Turntable isolation

One possible issue with any turntable springs is that they aren’t the correct spring rate to be able to handle the extremely low frequency of the seismic vibration. It’s neceassary to get the total Fr down to say, 2 or 3 Hz, ideally, before very good performance kicks in. Alternatively, you can load the existing springs with a heavy slab of granite to reduce Fr.

You might try what was used to good effect in the final iteration of the TNT turntables, that is, squash balls. At each corner a properly sized piece of wood with a central indent on top (such as you can make with a large counter-sink, a spade bit or a forstner) can hold the ball in place with the TT on top of it. Adding something heavy underneath can complete the base: a sandbox, a thick butcher block, a piece of granite or marble.

As others have mentioned, the spring and mass system has to be properly designed for it to effectively isolate. I still have an old VPI HW19 (original, not a later Mk X or Jr. version) in a closet, that I never bothered to hook up. It uses springs, but the arrangement just seems all wrong - there’s just no way it’s going to be effective. Thank god I eventually purchased a SOTA Star III, and got into vinyl right. The SOTA’s spring suspension is properly designed, and extremely effective at isolating. You'll be hard pressed to find anything more effective without getting into fancy rack systems. 

I've been considering two types of systems for isolation - active and passive.  Active versions are microprocessor controlled isolation systems (Herzan) and passive versions are spring based (MinusK).

The active solution is one-size-fits-all.  The passive solution, you have to order the unit according to the weight you are isolating.