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RF Choke Position


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Questions for those who are knowledgeable about electronics.

 

Background

I am working with an AX-Edge keytar and a wireless bass guitar rig (Nady, 484 MHz, diversity receiver). The AX output connects, via a 24" TS (unbalanced) cable, to the transmitter on the strap.

 

Initial tests, especially using a bass guitar voice, have resulted in a sort of whining noise at 5 KHz (and a harmonic at 10 KHz) when I turn at certain angles. Fiddling with the cable routing and transmitter position modify the whine, but they do not eliminate it. Notch parametric EQ has been less effective than I had hoped.

 

I get the feeling that the AX's CPU RF emissions may be affecting (or be affected by) the transmitter. So I dug up an unused ferrite RF choke, with which I am ready to experiment.

 

Finally, the questions:

  • Is the RF interference explanation plausible?
  • If yes to above, is an RF choke the likely solution?
  • If yes to both, then where should I put the RF choke -- near the transmitter input, near the Keytar output, or Doesn't Matter?
  • If No to the above, what do y'all suggest?

 

 

 

-Tom Williams

{First Name} {at} AirNetworking {dot} com

PC4-7, PX-5S, AX-Edge, PC361

 

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Do you get the hum with something else plugged into the wireless? A different keyboard or a guitar. I use three Line 6 wireless units and the only time I ever got interference was when the clubs wireless router was connected on a shelf right next to me at the far end of the stage. Luckily I was able to change channels on the unit that was humming. Have you tried changing channels on the Nady system or is it fixed frequency?

 

I doubt the Roland is causing the issue with the transmitter. I would bet the issue is something transmitting near you. Be it a wireless unit, a local transmission tower or some such.

Boards: Kurzweil SP-6, Roland FA-08, VR-09, DeepMind 12

Modules: Korg Radias, Roland D-05, Bk7-m & Sonic Cell

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Do you get the hum with something else plugged into the wireless? A different keyboard or a guitar.
The AX is my only portable electronic sound source.

Have you tried changing channels on the Nady system or is it fixed frequency?
It's fixed frequency. I chose it because it's nowhere near 2.4 GHz or even 900 MHz, the common U-NII frequencies. My only other overt RF source is an IEM that runs at 570 MHz or so -- and is turned off during these initial tests.

 

-Tom Williams

{First Name} {at} AirNetworking {dot} com

PC4-7, PX-5S, AX-Edge, PC361

 

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Not sure if RF interference is the cause - computer in the Keytar should not have all that much RF at such a high UHF frequency. However, these chokes don't cost a whole lot, and might help. I'd put as near the source as possible, because any additional wire means that the RF still gets radiated by air.

 

At that UHF frequency, even using a longer cable with the excess coiled up would effectively be a choke.

 

(I presume that the noise goes away completely if the keytar is directly connected by cable instead of going through the Nady system). Also, just to be sure that it IS coming from the keytar, you could connect the Nady to one of your regular keyboards that isn't portable normally (the lightest one), and then move around simulating the movement with the keytar).

Howard Grand|Hamm SK1-73|Kurz PC2|PC2X|PC3|PC3X|PC361; QSC K10's

HP DAW|Epi Les Paul & LP 5-str bass|iPad mini2

"Now faith is the substance of things hoped for, the evidence of things not seen."

Jim

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Wireless units are usually designed for guitars that do not have high frequency digital signals.

 

MBKeys is correct that a coil on the output cable will not eliminate radiation through the air.

 

Does the whine appear with other keys, and with any instrument with passive pickups?

 

If it appears with any instrument, the wireless may be defective. If it doesn't appear with passive pickups, it's the radiation from the CPU in the instrument and the wireless may not be designed for it.

 

What you are probably hearing is heterodyning between the wireless frequency signal and the CPU high frequency signals.

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Thanks to responders so far. More commentary is welcome.

Wireless units are usually designed for guitars that do not have high frequency digital signals.

Yup! I sorta knew it would be a bit of a crapshoot all along, although my greatest worry was that the AX output would be too hot for the transmitter.

Thinking about it a bit more, surely Roland knew that they would not want to jam transmission signals....

Does the whine appear with other keys, and with any instrument with passive pickups?

Ain't got no passive pickups -- Dammit Jim, I'm a keyboardist, not a guitarist.... -- but I will check out using a longer cable and other keyboards.

What you are probably hearing is heterodyning between the wireless frequency signal and the CPU high frequency signals.

Yep, although I lack the electronics vocabulary, that's my guess as well. It would be funny / ironic if it turns out that 900 MHz or (shudder) 2.4 GHz would work better. Those are actually easier to find.

-Tom Williams

{First Name} {at} AirNetworking {dot} com

PC4-7, PX-5S, AX-Edge, PC361

 

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Long shot, but is it possible to line the inside of the keytar enclosure with foil tape (like they use on guitar wiring compartments) and ground it?

Dan

 

Acoustic/Electric stringed instruments ranging from 4 to 230 strings, hammered, picked, fingered, slapped, and plucked. Analog and Digital Electronic instruments, reeds, and throat/mouth.

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Long shot, but is it possible to line the inside of the keytar enclosure with foil tape (like they use on guitar wiring compartments) and ground it?
Ground it to what, exactly? :laugh:

Can a Faraday cage work without an earth ground? :idk

-Tom Williams

{First Name} {at} AirNetworking {dot} com

PC4-7, PX-5S, AX-Edge, PC361

 

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Long shot, but is it possible to line the inside of the keytar enclosure with foil tape (like they use on guitar wiring compartments) and ground it?
Ground it to what, exactly? :laugh:

Can a Faraday cage work without an earth ground? :idk

 

Hmm, good question. I would assume that just tied to signal ground should do the trick, but I guess I never thought about it. But yeah, I would think all of that RF interference would be taken to signal ground, which is essentially zero. Afterall, it works on guitars that run wireless. Maybe others can chime in and confirm.

 

Dan

 

Acoustic/Electric stringed instruments ranging from 4 to 230 strings, hammered, picked, fingered, slapped, and plucked. Analog and Digital Electronic instruments, reeds, and throat/mouth.

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Can a Faraday cage work without an earth ground? :idk

 

Yes, it can. Think of a person that is protected from lightining inside a car. ECG (electrocardiograms) amplifiers are sometimes caged to improve rejection to 50/60Hz interference but the amplifier has to be isolated (1.5kVac minimum) from earth. Also during an RF immunity test of an equipment sensitive to some UHF frequencies (very intense EM field by the way), just wrapping (exploratory test) the equipment with aluminum foil was sufficient to stop the problems from happening.

 

Anyway in your case, the ground would be provided by the signal cable. But you could just wrap the keytar as a quick test to see if makes any difference and it is worth the effort.

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Lining with copper foil did make a significant difference in my Epiphone Les Paul guitar and Les Paul 5-string bass in the amount of hum (which was not huge to begin with). One of the guitar accessory companies (Adderio?) made foil with adhesive on one side. Removed all of the controls, covered all of the surface of the cut out wood area, and surface of the cover. Connected with short piece of wire to the ground on the 1/4" jack of the signal cable..

After doing so, I could turn up the amp to maximum (being careful not to jostle the guitar) and no hum.

Howard Grand|Hamm SK1-73|Kurz PC2|PC2X|PC3|PC3X|PC361; QSC K10's

HP DAW|Epi Les Paul & LP 5-str bass|iPad mini2

"Now faith is the substance of things hoped for, the evidence of things not seen."

Jim

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I'm liking these ideas. A little nervous about performing surgery on the AX before warranty is up, but it may be necessary to restore my mental health.

 

I assume that a metal shield -- even ungrounded -- would reflect (or transduce) RF; less clear is, if I ground to the signal ground, which then goes to the transmitter, have I simply direct-wired the ingress through the shield?

-Tom Williams

{First Name} {at} AirNetworking {dot} com

PC4-7, PX-5S, AX-Edge, PC361

 

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I have shielded several guitars in the past. Sometimes it works sometimes it doesn't, usually it tames it but never entirely eliminates it.. Single coil pickups are notorious for picking up emf. That's why they developed humbucking pickups. These have two coils that are would in different directions to cancel the emf hum. But some people prefer the sound of single coils myself included. Stratocastors have 3 single coils the center being the reverse wound to the other two. Using the middle in conjunction with the others removes the hum but again the sound changes. I eventually bought active pickups that solved the hum. But before you go opening up a new board and shielding it just hoping you don't short out the electronics, verify the Keytar is the problem not the uhf wireless. Because you want to know what tamed the hum in my bass guitar? Wireless! my bass and the amp are not electrically connected anymore. No more hum! A single $179 line 6 G10 system, 24 bit digital and worked like a charm. I have tested connecting this to my DM12, it also worked perfectly.

Boards: Kurzweil SP-6, Roland FA-08, VR-09, DeepMind 12

Modules: Korg Radias, Roland D-05, Bk7-m & Sonic Cell

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I'm liking these ideas. A little nervous about performing surgery on the AX before warranty is up, but it may be necessary to restore my mental health.

 

I assume that a metal shield -- even ungrounded -- would reflect (or transduce) RF; less clear is, if I ground to the signal ground, which then goes to the transmitter, have I simply direct-wired the ingress through the shield?

 

No it needs to go to signal ground. Don't think of earth, think of it as a zero reference, and you want to take the interference to zero relative to the rest of the signal.

 

Dan

 

Acoustic/Electric stringed instruments ranging from 4 to 230 strings, hammered, picked, fingered, slapped, and plucked. Analog and Digital Electronic instruments, reeds, and throat/mouth.

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