I think the use of stone top shelves has been discussed, try a search of the archives.
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I can post pages on that experience but the short answer is putting that much mass even slightly above the floor invites oscillation.
Think of an upside down pendulum. It's especially bad for turntables, less so for preamps but it has a sound that once identified is annoying.
Granite surface plates are a good idea that never quite worked out, at least for me. I don't know where you live but I still have one or more here that you may have for free.
Surface plates when purchased from machine equipment suppliers are inexpensive. As other have noted, putting a heavy surface plate on the top shelf could cause a pendulum effect. You do have an option of counter balancing the pendulum effect by placing additional surface plate on the lower shelves.
How is your floor suspended?
If you look at the range of hardnesses in turntable materials, granite is probabably harder and denser than any of them. In other words, its mechanical impedance (resistance to being set in motion by external vibration) is high. This is not what you want for a turntable base, as it will take the vibrations transfered from the turntable and bounce them back into the turntable to be picked up by the cartridge, effectively raising the noise floor of the extracted signal. A turntable base should absorb and dissipate the turntable vibrations.
I've gotten much better results from a butcher block style maple cutting board. Mine is 3.5" thick and audibly lowered the noise floor when I placed it under my turntable. You can spend a lot of money on a butcher block cutting board or save a bunch by getting it from Overstock.com. It will likely be much less than a finished solid maple board, and the butcher block construction breaks up resonances.
MIne is just like this one.
I did exactly what you propose. 3" granite surface plate on a machine stand from a company called Production Tool. Works fantastic with my VPI HW-19. Grizzly sells them nationwide. You don't have much to lose.
Try this and the next page.
Surprise some of u don't like granite platform . As I understd it HRS used granite for their platform as well except they added the " rubberised footers " . So the question is not whether granite is good . It's the implementation of the platform . Likewise there are a lot of opinions about merit of acrylic & glass platform & some manufacturers do used them eg Grand Prix Monaco , Mana rack etc
Audio blazer, don't forget that HRS uses a 0.75" granite platform set in a aluminum shell and sitting on a polymer bed and supported by load specific feet. The surface plate being discussed is 4 times the mass of the HRS.
Granite is useful for isolation, but IMHO, granite needs to be damped to prevent ringing etc.
I would suggest that you call Adona. I am sure that they would sell you one of their granite shelves. They are the best of both worlds. They bond granite to a layer of MDF with a special (proprietary?) polymer. Anyway, it eliminates problems with ringing etc. Visit the Adona website. I have been using one of their racks for abourt 4 years and like it very much.
A simple way to dampen ringing is to apply a constrained layer damper onto the horizontal support surface/shelf.
The one I found to give excellent audible improvement is from Machina Dynamica: VibraBlock Constrained Layer Damper.
Responding to the mention above where brass compression springs have been used,
The same Machina Dynamica can supply "cryogenically treated high-performance hardened steel compression springs"
cleverly named "Cryo Baby Promethean Mini Isolators."
These may be placed against the rigid flat bottom of a component to support the component which is resting upon a horizontal surface. Whether this horizontal surface is granite or wood may be a matter of what sounds best; I prefer wood.
Next place the VibraBlock constrained layer damper onto your chosen shelf.
I prefer what I hear with this damper placed onto a supporting shelf for a component, rather then placing a damper directly on top of a component.
I haven't tried the ball peen hammer test. I plan to use a thick maple butcher block next to the granite. The table will rest on three Stillpoint cones pointy side up.
The table, an original VPI Aries Extended,seems to really benefit from as much isolation and as solid surface as possible. Seems like every small change is audible.
I've been using 2" granite, layered with 3/4" Corian, separated by 1/8" felt, under my amps for a couple years. My VPI turntable is sitting on top of a cabinet I built(30 years ago), on 2" of granite, atop 1 1/2" of HDF, with Texas Instruments EMI/RFI Shield and 1/8" felt between. There is zero acoustic breakthrough or resonance, as cramped as my sound space is, regardless of listening levels. As Mr G mentioned: Don't hit your granite with a hammer, and it won't ring. I'm NOT going to worry about a resonant frequency of ONE Hz.
I have granite shelves in my equipment closet and have no ringing problems at all. I have rubber vibration pads for the appropriate weight between the shelves and the shelf support, a sheet of sorbothane secured to the bottom of the shelf that the turntable sits on to dissipation energy that the pads do not deal with. I have no ringing at all from any shelves.