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Post subject: Help! - Wireless mics signals bleeding into guitar amp!
Posted: Sat May 23, 2015 8:07 am
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FIRST POST:

I am currently gigging in the pit-orchestra at a local community Broadway musical theater, but having a major issue with my 2009 Frontman 212R: Wireless headset mics are used by the cast members and are quite noticeably bleeding into my amp. The signal is coming through both channels, depending on which is selected, and the reverb as well. Doesn't matter which input on the amp I plug into either. I am able to control it by turning the volumes or reverb to the "0" position, but that's just a workaround. (Messing with the volumes on a Frontman is a very touchy thing, as everybody is well aware of.)

I am using a 2004 Strat and a really high-quality Monster guitar cable with no pedals. I spent the better part of the last week shielding the Strat pickguard and body cavity, and resoldering all of the electronic connections (it's ok - I'm an expert solderer). Also potted the PUPs with paraffin/beeswax in the event of a microphonic issue. Guitar sounds choice, so no problems there.

On the amp, all pretty much I could try was to foil-wrap the reverb cable as seen on this video: http://www.youtube.com/watch?v=6VVLPQlMPVk , and also shielded the bottom of the reverb tank, which was open (used a cardboard backing wrapped with foil, grounded to chassis - reverb springs swing freely, I double-checked). I also took a quick look at the electronic components in the amp and didn't notice anything out of the ordinary - no noticeable loose components, cold solder joints or the like. The amp-chassis is completely factory-shielded on all sides, which I found impressive. And the amp is grounded properly at the theater as well.

As for the cable it's a Monster Rock Prolink 21' cable; but I have also tried several different other cables of varying lengths/quality to no avail. Haven't tried an RF choke on the cable, but read somewhere that sometimes tying (carefully...) an overhand knot at each end of the cable, about 3-4 inches from the ends can have a similar effect -- tried it, didn't work.

So, after all of the work I put into everything over the past week, you can imagine how crushed I was when I plugged in yesterday evening, turned on, and STILL had the wireless mics bleeding through my amp (and if anything at all, it was maybe a little WORSE than before even). - I am having to just leave the amp turned off when I'm not playing, then turn it back on with just enough time to sit up, turn up the Strat, and go. And of course as soon as the song ends, the amp goes back off. Blessedly it's a super-quiet amp other than the mic bleed-thru, so there's no popping, hum, or buzzes when I turn it off.

I know it's an RFI issue, but it's been too many years since the last time I tried building a RF filter, and honestly wouldn't at this juncture be exactly certain of where to insert the circuitry if I could remember. And I really don't want to take the chance on it affecting the amp's tone. - I have the amp schematic and an electronics background, so if anybody knows of, or can recommend any tweaks to the amp (ground this or jump this to that, etc) I would be much obliged and elated if it works. Not to mention very humbly grateful. -- I am also on a very limited budget, VERY limited, so replacing expensive parts will be out of the question unfortunately. And we all know the recent plight of Radio Shack, so even getting components without a 1 - 3 week delivery wait would be miraculous.

I will try to check this post at least once daily, probably more, over the next several days, and am hoping and praying some of you Fender "guru / space-age wiz kids" out there can help me out.

Thanks for reading this far - Cheers!

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Post subject: Re: Help! - Wireless mics signals bleeding into guitar amp!
Posted: Sat May 23, 2015 9:18 am
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Are you using any pedals or stompboxes?

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Post subject: Re: Help! - Wireless mics signals bleeding into guitar amp!
Posted: Sat May 23, 2015 11:33 am
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I wonder if poor contact with the input jacks, I mean the ground contact when an instrument is not connected to the input jacks .?
The input jacks would act as an antenna

Or missing ground at wall outlet at amp power cord ?

Try to plug a guitar at each input jack with guitar vol. to "0" . This will connect input jacks to ground


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Post subject: Re: Help! - Wireless mics signals bleeding into guitar amp!
Posted: Sat May 23, 2015 2:03 pm
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There are two approaches you can try to reduce bleedthrough on a 212R. I expect it will take both to make a significant difference because the interference is so close
The first is specifically for the 212, the second can work on any amp that picks up noises.

First is to build a shield over the input jacks and the IC right behind them. The 212 has noisy inputs and all the 212s could use this.
A piece of soft steel such as the jacket around a 9volt battery works well.
Loosen the jacks and controls to allow a little clearance.
Cut a notch in the steel to slip between the chassis and the jack. When you retighten the jack, it should hold the shield securely. Then bend the shield down to cover the IC and its related components. Bend the steel to bring the shield edges down to the board. After it is shaped, then silicone it down to the caps around the IC.
This will reduce buzz and hum generally from the input. It will probably reduce the radio interference, but not eliminate it completely.

The second approach is to build a noise cancelling guitar cable. You can't buy one, you have to make it. If you use pedals, then you will need to build two cables. Keep your cables as short as possible.

Start with a 10 foot microphone cable and cut off the XLR connectors. Get a couple good quality 1/4" plugs and wire the hot and ground as normal. The extra conductor is hooked to ground on one end and nothing on the other end. Be sure that end is insulated so it doesn't touch anything.
Mark that end as Guitar and the grounded end as Amp. Connect them that way and see if it doesn't eliminate the interference.
The way it works is to pick up the interference on the second conductor and insert it out-of-phase with the signal wire to cancel the interference. It's useful for amps that pick up radio stations and computer noise.


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Post subject: Re: Help! - Wireless mics signals bleeding into guitar amp!
Posted: Sun May 24, 2015 1:16 am
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Does your amp have shielded cable(s) from the jack to the input section?


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Post subject: Re: Help! - Wireless mics signals bleeding into guitar amp!
Posted: Sun May 24, 2015 1:37 am
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BMW2002Ti wrote:
Does your amp have shielded cable(s) from the jack to the input section?



Shematic show no shield cable like most (all ? ) Fender amp


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Post subject: Re: Help! - Wireless mics signals bleeding into guitar amp!
Posted: Sun May 24, 2015 2:32 am
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Are you using single coils? Do you have a hum-bucker guitar to try? My bet is that the signal-bleed is coming in through the guitar, not the amp. There might not be much that you can do about it with a single coil guitar, maybe using hum-buckers would help.

You could try simply unplugging the guitar cable from the amp when not in use. :idea:

Any chance of getting them to change the frequency of the wireless system?

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Post subject: Re: Help! - Wireless mics signals bleeding into guitar amp!
Posted: Mon May 25, 2015 8:37 am
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Thanks for the replies and suggestions! :) To address some of your questions:

shimmilou:
* My Strat is a stock Standard Stratocaster, with the exception of being modified for a bridge pickup tone control capability (bridge the 2 center posts on the 5-way switch...), and having been completely internally shielded against RFI. So yes, it has single-coils. Yes, I have a Les Paul with humbuckers, but I don't believe, due to some new information, that is my issue (this is addressed further below...).
* I'm turning the amp off when I'm not using it (which sucks!)
* Getting the current soundman to switch mic frequencies would take an act of Congress -- this is all "my fault," if you catch my drift... Not even willing to try to work with me on the subject, regardless of how good my equipment is or my background experience/knowledge of the subject.

BMW2002Ti / stratele52:
* On a Frontman, the jack is soldered directly to the electronic board, but "TimsAudio" has made a suggestion concerning that, that I find very intriguing.
* As I stated in my post, "the amp is grounded properly at the theater as well." - So I'm certain it's not a grounding issue. Also, the cable connectors and input jacks to the guitar and amp are clean as a whistle.
*Having more than just the one guitar in the "beyond tiny" orchestra pit is out of the question. I don't even have room for a backup just in case, and the drummer is using my guitar case as a back-rest!

TimsAudio:
* I find your suggestion about shielding the interior portion of the inputs most intriguing, and I fortunately do have the expertise to pull it off (a degree in electronics with 30+ years in the industry). However, as a warning to anybody else reading this post now or in the future: THIS MOD COULD QUITE EFFECTIVELY SHORT OUT THE INTERNAL CIRCUITRY OF YOUR AMPLIFIER, SO UNLESS YOU ARE EXTREMELY COMPETENT IN FOLLOWING TIMSAUDIO'S INSTRUCTIONS TO THE LETTER, DO NOT ATTEMPT THIS, BUT REFER THIS TO SOMEONE QUALIFIED TO DO SO!!! - With that aside, please see below in my further discussion section what I have since found out. I don't think this will be necessary, but I will probably do it anyway as a precaution against possible future issues that may crop up. (***Question: With the internal shielding of the Frontman circuitry [I was very impressed with this] is this mod necessary, or just adding more internal shielding?)
* I very, VERY much like the idea of the "noise cancelling guitar cable," and this may very well be my problem, which I will discuss further below...

Retroverbial (Arjay):
* No pedals, as stated in my post - the musical simply calls for that late 50s, early 60s Strat sound with some reverb from the guitar.

FURTHER DISCUSSION AND NEW INFO:
- During last week I moved the tremolo ground wire from the tremolo spring claw to the back side of the bridge to prevent the tremolo springs from acting as inductors.

-- This weekend I went back to the theater and tested a couple of thoughts I had during the week:
* First, I unplugged the guitar cable from the amp and did the "Spinal Tap Slide" (younger viewers may not get this, watch the movie "This Is Spinal Tap" [1984]) - sliding all of the front controls up to "10" with the amp on, and lots of actors speaking through their wireless mics during sound check. Other that a slight bit of hiss on the dirty channel (to be expected) the amp was impressively quiet, and I mean no sound at all, no noise, hum, crackles, or pops, and no wireless mic bleed-thru at all. I have never owned a amp this quiet in my entire life (and I've had one of just about everything out there at one time or another!). So this rules out the amp as the source of the problem! :D
* Next, I plugged a small, powered speaker that the drummer brought, into the Strat during the mic checks and there was no RFI at all from the mics - this rules out the guitar! :D
* Finally, I plugged just the Monster cable by itself, no guitar, into the amp and there it was - the wireless mic bleed-thru in all of its unwelcome nastiness. (And that was a quite expensive cable too - I would think it would be shielded beefily enough to block RFI...) :(

--- So I now believe what we're looking at is the guitar cable(s) - (I've tried several - same results) - acting as antennas and somehow amplifying the wireless mic signals, crazy as that sounds.

So, "TimsAudio," I have this idea and would like your (or anybody else's) opinion:
* What if I were to try soldering a 50 pf capacitor directly across each of the input terminals to the amp (one cap per input; a little more work and expense).
* Or just one cap across the terminals of the Monster guitar cable itself (a little less work and expense, if I can fit it under the shield)?
* Or (a further thought) across the output terminal of the guitar itself and be done with it?
* The capacitor should act as a direct path to ground for the wireless mic frequencies, but too small to affect the sound of the Stratocaster, or at least I would think so...
+ Would this color my tone in any way?
+ Is there very much danger of the cap(s) shorting out and causing damage to any of my equipment? (I would think not, as they tend to open in my experience - also with it being a low level signal we're dealing with here, I would think something dreadful would have to happen inside the amp itself to cause any kind of an excessive current failure that would fuse a capacitor - thoughts?)
+ What about an RF choke clamped onto the guitar cable?

Ok - so that's all I have for everybody to chew on at this juncture. Sorry I didn't think to perform any the "Further Discussion/Info" checks before starting the post but I was at a loss and in somewhat of a panic at the time... Count it up to being a "noob" on the forums!

Thanks again, and cheers!

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Post subject: Re: Help! - Wireless mics signals bleeding into guitar amp!
Posted: Mon May 25, 2015 8:44 am
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donehomr wrote:
Thanks for the replies and suggestions! :) To address some of your questions:

shimmilou:
* My Strat is a stock Standard Stratocaster, with the exception of being modified for a bridge pickup tone control capability (bridge the 2 center posts on the 5-way switch...), and having been completely internally shielded against RFI.
Is the job is well done ? How work the Les Paul ?


So yes, it has single-coils. Yes, I have a Les Paul with humbuckers, but I don't believe, due to some new information, that is my issue (this is addressed further below...).
* I'm turning the amp off when I'm not using it (which sucks!)
* Getting the current soundman to switch mic frequencies would take an act of Congress -- this is all "my fault," if you catch my drift... Not even willing to try to work with me on the subject, regardless of how good my equipment is or my background experience/knowledge of the subject.

BMW2002Ti / stratele52:
* On a Frontman, the jack is soldered directly to the electronic board, but "TimsAudio" has made a suggestion concerning that, that I find very intriguing.
* As I stated in my post, "the amp is grounded properly at the theater as well." - So I'm certain it's not a grounding issue. Also, the cable connectors and input jacks to the guitar and amp are clean as a whistle.
*Having more than just the one guitar in the "beyond tiny" orchestra pit is out of the question. I don't even have room for a backup just in case, and the drummer is using my guitar case as a back-rest!

TimsAudio:
* I find your suggestion about shielding the interior portion of the inputs most intriguing, and I fortunately do have the expertise to pull it off (a degree in electronics with 30+ years in the industry). However, as a warning to anybody else reading this post now or in the future: THIS MOD COULD QUITE EFFECTIVELY SHORT OUT THE INTERNAL CIRCUITRY OF YOUR AMPLIFIER, SO UNLESS YOU ARE EXTREMELY COMPETENT IN FOLLOWING TIMSAUDIO'S INSTRUCTIONS TO THE LETTER, DO NOT ATTEMPT THIS, BUT REFER THIS TO SOMEONE QUALIFIED TO DO SO!!! - With that aside, please see below in my further discussion section what I have since found out. I don't think this will be necessary, but I will probably do it anyway as a precaution against possible future issues that may crop up. (***Question: With the internal shielding of the Frontman circuitry [I was very impressed with this] is this mod necessary, or just adding more internal shielding?)
* I very, VERY much like the idea of the "noise cancelling guitar cable," and this may very well be my problem, which I will discuss further below...

Retroverbial (Arjay):
* No pedals, as stated in my post - the musical simply calls for that late 50s, early 60s Strat sound with some reverb from the guitar.

FURTHER DISCUSSION AND NEW INFO:
- During last week I moved the tremolo ground wire from the tremolo spring claw to the back side of the bridge to prevent the tremolo springs from acting as inductors.

-- This weekend I went back to the theater and tested a couple of thoughts I had during the week:
* First, I unplugged the guitar cable from the amp and did the "Spinal Tap Slide" (younger viewers may not get this, watch the movie "This Is Spinal Tap" [1984]) - sliding all of the front controls up to "10" with the amp on, and lots of actors speaking through their wireless mics during sound check. Other that a slight bit of hiss on the dirty channel (to be expected) the amp was impressively quiet, and I mean no sound at all, no noise, hum, crackles, or pops, and no wireless mic bleed-thru at all. I have never owned a amp this quiet in my entire life (and I've had one of just about everything out there at one time or another!). So this rules out the amp as the source of the problem! :D
* Next, I plugged a small, powered speaker that the drummer brought, into the Strat during the mic checks and there was no RFI at all from the mics - this rules out the guitar! :D
* Finally, I plugged just the Monster cable by itself, no guitar, into the amp and there it was - the wireless mic bleed-thru in all of its unwelcome nastiness. (And that was a quite expensive cable too - I would think it would be shielded beefily enough to block RFI...) :(

--- So I now believe what we're looking at is the guitar cable(s) - (I've tried several - same results) - acting as antennas and somehow amplifying the wireless mic signals, crazy as that sounds.

So, "TimsAudio," I have this idea and would like your (or anybody else's) opinion:
* What if I were to try soldering a 50 pf capacitor directly across each of the input terminals to the amp (one cap per input; a little more work and expense).
* Or just one cap across the terminals of the Monster guitar cable itself (a little less work and expense, if I can fit it under the shield)?
* Or (a further thought) across the output terminal of the guitar itself and be done with it?
* The capacitor should act as a direct path to ground for the wireless mic frequencies, but too small to affect the sound of the Stratocaster, or at least I would think so...
+ Would this color my tone in any way?
+ Is there very much danger of the cap(s) shorting out and causing damage to any of my equipment? (I would think not, as they tend to open in my experience - also with it being a low level signal we're dealing with here, I would think something dreadful would have to happen inside the amp itself to cause any kind of an excessive current failure that would fuse a capacitor - thoughts?)
+ What about an RF choke clamped onto the guitar cable?

Ok - so that's all I have for everybody to chew on at this juncture. Sorry I didn't think to perform any the "Further Discussion/Info" checks before starting the post but I was at a loss and in somewhat of a panic at the time... Count it up to being a "noob" on the forums!

Thanks again, and cheers!


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Post subject: Re: Help! - Wireless mics signals bleeding into guitar amp!
Posted: Mon May 25, 2015 10:29 am
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The test you performed with no plug inserted is not valid with this amp. There is a muting circuit that is disengaged by plugging in. It is connected to the ground switch of the input jack. The 212 is actually a very noisy amp. The muting circuit is a showroom trick to mask how much noise it actually makes.
For a valid test, connect a 1M resistor across a 1/4" plug and insert it and try again.
I think all Frontman 212 should have this shield installed to quiet the input stage. Some of them squeal when the volume is turned up.

And certainly, I'm assuming that the shielding work should be done without shorting out the amp. You can wrap the shield with electrical tape to prevent shorts. The closer you can get it to the IC, the better it works. I punched a couple holes to let it slide over the taller capacitors nearby.

I haven't had much luck with capacitors reducing RF noise. I've tried them and ferrite beads without much luck on many different amps.
A 10 or20PF cap shouldn't color your tone, nor will it short out at the low voltages involved. A snap on RF choke would be an easy thing to try.

There are very few cables that will shield against close-by RF transmitters. That's why I recommend the noise cancelling cable for your application.
I personally think that most cables are overhyped and overpriced. Oxygen-free copper is a hoax. All copper wire is oxygen free.
The determining factors are length of cable, it's capacitance, and the amount and quality of the copper braid used for the shield. You can only determine that by cutting off the heat shrink, unscrewing the plug cover and inspecting the amount and quality of shield material. The best cable has a tight crossbraid that covers 90% of the inner conductor. Many guitar cables have a crossbraid molded into the plastic cover to fool you into thinking its a good shield. Some have a carbon layer that does little or nothing to improve shielding.

In your situation in the pit, You might consider a noise cancelling cable with a foil shield. It's a stiffer wire, but better shielding.
The noise cancelling cable does add some capacitance to the input, but the tradeoff is a little less brilliance for no RF interference.


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Post subject: Re: Help! - Wireless mics signals bleeding into guitar amp!
Posted: Mon May 25, 2015 11:16 am
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TimsAudio wrote:
I personally think that most cables are overhyped and overpriced. Oxygen-free copper is a hoax. All copper wire is oxygen free.


+1

Just as with gold-plated pins on vacuum tubes -- a complete and utter scam.

Arjay

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Post subject: Re: Help! - Wireless mics signals bleeding into guitar amp!
Posted: Tue May 26, 2015 11:27 am
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TimsAudio:
Don’t I feel daft…?!?! -- I don’t know how I missed the muting circuit at the inputs, it’s plain as day on the schematic. I guess I was just so busy looking at everything else that I missed it; sorry about that, and thanks for pointing it out to me.

Don’t have a 1MΩ resistor right now, but will take your word for it. - Doesn’t surprise me about the amp being otherwise noisy, but it’s never been a problem so far at other gigs, just this one.

You are exactly right; I am up against a heck of a challenge here. The major problem is that the aforementioned soundman at this theater doesn’t really know what he’s doing. The wireless mics transmitter packs gain controls are all almost maxed out in an effort to minimize feedback (???), which doesn’t make much sense to me, and I think is probably the primary source of my dilemma. My equipment is getting bombarded with so much RFI that I seriously wonder if ANYTHING could block it.

I have some high quality 25’ Belden low-z mic cables, but I sure hate the thought of cutting one of them in half, as I use them occasionally for some extra DJ work at weddings and such. I don’t play again until Friday, so we have a little time. Do you think using one of them intact with the noise cancelling cable mod and a foil shield would be ok, or is there a reason you recommend the 10’ length?

Additionally, went to the only Radio Shack locally, without having to drive all the way down to Tampa, this morning in search of a 10 or 20pf cap, and all they had was a 100pf, which I think is a little high. Otherwise I will have to order one online, and to spend 15 cents on the cap and $5.35 for shipping seems a little ludicrous, not to mention the 1 to 3 week wait for delivery…

The snap-on RF choke was $6.00 at RS, to which I had to say, “Are you serious?” - I remember when they were 35 cents. I will pick one up if I have to, but I’m kinda’ with you on this one, I’ve tried them before with not much success (or even less).

Wonder if just sitting inside a Faraday cage would help?!!!

So, I’m getting ready right now to tear inside the 212R and make that 9V battery jacket shield mod you mentioned. Regardless of my current situation, it sounds like a GREAT suggestion as a possible guard against future problems that could arise. -- Pending your answer concerning my question above about the noise cancelling cable mod, I am going to hold off for now.

Anticipating your reply, and I thank you sir.

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Post subject: Re: Help! - Wireless mics signals bleeding into guitar amp!
Posted: Tue May 26, 2015 1:31 pm
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Fight fire with fire - get a wireless transmitter yourself. And turn the gain to max.


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Post subject: Re: Help! - Wireless mics signals bleeding into guitar amp!
Posted: Tue May 26, 2015 2:32 pm
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ChrisH1:
Actually I have a NADY wireless, and our drummer had that same thought... However knowing the kind of luck I've been having, the situation would probably get reversed and instead of the theater picking up my guitar through the sound system, I would probably end up having an even bigger problem with my receiver picking up the wireless mics AND my guitar; PLUS the added thrill of the problem with my amp ALSO picking up the wireless mics -- talk about double-jeopardy!!!

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Post subject: Re: Help! - Wireless mics signals bleeding into guitar amp!
Posted: Tue May 26, 2015 7:45 pm
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Well, most amps don't have or need a mute. It's an easy thing to miss if you're not looking for it.
You can use almost any value resistor for the test above 10K.

I don't think the gain of the wireless packs will affect the interference levels much. They are probably FM and the amplitude of the carrier remains constant with only the frequency changing.

The shorter the cable the better your sound and the less interference.
If you could get away with a 3 foot cable, its that much less of an antenna.

As far as your nice XLR cables, I suppose you could come up with some adaptors to do it, but I'm not sure if the grounding would prevent it from working.
There are noise canceling XLR cables with extra wires in them. They are usually long cables (50') and called "star quad" or something like that.
You can sometimes find 2 conductor with a foil shield at Home Depot or Lowes for home theater installations. Its very low cost.


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