You're hearing RF interference. The wires in your phono system are acting like antennas. Get Audioquest RF Stoppers or RF Stoppers Jr's via Audio Advisor, www.audioadvisor.com, or similar products at Radio Shack.
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The phono stage contains very high gain with high impedance which makes is very susceptible to RF pick up. You might want to try shielded cables between the turntable and headamp or preamp. You might also try twisting the interconnect cable and repositioning the cable to see if it will pick up less rf. Ferrite clamps on the cable might help if the noise is FM or TV.
Before you spend money on any cure, see if you can try it first. I had a serious problem with RF noise and the Audioquest RF Stoppers were a total waste. Most of the other suggestions seemed to help but the problem, as well as the cure was transient.
You may wish to also lift the ground, or, if you have cable tv, see if you have a circuit which does not share a ground with cable. Unfortunately, this option was not available to me.
What finally worked for me was a Linn Linto phono stage preamp. I tried about 4 other phono stages ranging from $100 to $1000 and only the Linto completely eliminated the noise. This may have to do with the unique electronics in the Linto.
The interference you are hearing is caused by a strong RF (radio frequency) signal that is finding its way into your phono stage, cartridge or cabling. Start by unplugging everything. Disconnect the phono stage from the preamp, the phono cable from the turntable and from the phono stage, and then turn on the just the preamp and amp. Listen for the offending interference on the input you use for phono. Next turn down the volume and hook up the interconnect from the phono stage. Turn up the volume and listen again. Then connect the phono stage to the interconnect. Listen again. Keep adding pieces until you find the one that is allowing the interference to get in. Once you find your "problem-child" component you can try three things:
Shield, re-orient, or replace.
Shielding means that you apply copper foil, mu-metal, or sheets of CRS (cold rolled steel) around the component to keep out the RF. Cables and cartridges can act like antennas and sometimes if you simply move them to another orientation or location, that will greatly reduce the RF problem. In the event that the problem is a cable, try a longer or shorter one. If all this fails, you can throw new components at the problem until it goes away.
The really frustrating aspect of RFI is that with a quality phono stage and interconnects I have the problem. But when I use the phono section of a cheap Onkyo preamp there is no problem! Of course, the sound sucks, so I'm still experimenting, trying to find a phono stage that doesn't do this or a way to position it to eliminate the noise.
Always try cheap fixes first. If all else fails, I suppose a Linn Linto would be nice! As nice a phono stage as it is, that's a rather pricey fix!
Here's something cheap and easy to try, if you are using a power strip:
1. Buy a hospital grade power strip (e.g., Wiremold)
2. Plug in components in this order, begining with the sockets closest to the power cord: power amp, preamp, CD player, other (e.g., phono stage, turntable).
I have the same problem with RF noise between a Kora Eclipse pre amp and my Music Hall MMF7 turntable. I found that shielding the interconnects work the best, but still does not completely remove RFI.
After getting fed up, I bought an ARC PH3 phono preamp. Connecting it to the line input on the Kora Eclipse, the TT is dead quiet now.
It's close to impossible to eliminate because you are using tubes. A solid-state unit would be quiet--and wouldn't sound as good.
Use the mute button on your preamp and try to reorient the phono stage to get the hum to an acceptable level. Move around cables--always with mute on preamp. Otherwise, you can get a big pop that will shut down your amp.
The first thing to check is the ground. Make sure you are properly grounded.
Second is the connection.
I agree with other posters that location and connection is first step. I have cured my RFI by tightening connection by just jamming the cables in harder into the integrated amp I'm using....then securing the ground (it had popped out during the process and I couldn't figure out why the tightening was no longer working as it had before).
To me it comes down to flawed design of certain TTs. My stuff is vintage. They probably learned to fix over time.