Dave, try relocating your TT onto a stand. This may change the nature of the vibrations that are reaching your platter. It is also possible that the TT is in a place in the room which has alot of bass excitation. Having the TT on a stand would allow some movement to experiment with the placement, and perhaps cure or reduce the problem. You might try just moving your speaker(s) temporarily to different areas of the room, and see if this cures it. If it does, then the feedback is related to the location of the TT with respect to the speaker location. Then you could move the equipment rack and TT to a spot that does not seem to cause the problem.
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Tom-here's my room/setup
At the long end of my room is a area that extends 3 feet longer than the room and is 6 'wide with same ceiling height of 7.5' There are treated windows.
The speakers are 4 feet from each side of the extention and 4 feet from the front wall.
The rack is in the extention as is all my dedicated outlets.The TT wall stand is 4.5 feet up over the stand.The steel support is bolted through my house wall and the nuts are on the outside siding.Every TT I've had I had acoustic feedback but was able to eliminate it with added weight on the TT shelf.3 TT's were non suspention and this is the 2nd suspention TT I've had.
I cant move my rack/stand d/t room setup.
I may need better advice how to set up the Delphi.
I have a ground from the interface box on the back of the delphi to the Creek.I added another ground tonight from the box to the TT steel support and it lowered the hum ~25% more.
Could the 65 gain from the Creek cause the hum?
I have a cheap linestage (tubes) could some better tubes (NOS) lower the hum?Its dead quiet with CD/tuner.
Without the clamp the acoustic feedback is really a non issue.I have to crank it louder than I would normally to get feedback.I would like to figure this out though so I can use the clamp.
Any other tricks to lower the hum with better grounding? Cheater plug maybe?
BTW Tom,The Denon is a special p.u. It reminds me of a Ortofon MC-20 super I once had,only smoother and more transparent.
Dave, the feedback issue is based upon whatever frequency vibrations are getting through the shelf into the TT. You might try using various weights on the shelf, and see if you can find a particular weight that brings it into a zone the TT suspension can handle. I think the reason that using the clamp makes it worse, is that the record is somewhat de-coupled when not using the clamp, so it doesn't get affected as much. From what you describe, the vibrations at a certain level and frequency are defeating the suspension and getting right up to the platter.
I think that there will always be somewhat more noise with a high-gain phono stage, but it shouldn't be a "hum". I don't think your Creek is the problem. But even a small grounding issue in the TT area will be fed through the high gain phono section and sound alot bigger than it might be otherwise.
Your room extension seems to be a "funnel" for reflected waves from the room, and focusing them on the equipment. This could be a large part of the problem. Try some standing diffusors or maybe even a bass trap in front of the TT/equipment. Even a couple of high back chairs in there to see if you can break up the wave-fronts, so you can see if there is any improvement.
Just a few guesses to try.
David, your "basic" room measurements that you provided ( 15 x 20 x 7.5 ) and speaker placement are an acoustician's nightmare. 15 is evenly divided by the 7.5 foot measurement and the 15 and 20 foot lengths share a common denominator of 5. This means that you'll have multiple nodes that tend to reinforce each other, so i would expect BIG peaks and dips in this room.
On top of that, you've got your speakers set up so that their is equal reinforcement from the boundaries at appr the same nodes. This is due to having four feet between the speakers and the front wall and 4 feet from the speakers and the side walls. For better results, you really do need to stagger those measurements.
As far as your wall mounted TT shelf goes, is the wall that you have it mounted to a load bearing wall or simply a "room divider" ? Walls that are not load bearing tend to flex and resonate quite a bit at low frequencies, which is exactly what you don't want with a wall mounted TT shelf. Since the mass of the wall is quite high ( compared to that of air ), it's quite possible that the resonance of the wall is tuned to the same appr frequency of the TT's suspension. The fact that you can remove the record clamp and not suffer acoustic feedback basically supports that point of view.
With the clamp or similarly weighted puck on the platter, you have a specific node that the TT's suspension is tuned to ( the suspension's resonance ). It does not matter if the record is clamped or not as the suspension is only looking at the mass that it sees. When you remove the clamp or "puck", you change the mass that is being suspended and the suspension is no longer "tuned" to the same appr resonance of the support wall. Hence, no acoustic feedback or it is at least reduced to the point of not being overbearing.
While you were able to add mass to the support shelf in the past, which changed the mass / resonance of the wall enough to get things to work, i suspect that the installation has always suffered from this problem and you've simply been "band-aiding" it without knowing it.
Personally, i think that your installation is a prime candidate for professional help OR a lot of hands-on studying / research by yourself. Between the room acoustics and placement of gear, you really are in a tough spot. You might get things to "work" to the point of not having noticeable feedback, but my guess is that in-room response is not as linear as you would like. Taking some in-room measurements may be an eye opening experience. While moving the speakers around and re-locating the TT shelf would help, you will still be left with a room full of reinforced nodes. As such, a good place to start can be found right here. I can see a LOT of room treatment devices in your future if you would like to get things rolling in the right direction. Good luck and hope this helps... Sean
Sean-Thank you for the information.
The wall the TT shelf is bolted through is load bearing.
The speakers are located 4 feet from the front wall and 2.5 feet from the side walls.
I mentioned the speakers are located 4 feet on each side of the room extention.Not the side walls.
I forgot to mention:
I have a closed stairway on the side of the room that is ~ 3 feet wide by 6 feet long.So ~2/3 of the room is 15' wide and 1/3 is 12' wide.There is a door on the stairs.The right speaker faces the door of the stairs ~ 8 feet away.I have very good imaging and can notice no ill effects from this.
I re-measured the lenght of the room and it is 19 feet long,not 20.
I have bass traps and first reflection point treatment along with corner tunes.
I should have been more precise,Im sorry.
I look forward to you giving me feedback (pun intended) on this information.
Thank you Sean.
David, sorry for the misinterpretation but i was basing my response on your first two posts that you provided here. While it seems as if you're doing the best you can with what you've got and have the basics covered in terms of room treatments, my assesment of the TT feedback / suspension resonance still seems to apply. As such, is it possible to "re-tune" the Oracle suspension in any way ? I am not familiar with this table, so i don't know exactly how it works. I have my doubts about that approach though as i doubt that you could really shift it enough to make a worthwhile difference. Sean