How to eliminate FM RF coming thru turntable?

Quoting from a response I made a while back when someone else had a similar problem:

I believe that oxidized or dirty contacts can cause that, particularly as here where low level signals are involved that are subsequently subject to high gain. The poor contact results in diode rectification effects, which demodulate the radio signal.

Try using some contact cleaner on all of the connections that are involved, including the ones in the headshell that mate to the cartridge pins, as well as any other connections that are in the path to the phono amp. Or, as a minimum, just slide each of those connections off, and then back on -- that might rub off the oxidation sufficiently.

Also, I assume that you hear this pickup with your FM tuner turned off. If that's not the case post back.

Good luck,
-- Al

A corrosion-formed diode can indeed rectify an AM station's signal so as to produce recognizable audio. In my experience what you usually get is a mixture of several stations, some louder than others.

However, it takes a lot more than a diode to pick up recognizable audio from a FM signal.

Hmm, yes, you have a good point there.

But still, if we understand the facts correctly (recognizable audio from fm, being introduced through the phono input, and present (I assume) when the fm tuner is turned off), SOMETHING in the phono path is somehow demodulating the signal. Not sure what that could be.

Regards,
-- Al

Are you really sure that it is an FM station you are hearing? There are two things that seem to point to AM -- the fact that it is being demodulated into understandable audio, as discussed above; and the fact that the problem occurs only at night.

My suspicion would be that the day/night difference would be caused by differing signal strengths before/after sunset. As you probably realize, AM stations typically propagate much further at night than during the day. With FM, I believe that would only be true to a slight extent if at all.

If the other suggestions don't help, you might try this: There are kits that are available (I used to have one) that provide small value capacitors on rca plugs, for the purpose of optimizing the capacitive loading that is applied to the cartridge. Or you can solder up something like that yourself. If you were to add say 50 or 100pf of capacitance (connected at the preamp input with a y-connector), without going outside the range of total capacitance that the cartridge is specified for, the extra capacitance might load down the rf pickup sufficiently to kill it.

Also, you could try to find the kind of ferrite-based rfi filter that has a donut-hole in the center, through which the phono cable would be routed. That might do the trick as well. I think some of them are specifically marketed for audio applications.

Regards,
-- Al

Here's an excellent paper, dealing with rfi problems caused by ham radio systems, but providing a lot of information that may be applicable to your situation as well:

http://www.audiosystemsgroup.com/RFI-Ham.pdf

Here's a link to the catalog of Fair-Rite, which makes a lot of these things:

http://www.fair-rite.com/newfair/index.htm

Radio Shack (you'll pardon the expression :)) sells some snap-together ferrite choke cores, part numbers 273-104 and 273-105 according to an older catalog I have.

Hope that helps,
-- Al

How about the other suggestion I had made, adding a small amount of capacitance via a y-connector at the preamp inputs (the capacitors being soldered to rca plugs that would plug into the y-connectors)? As I indicated, the only caveat would be to make sure you are staying within the range of total load capacitance recommended by the cartridge manufacturer. If you have a moving coil cartridge, load capacitance is most likely very non-critical.

Regards,
-- Al

Can you tell me exactly what model Grado it is, so I can look up its recommended range of load capacitance. Although many of the Grado's are fairly insensitive to load capacitance, which may be helpful. Also, how long is your phono cable (which adds significant capacitance)? I'll then try to pick out a suitable capacitor.

Your body has capacitance, and that is why touching the preamp affects the RF. When you say you connected it to a separate ground, if you just used a plain wire to do that, the wire's inductance may have made it ineffective at rf frequencies. What you should probably try is a braided ground strap instead, which makes for a much more effective connection at rf frequencies. Or as a temporary experimental arrangement, if possible, try making the ground connection with the shield of a coax cable.

I should have a little time tomorrow to research these parts further.

Regards,
-- Al

I have a 4mv Reference Sonata, which simply says "insensitive to load capacitance," and I suspect the Reference is similar in that respect. Also, your phono cable is very short. Both of these factors suggest that you can feel free to add any reasonable amount of capacitance, perhaps up to a few hundred picofarads (pf), without affecting the sound at all.

But it may not be necessary to do that, if a good ground strap would have a similar effect to what you found when you touched the phono preamp. It looks like the best way of getting that, without having to purchase a large quantity from an industrial distributor, is via e-Bay. You can search there under "ground braid." Here is one good example:

http://cgi.ebay.com/20-Tinned-Copper-Braid-3-4-Flat-Wire-85A-Max-Ground_W0QQitemZ370134600553QQihZ024QQcategoryZ26213QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

If that doesn't do the trick, and you want to try adding capacitance, it looks like no one makes anymore the kind of kit I referred to, that I had many years ago, containing a set of capacitors installed on rca plugs. So you'd have to make your own.

Here is one example of a y-cable that you would need; there are many others, as you no doubt realize:

http://www.bhphotovideo.com/c/product/488449-REG/Phoenix_Gold_ARX_569_Silver_500_Series_RCA.html

The most convenient and inexpensive way of getting the rca plugs and capacitors would be, if you can stomach doing so, from Radio Shack. Their part number RSU 11537479 is a set of four gold-plated rca plugs. Part numbers 272-123 (100pf), 272-124 (220pf), and 272-125 (470pf) would be reasonable capacitor choices. Start with the 100pf, and if that doesn't do it go up from there.

You can also find suitable capacitors, with similar values, here:

http://www.tubesandmore.com/

Or if you want to wade through a vast selection of potentially higher quality parts, from an industrial distributor, go to http://www.digikey.com

You would have to be prepared to solder the capacitors to the rca plugs, of course.

But it might be simplest to start by trying the ground strap approach (using it either in place of or in parallel with the ground wires you've tried -- it shouldn't matter which), which could very conceivably solve the problem.

Regards,
-- Al

Glad to have helped, and thanks for letting us know. This seems to be a not uncommon problem, and your info will be helpful to others.

Regards,
-- Al