Here's one for ya Part 2


So,this am the cartridge plays fine.I do have hum but only when I turn the pre up.No hum with a signal.But what I have now is a horrible case of acoustic feedback.So I took the table down and put a heavy patio block on the wall TT shelf.This has always done the trick with other tables.But I still have acoustic feedback.Come to find out I only have acoustic feedback with the clamp on! I tried it loose,tight still feedback.I tried a rubber puck I have also but still feedback.
With no clamp no prob.I have the Delphi level and set up so when I push down on the platter it bounces 3 times then stops.I was told this was correct.
Any thoughts?
I would like to thank Tom for all his help in my return to vinyl.Thanks Tom (aka TWL)
david99

Showing 2 responses by sean

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
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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
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