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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.
james1969

Showing 3 responses by atmasphere

atmasphere's avatar
atmasphere
12,115 posts
 
It's all resistors and it's designed to shrug off MHz range ringing because of it's crazy wide bandwidth.
We had to be careful too for the same reason.
atmasphere's avatar
atmasphere
12,115 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.
atmasphere's avatar
atmasphere
12,115 posts
 
I hate to say it, but this is a common problem is the audio equipment has poor RFI rejection.

There is this design aspect known as a 'stopping' resistor, that is used in tube and solid state circuits to prevent oscillation, and also has a big effect on susceptibility to RFI.

The resistor is in series with the signal and is placed as close as possible to the input connection of each amplifying device (grid of a tube, gate of a MOSFET and so on).

The resistor interacts with the input capacitance of the device, preventing it from causing oscillation by acting as a low pass filter. This is effect of RFI for the same reason.

The thing is, many audio designers don't employ stopping resistors (the Japanese really didn't use them at all for several decades) so you run into problems like this if you have heavy RF sources nearby and otherwise poor grounding.

The 'I hate to say it' part is you may have to change out gear to something more stable to get rid of this. May.

What happens if you operate your cell phone close to your stereo? Do you get interference?
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