high frequency intermittent noise

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

... the frequency my stereo is picking up is in the 700 - 800MHz range.

James, check out this link:

https://www.fcc.gov/general/700-mhz-public-safety-spectrum-0

The band extending from 698 to 806 MHz is used by public safety services, with part of it also apparently being used in some locations for cellphone or other mobile device communications.

Hope that helps. Regards,
-- Al

Jea48 11-17-2017
...how about when james1969 used the pro-audio monitors for the test. When he hooked them up he heard the same RFI noise. Just a guess the amps are SS.

Yes, I had thought of that, and the fact that the same noise occurred with both amps does make it seem less likely that the possible absence of stopper resistors is a factor. But note that Ralph’s comment referred to the use of such resistors in solid state as well as tube designs. (Of course, in the case of a solid state design they wouldn’t be referred to as "grid stopper resistors," that being the term I used in connection with James’ tube-based Lamm amplifiers).

Best regards,
-- Al

James, searching at eBay for "RCA shorting plugs" (without the quotes) will lead you to a number of sellers offering them at low cost.

Good point by Ralph (Atmasphere) re the possibility that the amp doesn't provide grid stopper resistors in its design.  If that proves to be the case, James, I would think that if you explained the situation to Vladimir he could have them installed at modest cost.  It would just involve the addition of a physically small low power resistor in series with the grid of each tube.

Good luck.  Best regards,
-- Al
 

...can an XLR, balanced input, be shorted out without causing any damage to the amp? My guess is yes it would be ok.

Yes, that should be ok to do, Jim. From the perspective of the amp, shorting the three pins on an XLR input connector together would not be much different than if it were to receive a zero volt signal from a preamp having very low output impedance.

Best regards,
-- Al

AM is WAY too low in frequency. Cell phones broadcast in the 700 to 2100 MHz range.

Thanks for your informative inputs, Kosst. But of course the bandwidth of James’ audio components most likely extends only up to a frequency that is even lower than both the 540 kHz he had his radio tuned to and the 455 kHz IF frequency which I believe is used by AM radios in the USA.

So if cellphone traffic is indeed the source of the noise, his components are presumably responding to relatively low frequency modulations that are present on the near GHz carrier frequencies, which **might** also be picked up by an AM radio.

Regards,
-- Al

If you have a portable battery powered AM radio, try tuning it to an unused frequency toward the low end of the band (e.g. 540 kHz), and see if it picks up the noise.  That might help to determine if the noise is getting into your audio system via the air or via the power line.  And if it turns out that the radio picks up the noise, you might be able to determine the direction it is coming from by trying various orientations of the radio's antenna.

Good luck.  Regards,
-- Al