dietermoreno

Looking for advice for cleaning cables and strings to prevent rectification of RF.

When I asked on Electronics Point forums why my guitar amp is amplifying voices on the radio, I was told that (1) using TRS cable instead of instrument cable is an antenna inviting in RF and (2) the RF invited in by the TRS cable is rectified into audio by envelope detection using the dirt on the cables.

Can strings pick up RF too?

What are some good ways to clean cables and strings so that rectification does not occur?

Is this one of the motivations of guitar effects boxes enclosed in steel, rather than connecting several effects pedals together in a series in the open with no insulation?

If I bought a 6 foot long wire from the hardware store and connected it to a guitar string in the magnetic field of the pickups, would RF from the 6 foot long wire be induced into the pickups?

Arby911

I'm not entirely sure you're serious here, but if you are you've been mislead greatly...

What amp do you have?

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667

Try a ferrite choke on the amps AC power cable, and maybe one on the instrument cable going into the amp.

http://en.wikipedia.org/wiki/Ferrite_bead

http://www.radioshack.com/product/i...22&locale=en_US

bburritt1

Whoa dude! Check your pups and make sure there grounded and wired properly. And check your cables. If you have cheap cables you will talk to aliens. (just tap the ends of the cable and see which one talks back through the amp. And get rid of that shiz!)

Strings have nothing to do with anything going on here. ^ p+1 the choke is nice to have to if your really worried about it.

Very common issue. Dont break out the tin foil!

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dietermoreno

What does a choke do?

So do you think dirt on the cables is not what is causing the rectification of RF into voices that the guitar amp amplifies? What else could be the culpret of the rectification?

why can strings not receive RF if they are oscillating in and out of a magnetic field generating alternating current at the same resonant frequency as the fret board position and string gauge and tuning string tightness allows?

Roc8995

Because they're grounded.

You're speaking gibberish here. 'Rectification' of RF? RF from dirt on the cables? Neither of those ideas make sense, even individually, and together they really don't make sense.

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667

Check the wiki link I posted.

DeathByDestroyr

As everyone has said, it's probably a grounding issue.

Cathbard

Rectification? Demodulation perhaps but rectification? Wtf?

Clean the sockets using contact cleaner on a plug, make sure your cables are good and check the earthing on the guitar.

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dietermoreno

I mean envelope detection. I said envelope detection in my OP. Rectification means making something into a form you want it to be converting from a sine wave, so then demodulation is a form of rectification, even though demodulation is not converting from AC to DC, but it is still converting an AC sine wave into a useable form.

So is contact cleaner actually something that is made for this purpose so that demodulation does not occur where it is not supposed to occur?

How do I check if the guitar is grounded? Why would the guitar not be grounded? If the guitar was not grounded then wouldn't no AC signal flow from guitar to amp because AC needs a ground to be reference to or it won't work?

Phil Starr

Play nice. Rectification is a term used worldwide to describe this process, I'm guessing that it dates back to the early days of radio when a diode (rectifier) was used to extract the audio signal from radio waves.

To the OP. any metal object of significant length can pick up radio waves. If this is fed along the signal path it will be 'rectified' at some point and come over as audio. Every single transistor junction can in theory do this. Each is in fact a diode. The amp should be designed to filter these rf problems out using capacitors to short out high frequencies or inductors/chokes to increase the impedance for rf.

All good cables are designed to be low noise by reducing rf pickup and reducing its transmission. The Earthing of cables etc is part of the process so a quick check of earths from the guitar through to the amp is always worth a try, not forgetting the mains sockets and leads.

By and large though if a cable is noisy it is on the way out or of poor quality. Chuck it out or mark it with plenty of tape and only use it for home practice.

There is increasing amounts of rf around us and some rooms and some points in rooms are becoming so noisy that even with gear in perfect condition you get rf pickup. You can try various clip on inductors and they might tune the problem out. Electric guitars were never designed for a world full of mobile phones and bluetooth. A squirt of switch cleaner won't fix this I'm afraid.

dietermoreno

So the transistors in the guitar amp can "rectify" RF (read as medium wave AM stations, at least that is what I'm getting) into audio by means of envelope detection like a diode?

I don't have a problem with signals from cell phones and blue tooth. The only stations I am picking up are AM stations. I think that cell phone signals and blue tooth have more complicated "detectors" required so a diode won't work to demodulate a cell phone or blue tooth signal.

Okay I read the Wikipedia article on how the RF choke works and I read the sources that the Wiki used. Please correct me if I am wrong in my explanation: The RF choke is an inductor, where it is placed in the circuit determines whether it is called a coil if it is part of a tuned circuit or it is called a choke if it is not a part of a tuned circuit and its goal is to block RF. The RF choke takes advantage of the fact that radio waves propogate on the surface of a conductor. So since the radio waves propogate on the surface of the conductor, puting a resistor on the surface of the conductor creates resistance to the flow of radio waves on the surface of the conductor, and an inductor does the same thing as a resistor for a cheaper price (cheaper since don't have to create a network with several resistors wired to surround the cable) since the inductor has inductive reactance, and in alternating current reactances are added together to equal impedence which impedence is the DC equivalent resistance.

So it appears that puting an inductor over a cable is a cheap way to block out radio waves from propogating on the cable.

I think I might like to buy several of these clip on inductors from Radio Shack for $5 each.

Then bye bye News Radio 780 WBBM if this works.


Conversely, to make sure that I really understand this well, say that my objective was to build a radio using my guitar and my guitar amp. If I wrap part of a 6 foot long wire around a cable on the point from guitar to a mixer (that has amplification and has demodulation from the transistors) with no chokes yet, then put chokes on the cable from the mixer to the guitar amp; I would be demodulating the RF in the mixer and amplifying the audio from the demodulated RF in the mixer and then the audio would pass from the mixer to the guitar amp with no more RF added into the circuit, correct?

AcousticMirror

rofl.

new amp.

good amps don't pick up the radio.

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667

Shoddy wiring/soldering is usually the culprit. This is how you check....

http://www.ehow.com/how_11367622_tr...multimeter.html

tubetime86

Good radios do.

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gumbilicious

none of this makes any sense to me.



i have only heard the term 'rectify' in terms of converting AC to DC. i am a little lost of how you would rectify a RF signal.

afaik transistors aren't the only culprit for AM radio signals playing through your speaker.

i have no idea how 'envelope detection' is related to a diode. a diode passes electric current in one direction and blocks current flowing in the opposite direction. i have heard of 'envelope followers' in effects applications, but this tends to deal with the volume/loudness/intensity of the input signal... i have no idea how a diode would be able to do that.



they operate on distinctly different wavelengths, it just so happens your guitar is acting like an antenna for a specific frequency... in the AM range. the signals are also quite different. i believe cell phones and blue devices use digital signals... which wouldn't sound like intelligible noise if picked up by guitar.



the purpose of a choke is evidently to get rid of "high frequencies" in an electrical signal (no idea how much a high freq is, it'd be helpful to know considering it could operate only in a area of frequency above your hearing capability)

chokes are there to reduce any electromagnetic interference.




RF is light/photons/electromagnetic radiation (whatever you want to call it) and light doesn't "propagate on the on the surface of the conductor".

brush up on the different between electromagnetic waves and electrical current. also look into how an antenna works.



i have no idea what you are talking about with resistors and inductors. inductors are more expensive than a resistor... i don't think a resistor would do anything wired around a cable, i think you have to run current through the resistor for it to do anything.

DC resistance is a limited case of AC impedance. AC impedance deals with resistance and reactance



i have a strong feeling this conclusion is not going to be as effective as you desire. try putting and inductor on your cable, let us know how it works. i am interested, the above paragraph sounds like snake oil but i am no EE.

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Cathbard

I was a tech in the comms game for many years and I have never heard demodulation called rectification.

Cleaning the sockets will just give you a better signal and often helps. Chances are highest that it's a dodgey earth and that can include the earth at the sockets.

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Gilchrist custom guitar
Yamaha SBG500
Telecaster
Ibanez Iceman
Roland GP-8
Quadraverb
Abbey Harmonic II
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Marshall 1960A

Cathbard Amplification

dietermoreno

I'll post recordings of the noise if it helps for you to listen to what noise we are dealing with.



Here is one where I am playing guitar. You only need to listen for about 10 seconds after the song starts and then you will get the idea, and you can also listen to the background noise before the song starts.
http://www.youtube.com/watch?v=EQKEyvC7_aY



Here is another one where the guitar playing is a little bit better for you to hear the background noise and the distortion it causes to the guitar.
http://www.youtube.com/watch?v=VKZdB8WRLDM


Here is a video where I only recorded the background noise coming in through my guitar and playing with the pots to create an LCR circuit to tune to the local AM news station in Chicago, WBBM 780khz. I amplified the quite audio from my guitar amp in Audacity from -45db to 0db.
http://www.youtube.com/watch?v=PXSRgMk3cN8


To better hear the radio tuning, I later further did low pass filters and high pass filters in Audacity to block out any remaining RF higher than the range of the human voice and block out any remaining mains hum and that allowed me to amplify further by 65 db without clipping, but I didn't upload that audio to YouTube yet.

fly135

Instead of addressing the esoteric, I'm going to address the obvious.... Why would anyone use a TRS cable on a guitar?

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