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high frequency intermittent noise

I have a noise issue that is intermittent.  Here is what the noise sounds like:

https://clyp.it/4b233bmm

Here is what I know so far:
  • The sound affects all components and is compounded if all components are turned on.  I have turned off my preamp, phono preamp, leaving just my mono blocks on, and the noise still appears.
  • I have turned off everything and unplugged everything in the house including my dimmer switch, and the noise still appears.
  • I have a pair of pro-audio monitors, self powered with class AB amps, and when I plug those into the same outlet, I hear the same noise coming through the pro-audio monitor.  So this rules out my big system.
  • The noise is primarily during the day and goes into the evenings, weekends too, early mornings it does not appear.
  • I live in a pre-war mid-rise building.  I have no ground, I'm using a Nordost QKore grounding system.  This did reduce the noise floor quite a bit, but has no affect on this intermittent noise.
  • I have a cell phone tower directly across the street from my building in Manhattan.
  • Looking at a real time analyzer, I see peak at 2kHz when the noise appears.
128x128Ag insider logo xs@2xjames1969
almarg's avatar
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almarg
9,670 posts
 
almarg and or kosst_amojan, or others,
Would the copper screening work well as a shield and reject RFI from entering through the glass window?
Yes, Jim, I would think that a grounded copper screen would be effective against 700 - 800 MHz, since the openings in the mesh are far smaller than the wavelengths of those frequencies (which are about 17 and 15 inches respectively). Although as noted at the FCC link I provided earlier those frequencies can "penetrate buildings and walls easily," so keeping them from getting in via the window might not be all that helpful.

Good suggestions, though, in your previous post.

Best regards,
-- Al

shadorne's avatar
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shadorne
7,477 posts
 
This RF/EM signal must be picked up by the voice coil in the tweeter and it is likely a powerful megaHz signal and would be inaudible if it didnt cause problems for the amplifier(which tries to surpress distortion with negative feedback so as to match input) and the problem manifests itself in the audible range somehow due to signal saturation in the amp section making it a broadband signal.

Try adding a choke across the speaker wires to short out the HF signal.

http://audiosystemsgroup.com/RFI-Ham.pdf

See Fig 4 for an example...
james1969's avatar
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james1969 OP
347 posts
 
Try adding a choke across the speaker terminals to short out the HF signal.

@shadorne 

What is a choke?  How would you implement this?  I'm not an electronics guy, learning-as-I-go-here.
atmasphere's avatar
atmasphere
11,862 posts
 
I got my phone steaming videos last night and put it INSIDE my F5 right up against the left amp board.
Nelson knows his stuff.

If the amp is not properly stopped, it will be almost impossible to make this problem stop (if you will pardon the expression...).

Sometimes the amplifier circuitry is OK, but RFI gets rectified right at the input. RF beads of the right value can help, if added right where the signal gets to the amp circuit.

If the amp makes this sound with the inputs shorted, there really isn't much you can do about it but replace the amp or have it repaired.
erik_squires
16,245 posts
 
Um, do NOT "short" your speaker wires!

The example in fig. 4 is fine, but it is most certainly NOT a short, it is the opposite, a choke.

Given however that you even have powered speakers having this issue, the problem is probably too late by the time it gets to your speakers. You need to think further upstream.

I did by the way recommend cheap RF chokes above.

Best,

E
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