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Phono interconnects acting as antenna, adding noise to system

Have interesting problem with cable related noise. I have a coincident phono amplifier and biamped coincident speakers with dedicated 20 A circuits from a separate panel share a common star ground (independent from rest of house).  

There is not ground hum, but when phono to amplifier interconnects connected to amplifiers a course EMF like noise occurs.  The cables seem to be acting as antennas since the noise does not change if disconnected from phono amp but stops if disconnected from power amplifier.

Interestingly, the interconnects are 12feet long running in galvanized conduits under floor to the phono amplifier

The noise is louder if i run in air outside of the conduits.  When in conduits only 3 feet at either end is exposed.

does anyone have any suggestions?  read that there are braided copper sleeves could try with exposed part?  use tinned copper tape?   or is there something else going on?
128x128jricci
reubent
7,004 posts
 
jricci,

Does it only make the noise when you have the phono input selected, or is it when any input is selected? I ask because I just remembered that my system has a nasty noise when the phono is selected and I turn on the TV. Not sure what is causing it, but I just turn off the TV and it goes away. I don't know if the cable or the cartridge are picking up the interference. Regardless, turning off the TV resolves the issue. No other inputs are affected, just phono.
erik_squires
16,119 posts
 
@reubent Check out my blog post on ground loops. :) Cable TV often causes that.

http://pqltd.blogspot.com/2016/11/quick-safe-and-easy-fixes-for-ground.html

Jricci,

Let’s assume it really is EMI/RFI that you are hearing, and not a ground loop issue.

With RCA lines running that long it could be an issue and may not be solvable with just cables.

Make sure your cable is 2 conductors + shield. Connect the shield and - side at the preamp side only. This may not fix the problem, in which case your choices include:

  • Converting the signal to balanced/XLR outputs and run that XLR to amps that accept XLR. (this can also be done with matching transformers at both ends)
  • Moving the preamp and amplifiers so they are much closer together.
Also, before you do any of this, make 100% sure you don't have significant noise pick up in the preamp itself. I've seen cases where this occured as a result of being too close to a transmitting station.

Best,

Erik
erik_squires
16,119 posts
 
I just noticed that the Coincident statement line stage does in fact include balanced outputs built in.

One last thing, running audio cables in metal conduits can roll off the highs due to capacitive coupling.
jricci's avatar
jricci OP
10 posts
 
The noise occurs if interconnect connected to amplifier

Doesn't matter if
-connected to phono amp
-phono amp on or off
-phonoamp volume up or down
-phono or bypass input selected


erik_squires
16,119 posts
 
Jricci,

But it doesn't happen if you use a short cable?

Erik
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