[sstalwar]

I am having hard time diagnosing a slight hum in right channel speaker that goes up with volume and disappears at the final peak volume level. Hum stays even if I change the input on the preamp. I have tried changing cables and cheat plug but hum stays. I have a feeling that this noise is picked by the phono DIN cable as it runs in close proximity to either turntable motor or amplifier transformer on equipment rack. moving the cable does make some difference, so may be I should get better shielded phono cable. I don't have much room to place the equipment differently on rack. But, before getting a new cable I wonder if I should measure the EMI or identify the source. May be it is induced by the amp transformer or may be it is getting in power source (wall outlet) and carried over. How can you measure noise in electrical line (EMI/RF)? Lastly, I find power conditioners degrade sound in my system. Can you guys recommend a simple power strip that kills EMI but don't do any other conditioning? Any other ideas? Help is appreciated. Thanks.

[SpatialKing]

The least expensive EMI test unit available is the basic battery powered AM radio.   Tune it to a non channel, so  you hear just background static and move it around the cables and places you think are emitting EMI.   It will pick it up if it is there.    The channel to which you tune doesn't matter.    Once you find the source, you can figure out how to eliminate it or mitigate it.    The reason the hum drops at full volume is the impedance of the volume control is minimum there also along with the lowest volume setting.  The impedance is max'd at center of volume rotation.  If the hum is present when you change inputs on the preamp, then you the noise pickup is probably not the phono input, unless it is much worse on phono?   BTW, it has to be battery powered and it has to be on the AM band, not the FM band.

[sstalwar]

Wonderful. I will try this. Thank you for helping.

[HAL]

First you might try isolating the phono cable as a source of the noise.  Just disconnect the cable and the turntable ground from the preamp.  If the hum goes away, you need more shielding in the phono cable.  If it still hums, something else is the source of the hum.

[Speedskater]

EMI will sound more like 'white noise' or hash or spikes than hum.

[sstalwar]

Thanks for responses. Is the buzzing sound from tweeters from EMI?  (not HUM but zzzzz sound; just like some experience zzzz sound with light dimmers) I thought that was tube rush. In my case dimmer is not there.  I hear this annoying buzzing sound when system is idle. I have fixed the hum (magically ?) by moving my phono pre at an angle. I am sure it is due to interconnects/power cord interaction and turning phono pre moved the cables in some way and hum is gone. But buzzing issue remains.

[Speedskater]

Thanks for responses. Is the buzzing sound from tweeters from EMI?  (not HUM but zzzzz sound; just like some experience zzzz sound with light dimmers)...... But buzzing issue remains.

If it's coming from the tweeters, it's probably interference/RFI (EMI).
Something near-by is causing it.  Is it 24/7 ?
Unplug or flip circuit breakers and see if it stops.

[sstalwar]

Yes, it is from tweeters. System is tube based so is not 24/7 on. It appears when I turn on the system. It is there most days but I would say 2 out of 10 days are lucky and the buzzing is not there (mysteriously). You mean flip the breaker of and turn back on? I shall try this. thanks.

[HAL]

Buzz or intermittent noise as described can also be from the power supply of a component.

Usually caused by power supply caps that are starting to have the electrolyte dry out.  Might be a good time for a checkup.

[Speedskater]

You mean flip the breaker of and turn back on? I shall try this. thanks.
No I meant flip other breakers off one at time and see if the buzz stops.
Almost any modern device or unit that runs on electricity can generate interference. With some devices switching them off doesn't stop the interference. 
Some problem children: appliances, HVAC systems, solar power systems, other entertainment units, Set Top Boxes, computers and wi-fi, high tech lighting, wall wort's.

[Arno P]

[Arno P]

Try one of these?

[Big Red Machine]

Unshielded cables rubbing against other cables typically cause me this type of noise. Then I kick myself for buying cables with zippo for shielding.

[sstalwar]

Thanks for all the responses. The problem lied with cables rubbing (poor shielding I assume). I do think that  HVAC system may have a role to play as the mechanical room with air handlers are in adjoining space. I re-did the cables and it is quiet on most days now. Thanks guys.