Periodic noise

Thank you kindly, Ptss!

If anyone is curious as to what led me to suggest the security system possibility, I have no particular knowledge of such things, and I don't have one in my house.  What I did was to first Google one or two terms I thought might provide some clues, which turned out to not be helpful.  I then Googled "every 8 minutes" (with the quotes) together with the word "signal."  That led right to the security system device I linked to in my first post above, that is described as transmitting a signal every 8 minutes.

Regards,
-- Al
 

Good question, dbtom2, but I guess the answer is "no" since Mr. H stated in the OP that the issue occurred with multiple phono stages, and only when a phono stage was connected and selected.

Regards,
-- Al
 

P.S: FWIW, I note the following statement in the datasheet for the security system device I linked to earlier, which like your device transmits every 8 minutes. This is in addition to a separate statement in the datasheet indicating compliance with FCC Part 15 requirements for a Class B digital device, which are intended to "provide reasonable protection against harmful interference in a residential installation."

Radio frequency radiation exposure information according 2.1091 / 2.1093 / OET bulletin 65
This equipment complies with FCC radiation exposure limits set forth for an uncontrolled environment.
This equipment should be installed and operated with minimum distance of 20 cm between the radiator and your body.
This transmitter must not be co-located or operating in conjunction with any other antenna or transmitter.

20 cm is about 8 inches.

Regards,
-- Al

Now it is close to our bedrooms. And I wonder, how healthy is this, with such strong bursts?

As can be seen in some past threads here, it is not uncommon for people to hear interference from radio stations when listening to phono sources.  A factor in that, obviously, is the very high gains the signals at phono stage inputs are subjected to.  In this case the issue arose only when the security device was in the same room as the phono stage, and only when that source was selected.

All of which would seem to suggest that the signal strength involved is very weak.  And I suspect that it has to be, in order to meet FCC Class B certification requirements that are intended to protect against interference with radio and TV reception beyond distances of about 10 meters.  And any potential for effects on health would presumably be further weakened in proportion to the low "duty cycle" (ten or fifteen seconds out of every 8 minutes).  So personally I wouldn't be concerned.

It might be worthwhile, though, to try to find technical specs for the particular device via Google, or in the manual if you were given one.  If an indication is provided of how much power it transmits, and at what frequency, it might be interesting to see how that compares with typical wifi power levels and frequencies.  My understanding is that laptops may transmit a few tens of milliwatts in the 2.4 GHZ band, at a distance that is of course much closer to the user than the security device is likely to be.  Also, I have a 2.4 GHZ wireless access point in my house which I understand to be considerably more powerful than most, and it can transmit up to a little under a watt when necessary. 

Regards,
-- Al
 
P.S:  Cymbop, thank you kindly.  However, I'd much rather listen to my stereo system than have to deal with all the problems the President has to face :-)

Could be a nearby security system device, such as this one, which is described as transmitting various information every 8 minutes.

Good luck.  Regards,
-- Al