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Author Topic: AM Radio bleed out of the amp...  (Read 1620 times)

frank kayser

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AM Radio bleed out of the amp...
« on: October 19, 2020, 11:43:14 am »

Hi again,

I did an outdoor church service yesterday.  Funny thing, when we turned the small amp on for the keyboards, out came a radio broadcast.  Loud and clear. The keyboard and/or amp seemed to be the only thing affected.  The guitar amp and mixer seemed to be unaffected.

After changing the amp out, the reception was reduced to an acceptable noise floor; still there but only noticeable during quiet periods within 10' of the speakers.  The celebrant said they've had problems before, and pointed out an AM radio station antenna, the base of which is 100 yards away.  D'Oh!  Never saw it.  (DANG Trees!) WFAX 1200AM operates at 48/5000 watts (night/day).  At least it is not a 50KW powerhouse... They also broadcast at 100.7FM at an unknown power level.


So my uneducated guess is that the mixer is adequately shielded, and the XLR cables are doing their job.  Both the keyboard and guitar are on Radial Stage Bug DI (SB-1 Active). I would further assume signal could be leaking into instrument cables and improperly shielded amps or the keyboard itself.  Given that the output strength of the radio reproduction was reduced, but not eliminated, I'm thinking all three were at issue. 

Unfortunately, I lacked time to troubleshoot properly.  Get it working adequately, and move on.  They threw mw a bunch of curves.  Never happens, right?
In two weeks, during the next service, I expect the ticking time bomb to exploit the weakness yet again.  I may not be as lucky this time.

So, what can I do?  Anyone have suggestions as to what to avoid, and mitigation of effects?
So I'm guessing:
- Keep instrument cables as short as possible.
- Use the amps we know work.
- Build a fine-mesh-screen faraday cage around the keyboard, player, cables and amp?  I'd guess I'd need screening on the bottom, too...
Anything else?

A note: The keyboard has 120v power supply on board,  The IEC input socket is pulled out from the chassis, and the wires are still insulated, but not within the shielded enclosure.  I don't know if that matters...

In two weeks, the main celebrant mentioned he would like a LAV mic - I have three flavors:
1. Senn UHF Wireless (band A)
2. AT System 10 (2.4ghz)
3. Hard wired.

As the celebrants all go to the far edges of the congregation for communion, wireless would work the best, from a celebrant standpoint.  It would be additional risk with wireless.  Wired would be constricting.

I know the BrainTrustę here have faced this many times.  Thanks in advance for anything you have to offer.


frank




« Last Edit: October 19, 2020, 12:21:28 pm by frank kayser »
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Steve Litscher

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Re: AM Radio bleed out of the amp...
« Reply #1 on: October 19, 2020, 01:10:28 pm »

Following this one out of curiosity because a few years ago, I worked with a keyboard player who had two (2) amps that would pick-up local AM radio stations, clear as day. None of the other "musicians" (it was at best, a really crummy bar band) had issues with AM bleed in their amps. One was a Fender hybrid (digital/tube), and I can't for the life of me remember what the other one was.

The aforementioned keyboardist did have a penchant for picking up amps/gear from Craigslist and The Recycler and whatnot... no telling what they'd been through prior to him taking possession. So glad I don't do that gig any longer. :D

John L Nobile

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Re: AM Radio bleed out of the amp...
« Reply #2 on: October 19, 2020, 01:53:38 pm »

I used to pick up radio stations from my Leslie back in the day. I think changing the tubes with new ones fixed it. But it was cool listening to pop songs with tremolo and noise.

I'm assuming you have tube amps. If not, my next suspect would be the 1/4" cable from the keyboard or any others in the chain. I've never had that happen with an XLR cable but I would think it's possible if the cable is damaged or miswired so you may want to check the ones connected to the amp.
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Matthias McCready

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Re: AM Radio bleed out of the amp...
« Reply #3 on: October 19, 2020, 02:04:21 pm »

I had an Ampeg that used to do this  ;D
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Mike Monte

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Re: AM Radio bleed out of the amp...
« Reply #4 on: October 19, 2020, 02:28:12 pm »

Hi again,

I did an outdoor church service yesterday.  Funny thing, when we turned the small amp on for the keyboards, out came a radio broadcast.  Loud and clear. The keyboard and/or amp seemed to be the only thing affected.  The guitar amp and mixer seemed to be unaffected.

After changing the amp out, the reception was reduced to an acceptable noise floor; still there but only noticeable during quiet periods within 10' of the speakers.  The celebrant said they've had problems before, and pointed out an AM radio station antenna, the base of which is 100 yards away.  D'Oh!  Never saw it.  (DANG Trees!) WFAX 1200AM operates at 48/5000 watts (night/day).  At least it is not a 50KW powerhouse... They also broadcast at 100.7FM at an unknown power level.


So my uneducated guess is that the mixer is adequately shielded, and the XLR cables are doing their job.  Both the keyboard and guitar are on Radial Stage Bug DI (SB-1 Active). I would further assume signal could be leaking into instrument cables and improperly shielded amps or the keyboard itself.  Given that the output strength of the radio reproduction was reduced, but not eliminated, I'm thinking all three were at issue. 

Unfortunately, I lacked time to troubleshoot properly.  Get it working adequately, and move on.  They threw mw a bunch of curves.  Never happens, right?
In two weeks, during the next service, I expect the ticking time bomb to exploit the weakness yet again.  I may not be as lucky this time.

So, what can I do?  Anyone have suggestions as to what to avoid, and mitigation of effects?
So I'm guessing:
- Keep instrument cables as short as possible.
- Use the amps we know work.
- Build a fine-mesh-screen faraday cage around the keyboard, player, cables and amp?  I'd guess I'd need screening on the bottom, too...
Anything else?

A note: The keyboard has 120v power supply on board,  The IEC input socket is pulled out from the chassis, and the wires are still insulated, but not within the shielded enclosure.  I don't know if that matters...

In two weeks, the main celebrant mentioned he would like a LAV mic - I have three flavors:
1. Senn UHF Wireless (band A)
2. AT System 10 (2.4ghz)
3. Hard wired.

As the celebrants all go to the far edges of the congregation for communion, wireless would work the best, from a celebrant standpoint.  It would be additional risk with wireless.  Wired would be constricting.

I know the BrainTrustę here have faced this many times.  Thanks in advance for anything you have to offer.

frank
I am no expert.
My guess: an unshielded cable between the keyboard and the amp.  It may act like a radio antenna.

I witnessed this happen years ago....; one time it was an unshielded inst cable, the other time it was traced to a Radioshack four channel mic mixer...the mics had 1/4" jacks...
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Jamin Lynch

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Re: AM Radio bleed out of the amp...
« Reply #5 on: October 19, 2020, 02:36:24 pm »

I remember picking up CB radios back in the day. Usually turned out to be an unshielded patch cable somewhere.
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John Sulek

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Re: AM Radio bleed out of the amp...
« Reply #6 on: October 19, 2020, 02:47:49 pm »

I would check that the keyboard player does not have a 1/4 to 1/4 speaker cable mixed in with the shielded patch cables to the amp. There are some that look exactly like shielded cables until you look inside the jack barrel.
And check that the jacks on the amp are tightly attached to the chassis and there isn't any corrosion on this connection.
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John Roberts {JR}

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Re: AM Radio bleed out of the amp...
« Reply #7 on: October 19, 2020, 03:03:45 pm »

It's called rectification when an active stage is not fast enough to cleanly reproduce the AM radio signal so it slew limits, effectively decoding the AM content.

Yes, typically RF comes into inputs, but a few cases of RF coming into speaker wires have been seen.

JR
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Daniel Levi

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Re: AM Radio bleed out of the amp...
« Reply #8 on: October 19, 2020, 04:43:26 pm »

I know residents of Hooke in Dorset (UK) were complaining that they were getting Russian radio coming out of their toasters ans their remote car keys weren't working, I believe this was to blame,
http://tx.mb21.co.uk/gallery/gallerypage.php?txid=1662&pageid=2500

Another solution that can work for RFI are neutrik EMC XLR connectors http://www.neutrik.co.uk/neutrik/products/xlr-connectors/xlr-cable-connectors/emc-series
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Russell Ault

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Re: AM Radio bleed out of the amp...
« Reply #9 on: October 19, 2020, 05:07:16 pm »

[...] So, what can I do?  Anyone have suggestions as to what to avoid, and mitigation of effects? [...]

Ditch the keyboard amp for a DI (I've never understood the need for keyboard amps; isn't that what monitors are for?) or, better yet, ditch the amp and get a keyboard with balanced outs; common mode rejection is your friend.

[...]
In two weeks, the main celebrant mentioned he would like a LAV mic - I have three flavors:
1. Senn UHF Wireless (band A)
2. AT System 10 (2.4ghz)
3. Hard wired.
[...]

Option 1, with option 3 as a backup (I don't see any advantages to option 2).

-Russ
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ProSoundWeb Community

Re: AM Radio bleed out of the amp...
┬ź Reply #9 on: October 19, 2020, 05:07:16 pm ┬╗


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