I've just gotten finished, (or so I thought) upgrading and servicing all th e equipment in my entertainment center. I just replaced my mediocre speaker s with a pair of nice 12 inch JBL's that were abandoned in the building we just bought. My turntable is an old Thorens belt drive, supported with a spring arrangement that I thought would absorb any vibration. The turntable is in a cabinet about a foot off the floor and right next to the left chan nel speaker. If I crank up the volume when listening to a record there is d istortion, the severity directly proportional to the increase in volume. At first I wasn't certain about this but you can actually "feel" the vibratio ns on the turntable base. There are these springs supporting the platter an d arm and these springs are "stuffed" with a foam material but this vibrati on is coming through anyway. I know that one solution is to relocate the sp eaker however the room is not large enough to do this. I was thinking of pu tting foam under the speaker which would be easy and perhaps even a soft fo am block under the turntable as well but I'm afraid that trying to support and level a turntable supported like this could be a nightmare. Does anyone have any ideas? Thanks, Lenny
On Wednesday, December 18, 2013 8:10:39 PM UTC-5, snipped-for-privacy@gmail.com wr ote:
the equipment in my entertainment center. I just replaced my mediocre speak ers with a pair of nice 12 inch JBL's that were abandoned in the building w e just bought. My turntable is an old Thorens belt drive, supported with a spring arrangement that I thought would absorb any vibration. The turntab le is in a cabinet about a foot off the floor and right next to the left ch annel speaker. If I crank up the volume when listening to a record there is distortion, the severity directly proportional to the increase in volume. At first I wasn't certain about this but you can actually "feel" the vibrat ions on the turntable base. There are these springs supporting the platter and arm and these springs are "stuffed" with a foam material but this vibra tion is coming through anyway. I know that one solution is to relocate the speaker however the room is not large enough to do this. I was thinking of putting foam under the speaker which would be easy and perhaps even a soft foam block under the turntable as well but I'm afraid that trying to suppor t and level a turntable supported like this could be a nightmare. Does anyo ne have any ideas? Thanks, Lenny
I'll try removing the spring foam and see what happens. Thanks, Lenny
One approach I've seen used is to add both mass, and decoupling, to the turntable.
Mass: go buy a sheet of rock. Stone. Cement. Something like that. If you want to be elegant, go to a stonecutter, pick out a piece of marble or limestone or granite whose color appeals to you, and have a slice cut and polished which is just the size of your turntable base. If you're on a budget and don't care terribly about looks, get thee hence to a home-improvement/garden store and buy a concrete or brick "paver" roughly the size of your turntable base (18"x18" is a good size to look for). All else being equal, thicker is better than thinner.
Decoupling: go to a bicycle store, and buy a small-diameter heavy-duty bicycle inner tube. If you bought an 18"x18" paver or marble slab, a
16" inner tube would be a good size to try. You want it to be just a bit smaller in diameter than the smaller side of the stone slab, so that the slab will sit on it in a stable position without the tube "bulging out" in any direction.
Partially inflate the inner tube. Lift your turntable out of position (unplug first, of course). Put the partially-inflated inner tube in its place. Put the stone slab on top of the inner tube, and put the turntable on the stone. If it doesn't sit entirely level, lift the turntable and slab and try moving an inch or so in each direction to get it to level out.
If the weight of the slab makes the cabinet collapse and dump your turntable onto the floor... oops :-( Sorry about that.
This approach will greatly reduce the amount of structure-borne vibration which gets up to your turntable through the cabinet or shelf.
It won't do anything much to reduce airborne vibrations (acoustic pickup). Removing the dust cover entirely when you're playing music can help to some extent.
A somewhat similar approach involves building a heavy platform for the turntable to sit on, and suspending it from above using rather long springs.
In either case, you want the resonant frequency of the mass/spring system (whether it's a metal spring, or the "air spring" of an inner tube) to fall below the lowest bass frequency from the speakers. This creates a "low pass" filter which will tend to attenuate the bass before it feeds back into the turntable system. It's also good if it falls below the resonant frequency of the arm/cartridge system, so that the two resonances don't reinforce one another and cause excessive stylus "wobble". If you can get it to the point where it "bounces" 2-3 times per second and no faster, it should help stabilize the turntable's playback quite a bit.
You're probably not going to like this, but the easiest -- and maybe the best -- solution is to digitize the records and play the resulting files. No acoustical feedback. No (more) degradation from the stylus damaging the groove, either.
Years ago,when I still used vinyl records, I became concerned about the exact same phenomenon, but when it was at a level where it couldn't be
*overtly* heard, but still was causing a modification of the desired output signal. I figured that it just had to be there and I wanted to measure it.
I came up with a scheme for placing a second cartridge on the record that rotated with it, and taking the output of that as the signal to be measured. Not only would it detect the acoustical feedback from the speakers, but also the direct vibrations of the active stylus in the groove, which has to be creating stuff that bounces off the edges of the disc and back to the stylus as distortion.
Then CDs came along, and I totally lost interest in the whole project.
Reverse the phase of ALL the speakers, then they wind in phase with each other but in the oposite phase regarding the phono cartridge.
I've done this a whole bunch of times and sometimes it does have a quite significant effect. The problem is that it's a 50/50 chance that it could get worse. In that case reverse them all back.
Concievably you could accomplish the same thing buy reversing both channels of the cartridge itself, but it is not designed for that. Some of them automatically ground the shell to one of the - terinals. Even if not it still kinda beelongs one way.
Ergo, you reverse the phase of the speakers. All of them. Unnastan now kid ?
I'm reminded of the old joke where a patient tells his doctor that his arm hurts when he holds it vertically. "Then don't do that!"
I remember when H H Scott made console phonographs. They went through quite some trouble preventing feedback when the turntable was in the same cabinet as the speakers. And I doubt their speakers went as low as the JBLs.
Some of the suggestions are good (such putting a huge chunk o' granite under the 'table), but you really need to move the turntable away from the speakers -- and the walls.
wrote in message news: snipped-for-privacy@googlegroups.com...
I've just gotten finished, (or so I thought) upgrading and servicing all the equipment in my entertainment center. I just replaced my mediocre speakers with a pair of nice 12 inch JBL's that were abandoned in the building we just bought. My turntable is an old Thorens belt drive, supported with a spring arrangement that I thought would absorb any vibration. The turntable is in a cabinet about a foot off the floor and right next to the left channel speaker. If I crank up the volume when listening to a record there is distortion, the severity directly proportional to the increase in volume. At first I wasn't certain about this but you can actually "feel" the vibrations on the turntable base. There are these springs supporting the platter and arm and these springs are "stuffed" with a foam material but this vibration is coming through anyway. I know that one solution is to relocate the speaker however the room is not large enough to do this. I was thinking of putting foam under the speaker which would be easy and perhaps even a soft foam block under the turntable as well but I'm afraid that trying to support and level a turntable supported like this could be a nightmare. Does anyone have any ideas? Thanks, Lenny
It's been common practice for decades now in nightclubs, to sit the Technics
1210 turntables on a heavy slab of whatever material is available, to reduce such feedback from large PA systems in close proximity.
I think Jurb's advice is well worth trying. You might find the turntable is currently in one of the room's many resonant bass peaks, and reversing overall phase might put it in a trough. Or you could move the turntable and speakers as much as is possible to try and get them out of this peak, perhaps reversing phase as well to see what works best.
As Jurb implies, it is always possible you are in a trough already, and anything you try could make things worse, but that would be the absolute worst case scenario.
** If low frequency sound waves were vibrating the LP direct - there would be no fix other than removing the TT from the room. So this is not the usual case.
Turntable feedback IS a result of the room's *structure* vibrating in sympathy with standing waves at particular wavelengths - mainly the floor if the TT is supported by that OR the walls if a shelf is being used. The frequencies of such standing waves depend on the room's dimensions and the speed of sound.
Any TT must be isolated from these vibrations and the oldest and by far most effective way is by use of coil spring.
IME, the ideal set up is 3 conical springs and enough weight above them to result in vertical oscillation at about 2 Hz. When displaced, the TT system should move freely and oscillate up and down for a couple of seconds - so any connecting cables must be looped and able to follow the movement easily. No damping material should be used.
If done correctly, room surface vibrations no longer affect the TT - even jumping on the floor - so the chance of actual feedback is eliminated.
On Wednesday, December 18, 2013 8:10:39 PM UTC-5, snipped-for-privacy@gmail.com wr ote:
the equipment in my entertainment center. I just replaced my mediocre speak ers with a pair of nice 12 inch JBL's that were abandoned in the building w e just bought. My turntable is an old Thorens belt drive, supported with a spring arrangement that I thought would absorb any vibration. The turntab le is in a cabinet about a foot off the floor and right next to the left ch annel speaker. If I crank up the volume when listening to a record there is distortion, the severity directly proportional to the increase in volume. At first I wasn't certain about this but you can actually "feel" the vibrat ions on the turntable base. There are these springs supporting the platter and arm and these springs are "stuffed" with a foam material but this vibra tion is coming through anyway. I know that one solution is to relocate the speaker however the room is not large enough to do this. I was thinking of putting foam under the speaker which would be easy and perhaps even a soft foam block under the turntable as well but I'm afraid that trying to suppor t and level a turntable supported like this could be a nightmare. Does anyo ne have any ideas? Thanks, Lenny
I have a question for Phil. As I had mentioned earlier there are soft foam rubber inserts inside the three coil suspension springs on my turntable. Yo u had mentioned to remove these. I am going to try that but haven't had a c hance to try this yet, but why did they put that foam in there in the first place? Lenny
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