Please Help Ambient Bass Resonance
Check the ground between your tt and preamp. The signal seems to be from the platter from what you are saying. If you want to test it to confirm take a piece of wire connect it to the same preamp ground and touch your platter while the cartridge is lowered but not while its spinning. If that takes care of it, call vpi and see what they suggest for the best grounding solution. If that's not the issue, make sure your cartridge is hooked up correctly, I know that seems silly but I've made that mistake before when swapping out a cart. Hope you get it worked out. Regards, Bill |
Hi Bruce, I don't know what the answer is, but some of the thoughts expressed in this thread may prove to be relevant. Best, -- Al |
What you are experiencing is indeed feedback into your cart. It is very common in LP playback. There are many ways to solve this. The key is to place the TT far enough from the standing wave or at a base null. Since you didn't post a picture of your setup, it is difficult to imagine where the problem might have been. I am going to list a few things for you to try out. If your TT is inside a cabinet, take it out. Cabinet will increase resonance. If your TT is placed at a corner of your room, move it out of the corner. Standing wave is stronger at the corner. If your TT is placed very low close to the floor, move it to a higher place. If your TT is placed in between two speakers, move it in and out a foot or two to see if the situation improves. With bass heavy music playing, walk around the room and listen at the hight where your TT can be placed and listen for bass null. If you find one, that will be the place to move your TT to. |
Thanks for all of the suggestions. I think the idea that had the most chance of working was moving the TT further away from the speakers. It didn't really make a difference. The best thing I've done so far is to use an extra rubber washer with the Delrin clamp. For those who own a VPI table, you'll know that the table comes with a single rubber washer which is put on first, then the record, then the clamp. I had an extra washer, so I put the extra washer on top of the record, directly underneath the clamp. That seemed to raise the feedback threshold well beyond the level I would ever listen to the music. Btw, I've used MM carties before, Can't say I recall having the problem. Also, I used to use the Soundsmith Zephyr, a MI cartridge, and the Dynavector, a MC cartridge. Again, I don't recall experiencing the problem. Just a thought -- I have a 3 gram headshell weight. I wonder if using it to increase the effective mass of the arm would make a difference?? Just thinking out loud here -- where the heck is the Delos picking up the ambient sound? Yes, there are motors throught the house and laundry room, but could that be the source? Or is it possible there is electrical inductance between the tone arm cables and maybe a power line that in some back is feeding a 60 Hz current back into the moving coil parts of the cartridge, thereby causing vibration to feedback through the system as Al described?? Al what do you think?? Thanks again. |
Hi Bruce, I doubt that ambient sounds or 60 Hz coupling have anything to do with the problem, although experimenting with the turntable ground, as was suggested above, would seem to be worthwhile. Perhaps even trying having no ground connection to the preamp. Obviously, have the system turned off while you are making any of those changes, and have the volume control turned down when you initially power up. Regarding ambient sounds, as I indicated in my first post in the thread I linked to earlier: A high gain feedback loop that is unstable at some frequency or frequencies does not require much if any input stimulus for oscillations to occur.... So what is probably happening is that some extremely tiny (and inevitable) low frequency vibration causes the cartridge to output a tiny signal, which is amplified by a high gain factor, resulting in an output from the nearby speaker that causes further vibration to be mechanically transmitted to the cartridge, which is further amplified by that high gain factor, etc., etc. The root cause of the problem is that the overall loop is unstable at some low frequency or frequencies, when the overall gain through the loop is above a certain amount.The only suggestion that occurs to me at this point, besides the grounding experiment, is to continue to experiment with turntable placement, and the related suggestions the others provided above. Best, -- Al |
Thanks Al, I tried the grounding idea already. No change. Not a lot of options about moving gear around. My wife finished the basement, carpet furniture, etc and there's simply not much I can do. So far, the double rubber washer trick seems to help a bit. At least the feedback oscillation doesn't kick in unless I turn the linestage gain up to ridiculous levels, which I never would use. There's no discernable feedback at "normal" listening levels. Curiously, the peripheral ring seems aggravate the problem. The delos is a nice sounding cartridge. As long as the problem doesn't get worse, I can live with it. Thanks for the help. |
What is your turntable sitting on.?.... The more solid the supporting table the better and cleaner the sound. VPI's....all of them ....need a solid foundation to the center of the Earth. Also, if you have feet on your Classic that is the traditional mini-feet, a remarkable, very worthwhile upgrade is to replace those feet with Bearpaws. These are very heavy solid brass cones that simply screw on your table after the mini feet are screwed off. I'll bet either of these and certainly both will solve your dilemma The solid supporting table should be tried first. |
String..., I think you're onto something. I just tried something that is a temporary patch that seems to have nearly eliminated the bass feedback. I have furniture pads that are used to stick onto the bottom of heavy furniture to prevent scratching hardwood floors. I doubled up on the pads and rested the mini-feet on a small part of the top pad. It seems to have nearly eliminated the problem. You are right about the table too. It's an IKEA kitchen table. Not very sturdy or heavy. I could feel the sound vibrations on the table top, which I bet traveled right through the TT mini feet to the rest of the chasis. Right now, I may have cobbled together an ineligant temporary solution, but I need a permanent fix. What about using a heavy piece of concrete slab that sits on styrofoam? Just sketching. Any other ideas? Isn't there something called sound bricks?? Thanks. |
Sound-Real-Audio, thanks for the thought, but I don't think my phono pre, linestage or amp clipped. If I didn't mute the phono stage, who knows what would have happened. But I muted the phono pre as soon as the oscillation became louder. I think String... is on to something, especially in light of my meatball fix. I've got to do a much better job of isolating that turntable from the IKEA kitchen table. The IKEA table is like an antenna that is transmitting vibrations into the TT via the mini feet. Thanks again, Bruce |
Bifwynne....your light Ikea table may be perfect for a Linn, however, as I said before...you have to have a solid support for your VPI. If you have a light budget, even get pavers, cinderblocks, etc. to support your VPI. Those isolation pads are not the audiophile answer. As I said before...a SOLID path to the center of the earth is what you are looking for. |
Glad the "pads" are helping out the situation. They are decoupling the TT from the resonances of the shelf that it is sitting on. Something I suggested you look into in an earlier post. It is the opposite approach of Stringreen which seeks to couple the shelf and table to "the center of the earth" which is highly effective but can be costly or impossible to do in some situations. If you can't switch to a much more solid base you can experiment a bit by adding some other dissimilar materials with the "pads" to possibly gain even better isolation from the shelf. You can try layering say a maple block on top of pads than another set of pads then the TT. Some TT makers build their plinths in a similar manner. |
