How to eliminate TT feedback/vibration

If possible, have the wall shelf attached to an outside wall or strucural inside wall as mentioned(behind the speaker plane). Try to set up the shelf so that it is not rigidly coupled to the wall, don't over tighten the screws and perhaps put rubber washers as spacers behind brackets at wall. Then decouple the table from the shelf surface by putting a seperate MDF board or buthcher block on it seperating it with cones,vibrapods,small spikes,rubber/plastic pads etc. Take the lid completely off while playing(antenna for airborn vibrations) also mentioned. Give the cartridge bolts a good tightening and make sure the tonearm bearing is not loose. If this don't help I would say you need a better table.

Me, I will build one (OP amps base) if I need one. There's a lot of circuits floating on the net somewhere. It is much cheaper to DIY.

If you do not want to deal with the hassle, just buy one (KABUSA carries them just like Bob already posted). Or for the best solution, get a new preamp that has one(i.e. Cambridge Audio 640P for a cheap alternative).

Of course, the cheapest way is to move the TT further away from main speakers, hoping that it will solve the problem.

It may be the turntable. I had an old PL-50 years ago and it had an aluminum platter that rung like a bell. If you could borrow a different turntable and set it up on the same shelf at least you could narrow down the problem.

If possible, try lowering the stylus on a disc without the platter turning, and then rapping on the wall, platter, plinth, platform etc with volume at realistic loud levels to get some insight into what is happening.

This may help you avoid less-promising approaches, and more quickly evaluate potential cures.

Keep in mind also that although a rumble filter will probably help, ideally you would eliminate the problem at it's source (assuming that's feasible), since your cartridge and phono stage are processing that rumble even when you filter it out.

I will try to get a pic of my set-up on line. I actually improved my problem by spiking my TT on a piece of MDF and put the MDF on some foam rubber. I can now increase my volume several db without any feedback and the spikes are responsible. What I need is some type of jell material I can place under the MDF board. Can anyone think of a soft jell material I can put under the MDF board? You know I had a friend that hung his TT from the ceiling and it worked great. I may try that too. Thinking as someone suggested part of the problem may be in the actual TT construction. I can peck on the TT at high volumes and hear it. As I mentioned, going to audition a Clearaudio CMB Friday. Guess I will see then. As far as filters go, I already have a rumble filter on my cheapo Balleri VP130. I kid you not, for the $$ that phono stage is well worth the bucks. Thinking about going to the YS Concerto Plus. I don't want to invest bookoo bucks at this time. Actually read a review where the YS won out over the Audio Accoustics PH3. Anyway-I'm making progress on my feedback problem. Will keep you all updated. Thanks for all the ideas.