Interference (poss Radio Freq) on a Fender

[ThomJ]

Okay guys, might need thinkin caps on for this one.

 

A brand new Fender Strat is picking up some noise, having spoken to guitar specialists it seems that these beasts are know for noise. A fifty year old design and has some earth issues.

 

However, having got around (or by) that one it now seem to be picking up some external noise when in a particular practice room. If the guitar is moved/turned around the signal gets stronger and weaker.

 

I seems to me like it is picking up a radio signal and turning it act like a direction aerial.

 

Q1 - Is my theory sound?

Q2 - Is there any way around this problem (short of moving the practice location)?

 

Please be gentle

 

Thom

[dunk_1984]

Lead line the room?

 

Does the particular room have a hearing loop/transmitter somewhere near by?

[basilbrush1982]

I was also going to say find out if there's an induction loop system installed.

[paulears]

or a medium wave RSL radio station? Fender's single coil pickups, like most of these designs love computer monitors, transformer in amps and anything with a magnetic field. There are a few capacitor tricks that hep if it's really bad, but distance usually sorts it. If your problem is picking up music, then it's a bit more tricky as on it's own, the guitar is simply acting as an aerial for the kit with poor RF immunity - so look at the amp - and posibly try another to see if the problem reduces/goes away.

[Josh 2]

Excuse me for jumping in because I'm no expert.

 

However, I have found Fender (Strats and Precision Basses) suffer more than most. I also have a theory that that strings and even the truss rods act as some kind of directional aerial which as paulears says, picks up stray RF which gets better/worst as you move the guitar... often near amps.

 

The Fenders that I have had to pieces, have had the pick-up/pot recesses painted with what I assume is a kind of shielding(aluminium coloured) paint. Don't know if it would help in your case, but I've had some good results by lining the recesses with copper foil.

 

Josh

 

EDIT one spelling mistake, left all the others ;)

[Dan Appleby]

I seems to me like it is picking up a radio signal and turning it act like a direction aerial.

 

I have come accross this problem before... it's quite common with older guitars with single coil pick-ups (like a strat), which quite easily pick up hearing aid inductoin loops and other interference (don't ask me how) - Rob Beech with back me up here! We had a problem with an old viola bass which we just couldn't get to stop feeding back through it's cab. After much head scratching we realised we could hear the PA coming through the bass cab - very odd we thought... Unplugged the bass guitar - feedback stops. We then noticed induction loop amp in the venues amp rack room - turned it off, plugged the bass guitar back in.... no feedback. If you do a bit of searching on BR I think that has been covered somewhere before!

 

Can't really say for radio though.

[Rob_Beech]

The same applies with some radio frequencies, but induction loops are the common one. It all depends how the rest of the system is wired. Some induction systems are sent through the earth wire in a mains electrical circuit. You may find that plugging into somewhere else on the loop with something hardwired may cause this problem. This is where the example Dan gave above with the hofner bass caused the problems. Because it was directly connected to the PA (DI box) it caused this problem.

 

You may also get problems if there is a fault with a guitar lead. If the centre core is failing you sometimes you still get the signal through and pick up interference.

 

Turning off a loop will normally stop the problem but often the idea of having the loop is so its used. The direction in which the loop run can make a large difference.

[KevinE]

...and some cheap freebee leads will do this from new as their screening can be particularly poor.

 

If it changes as you orientate the instrument then it's obviously induced hum, can't you introduce a humbucker? An additional inductor connected antiphase to the pickup but mounted in the same plane, allows the signal through but opposes the hum.

[p.k.roberts]

There's a little item about guitar screening in the current 'Sound On Sound' magazine (page 66 if you find yourself in WH Smith). It suggests lining the body cavities with self adhesive copper tape (and earthing it). Apparently this stuff is much cheaper if you buy it from a garden centre (where it's sold as a slug repellent) than the same product bought from as a guitar accessory.

[Simon Lewis]

OK...

 

Firstly, all wiring can act as a radio antenna. Radio frequencies can induce current in a wire (and if RF is present in a wire, it can transmit them too).

Secondly, twisted pair cables (with balanced inputs) are far more effective at reducing noise and RF than shielding. However, for the Fender Strat's high impedance single coil pickup, this isn't appropriate.

 

Shielding may help, using stacked pickups may help, making the guitar active may well help too. It's worth checking the amp - as this may be "pre EMC" and susceptable to receiving interference. Dry joints can cause rectification of the radio signal - making it audible in the circuit. Guitar leads can also cause problems, so it's worth trying a different one. Many leads have a black carbon loaded skin over the central conductor insulation that many assume is just part of that insulation. It isn't. It needs to be peeled back so that it doesn't touch the tip connection of the jack plug. Also, many guitar leads ended up with non conducting copper cores when the poor quality rubber sheath (with sulphur impurities) casued the formation of copper sulphide - which isn't a terribly good conductor!

 

Thirdly, the Pin One problem can allow RF noise to enter the PSU or audio input end of the amplifier circuit, instead of being run through the equipment chassis.

 

The loop issue is a common one, and does cause problems with single coil pickups / poorly screen amps. Shielding isn't really the answer - that only works effectively for electrostatic noise (unless you line the guitar with mumetal).

 

If it is a loop causing the problem, it's probably not good to just turn it off (unless you are sure that it's not in use). Orientation of amp and guitar can make a difference. Furthermore, having the loop plugged into the mains some distance from the rest of the audio system can cause real problems with bleed. I'm not sure if that was a pin 1 issue, or not, but they work better powered from one source.

 

Lastly,

Some induction systems are sent through the earth wire in a mains electrical circuit.

 

NO! How can an amp drive its output (say up to 10A) into the mains earth??! Even if you could, how do you stop that current flowing anywhere else the earth is connected to? Loop amps that have their output connected to earthed pipes or other earthy connections typically shut down or blow fuses!

I suspect what you have seen is either the use of earth cable as a loop wire, or the use of three core cable to achieve a three turn voltagae driven loop. Such a loop isn't very good anyway, but it can look like it's connected to earth.

 

Turning off a loop will normally stop the problem but often the idea of having the loop is so its used. The direction in which the loop run can make a large difference.

 

The layout of the loop makes a significant difference - in fact it is possible to use a variety of techniques to make very low spill systems. The loop signal itself is a current source AC signal, not DC, therefore "direction" isn't an issue.

 

 

Hope this helps...

 

Simon

[Rob_Beech]

That could have been what I have seen, although I actually thought that I'd been TOLD that by somebody rather than seen it.

 

Layout is what I meant rather than direction. wrong choice of words. Where the loop is and isn't and does and doesn't travel in a particular room.

[ThomJ]

Thanks Guys all very instructional and helpful.

 

I do know that there is not and induction loop in this room, but we think there may be a pirate radio station in the area.

 

Any thoughts

 

Thom

[KevinE]

If it's RF pickup rather than mains pickup, (easily recognised with experience) the amp input stage will have to essentially 'detect' or rectify the RF signal before anything at audible frequency is heard since RF by its very nature is itself way above audible frequency (or it wouldnt travel far). This does happen, some unbalanced mic channels do it especially on cheap desks.

 

It is almost always cured by 'killing' the RF carrier by fitting a small (few nF) ceramic capacitor across the guitar or mic lead either in the plug at the amp end or inside the amp on the back of the socket. The capacitor shows a low impedance at RF and shorts out the signal, leaving audio frequencies untouched.

 

This technique won't kill 50Hz mains hum and your best bet then is the humbucking coil method if the hum is being introduced in the pickup itself. A quality lead will stop it if that's the problem. Bear in mind your amp may itself be giving off magnetic interference (off the mains transformer) which could be inducing the noise and amplifying it.

 

Mains hum can also be radiated from fluorescent lights from the floor below or above as they contain large inductors (ballasts)...but thats about it in a domestic environment.

 

 

 

 

 

RF interference sounds like a hiss or badly tuned radio, clicking, motorboating or whining. Mains hum is just that..sounds like a set of electric hair clippers. You need to decide which you have!

[kev-no1987]

I have had the same problem with a P-bass, The best solution I found was not it use cheap leads, spend a bit money on the £15-£25

 

Planet waves, Whirlwind, monster Are your best bet in my opinion.