rights to someone to erect an FM broadcast radio antenna no more than 300 feet from the sanctuary. :rollno::rollno:

Over the years, the college bought us a new sound system, and we've done just about everything we can think of to prevent our equipment from channeling KLOVE every time we attempt to play.

Fast forward 10 years and we have just about everything silenced except for my bass. Recently we had a professional sound tech spend about a week at our church diagnosing and tweaking. We now have more sound - more clear sound - than we've ever had before. And yet again... even with decent cables, the only way we can even get the KLOVE music down to a bearable level is to put a direct box inline. Let me see if I can elaborate on my rig...

Dean Rhapsody 5-string w/semi-active pickups (EMG-HC pickups), with a pedal board with a Plant Waves tuner, an Ernie Ball volume pedal, followed by a Boss OC-3 Super Octave, and a Beheringer Bass Overdrive. This then gets piped out to a bass effects unit (not mine; I don't know what it is to be honest). From the effects unit, signal goes straight to the mix board. Right now, the signal goes from the pedal board to a direct box, THEN into the effects unit. Without the direct box, the radio station is as clear as you can imagine in the house; with the direct box, I can hear the radio station, but the audience doesn't/can't.

We're using in-ear monitors, so I can hear things that the audience doesn't hear. With the direct box inline before the effects unit, the RF is only noticeable a) when my bass volume is all the way up; if I turn it down just a fraction of a turn, I can hear it drop completely off, and/or b) if the direct box is out of the picture. But if everything is full volume, I can hear the radio station, though the direct box keeps the audience from being able to hear it.

This is driving me crazy. The sound tech told me I needed to replace my cable from my bass to my tuner with a balanced cable. I tried that, and absolutely nothing... should have realized there's no way a stereo cable is gonna work on my bass. Currently playing on a Monster cable.

I'm wondering if pickups are the issue, cable, pots, or something else? I've been in music most of my life, but that in trumpet. I play my bass, but don't have a good grasp of the electronics in the instrument, so please be gentle. I assume that since there's a small turn of the volume knobs that causes the radio station to come alive, there's probably something going on with the pots. Without the direct box, this thing is like a radio! So anything I can do to kill this radio station in my bass... well, let's say I'm real interested in putting an end to this madness.

Can anyone please help? :help::help::help:
 
#2
The key to this is the di box, notice that's what's making the biggest difference.

But here's a question, what exactly is it doing? With a more typical di setup, the di would send the signal directly to the mixer, however, with the extra effect box behind it, I wonder if it is. Also, if what you hear in ear is drastically different from the audience, what's different in the monitor train than the main train?

Going out on a limb, it sounds like perhaps the di feeds the main, and the nameless effects unit the monitors, or vice versa, which would lead to two main theories from me. Either the balanced line of the di (to mains) is clean, and the line out line (to the effect) is getting rf (possible, if the line out cable is insufficiently shielded, or something is off in the circuitry) or something is going on in the effects unit. If this is the case, what exactly is the function of this extra effect? Have you tried it without it? Why is it set behind the di rather than with all the other effects.

If none of this is true, then it could be a shielding issue on the bass (which the di might provide enough of a buffer to reduce/almost eliminate), or some combination of the other pedals/cables in the train. Start diagnosing this by going from the simplest setup and moving up from there. Can you plug your bass direct to the mixer, or with just the di to the mixer? If there is no rf, start adding the other components until you find the offending unit.
 
#3
Have you tried different pickups (different basses)?

Eliminating possibilities, by trying every option to narrow the source of the RF pickup is your only option.

It does sound like you've been trying to do this for years

I wonder if the bass is picking up something.

Mind you, a guitar player should get the same issues using magnetic pickups.

What about trying a decent digital wireless, like a G50, or one of the new Sony/Shure models?

Failing that, you could build a Faraday cage into your building (!? Maybe not).
 
#4
If the bass is okay, played directly into the DI, the bass is NOT the problem ...

I would first search the problem in your effect board ...
This may be the best way to get your sound, but many connectors, cables, different housings +++
Maybe one good effectboard could give you peace of mind ...
Bass > cable > effectboard > (DI) > PA

I set "DI" in brackets because high quality effect boards already have a built in DI.
For instance the BOSS GT-6B. It has multiple effects, the pedal can be used as a volume pedal, it has amp simulation and (mono or stereo) DI.

So, if your bass isn't the problem played into a DI box, there would only be one cable from the bass to the effectboard. The signal already leaves the effect unit as a DI-signal.

I bought a BOSS SE-50 20 years ago and it is still nearly up-to-date with its effects.
I also have the BOSS recorder and the bass section with it's amp (and cab) simulators is just killing!
The problem with the GT-6B might be that it can do too much! Even with an old SE-50 you are lost without the manual.

There are surely other very good effect boards with simulators and DI in it.
Test them and find out your board. Ask to play it in this room to find out if it solves your problem. Many shops have "money back" guarantee also for dislikes. A shop near by would surely let you test when you tell the story?

I think KISS is the solution to your problem.
Keep It Short & Simple ...
 
#5
Your sound tech is a little suspect... A balanced cable does nothing if the output and input it's connecting aren't balanced as well.

As far as picking up the radio station, wireless may help. It's possible your instrument cable is acting as an antenna.

It's also possible the building's grounding scheme may be a problem. Building codes specify grounding for safety, but there are a few ways to do it and not all of them are proper for RF rejection. It might be prudent to consult an electrician who's familiar with requirements for audio.
 
#6
Old business telecom tech here. Sitting under a radio tower is every technician's nightmare.

Shielding/grounding under those circumstances are much more complex than even most audio technicians realize, and a bass guitar's high impedance electronics are going to be the weakest link in this situation. Next will be any devices with significant gain and poor shielding, ie; the overdrive pedal.

The simplest solution is turn the bass volume down a bit...
 
#7
steve_rolfeca said:
Old business telecom tech here. Sitting under a radio tower is every technician's nightmare.

Shielding/grounding under those circumstances are much more complex than even most audio technicians realize, and a bass guitar's high impedance electronics are going to be the weakest link in this situation. Next will be any devices with significant gain and poor shielding, ie; the overdrive pedal.

The simplest solution is turn the bass volume down a bit...
Click to expand...

What he (and you: "With the direct box inline before the effects unit, the RF is only noticeable a) when my bass volume is all the way up; if I turn it down just a fraction of a turn, I can hear it drop completely off") said!

It's a church, not Wembly Stadium, right?
 
#8
Your church is located in electromagnetic Hell! At my last church, long before bass guitars were allowed in the door, we had periodic radio interference in the PA. We eventually discovered it was coming from a local amateur radio operator and when he was told of the problem he just volunteered to stay off the air during services. I don't suppose the radio station will be so accommodating.

Most likely the remaining problem is in your bass. It depends on the volume control setting after all. Turning down the volume control a smidge seems like the solution but when I play at chruch I use the volume control as an on/off switch. When I am playing it is full up and the sound team sets the level. When I am not playing it is full off to keep it quiet. Hitting full on is easy, hitting some point just below full on consistently not so much.

I would try a passive bass. It takes a semiconductor junction to rectify RF and turn it into audio. Actually to do that with FM also requires a filter circuit. There are filters and semiconductors in the active bass, perhaps the DI box, and the house sound system. A passive bass and its cord can act as an antenna but it cannot rectify. If the radio station goes away with a passive bass then you may need to shield the active bass better or put some small capacitors on the preamp to eliminate the RF.

Ken
 
#9
Well, I'll try a number of different things tomorrow prior to rehearsal. Don't know if I can track down a passive bass or not, but if I can, I'll try that too. May have to just live with what we have; with the DI box, at least the RF is down to just an inner ear monitor nuisance and no longer in the house.

Thanks all...

BTW, I've considered a Faraday cage, but it would be a bit of a detractor to the service... :D
 
#13
a) i seriously doubt a passive bass would perform better... a pickup is an antenna after all, and a humbucker is designed to reject external signals :)

b) if it sounds better when you turn the bass down, turn it down and turn the amp up...

c) a balanced cable does nothing if both output and input are not balanced...

d) you are playing at a church, not glastonbury..eliminate the pedal board totally. Your bass should go to a DI using as short a cable as possible and then run balanced to the mixer...use the preamp on your bass for tone control...

e) the IEM interference could well be the s/n ratio in the IEM system.. make sure you have good line of sight, the antennas on the transmitter are located correctly. and if channel switching options are available try that... if the antennas are moveable, try rotating through 90 degrees..like tuning (out) a station with rabbit ears on a TV

if none of that works, try a silver foil hat!:)
 
#14
oldleftybass said:
Just tell them it's to ward off evil spirits....
Click to expand...

This is closer to the truth than any of the advice so far! I tell you all truly that shielding a bass and amp from a strong radio station is very much like magic! Chances are once you get it fixed you'll have NO idea what actually fixed it no matter HOW much you know about electronics! I am experienced in these matters!

What happens is that the RF from the radio station gets into preamps which will detect the signal and can turn into a radio.

The answer is first of all to try to short all radio signals to ground. You do this by putting 100 picofarad caps all over everything in sight. These are small enough they should have no effect on audio or tone. Start some with across the bass output jack and across the amp input jack(s). Then be sure the bass parts are shielded as best you can with COPPER foil with soldered seams! The ideal is a totally water-tight metal box! You can't really do that (because of tops of pickups etc) but do as much as you can including pickup wires etc.

You need to determine just where the radio detection is taking place. In the bass amp input or in the bass preamp? Since a DI stops the problem I suspect that it is in the amp. I suspect what is happening is the that radio frequencies cannot go through the DI (especially if it's a transformer type) and that blocks the radio. But then a DI doesn't go to your amp but to the board which may not be susceptible to the radio effect. Not all preamps do the radio thing. For these things try a quality foil-shielded cable and even better loop it through a couple of ferrite cores at each end. Even better at a computer store you may be able to buy cores that snap onto cables to block RF. Put one on each end of the guitar cable.

The fact that turning down volume just a tad is interesting. That could mean you have parasitic oscillations in the bass! I'd suggest trying a different bass (borrow one) especially a well-shielded passive one and see what that does. Or it also might be affecting the amp input circuit. As I said at this point it's ALL magic! RF will do amazing things you'd never expect like crawling down the OUTSIDE of the cable (hence the clamp on cores). Note: parasitic oscillations are preamp oscillations at frequencies you can't hear but will detect radio signals. My alembic did this for certain tone settings until the factory sent me a new preamp.

Hence the fix if really just trying a million things and keeping those that help and ignoring those that don't.

All I can say is "good luck"!

(my house is right in line with a megawatt FM Rock station antenna and I can tell you I had hell to pay to get the radio out of my G&L basses!)
 
#15
Just thinking out loud here about the tinfoil hats....

When you're troubleshooting something like this is there any benefit in just taking a big piece of foil, or copper plate, or whatever shielding material you have and having someone move it all around the equipment to see (hear) if it has an effect anywhere along the line? would that help narrow it down? Or is the RF too non-directional?
 
#16
oldleftybass said:
Just thinking out loud here about the tinfoil hats....

When you're troubleshooting something like this is there any benefit in just taking a big piece of foil, or copper plate, or whatever shielding material you have and having someone move it all around the equipment to see (hear) if it has an effect anywhere along the line? would that help narrow it down? Or is the RF too non-directional?
Click to expand...

Actually tinfoil hats may do more harm than good! :D

Link Removed

The problem with high frequency RF (FM band) is that it will crawl all over EVERYTHING. It will crawl along conductors (outside of a cable) or it will crawl along dielectrics (non-conductors). It doesn't do what you'd think it does which is why it's so hard to deal with.

Real shielding is the water-tight metal box thing usually sealed with a brillo "o" ring or "finger stock" to make contact along the edge. All wires into and out of the box must pass through an RF filter. A 100pf cap is sort of a filter but real interference takes real filters. These are commercial and not too expensive.

It's all bad news and nothing is straight-forward or easy about it.
 
#17
oldleftybass said:
When you're troubleshooting something like this is there any benefit in just taking a big piece of foil, or copper plate, or whatever shielding material you have and having someone move it all around the equipment to see (hear) if it has an effect anywhere along the line? would that help narrow it down? Or is the RF too non-directional?
Click to expand...

This is a common technique in solving EMC problems but waving a sheet of foil around will do nothing. You have to completely wrap one piece of the system or another in foil to get much shielding. The tinfoil hat report was mostly a joke but since they went to the trouble to take data they should have noted that you do not get "gain" from aluminum foil. Their result may have been insensitive to the location of the receiving antenna but it would have been exceedingly sensitive to the location of the transmitting antenna. But their results do prove that if you want to shield with aluminum foil an open shield is useless.

RF shielding is not magic but it does require a deep understanding of electromagnetics and an extreme attention to detail. We are not going to scratch the surface here. However if you want to use foil you need to wrap the whole thing you are trying to shield and then wire penetrations can negate the whole shield. If you are wrapping items whose only ins and outs are shielded instrument cables you need to wrap the foil around the metal (you'd better hope) barrel of the cable connectors to ground it to them. If you can do that you have a pretty good shield but you still cannot be sure that the equipment at the other end is not coupling RF into the cable. Instrument cables are broadly similar to the cables used to carry RF signals in radio systems and if you inject an RF signal into them they will conduct it quite nicely.

Ken
 

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