i own 2 vintage tt's with Dobbin's plinths; a Garrard idler drive 301, and an Technics SP-10 Mk3 DD. i did also have a Dobbins plinth'd SP-10 Mk2.
so why do these plinths improve performance over the basic stock tt or OEM plinths?
my opinion, already touched on by others here, is that 60's, 70's and 80's 'even SOTA' tt's had their strengths and their 'areas of potential improvement'. obviously the strengths were that much more R&D could be directed to building motors since these companies were much more able to spend that money than the typical tt builder of today. OTOH their weaknesses were that the case work was an afterthought in comparison. system performace (cartridges, arms, phono stgaes) did not necessarily reveal limitations of the build quality back then.
it's not simply adding mass, or adding isolation. it's more a matter of engineering the precise plinth construction that will optimize the drive system. what is the best combination of materials put together in a particular way to allow the tt to have maximum livelyness, low noise, and drive which sounds the best.
i know that Steve Dobbins built many dozens of plinths before he sold his first plinth for the SP-10 Mk2. he did the same for the Garrard 301, and then again for the SP-10 Mk3. i know he experimented with many materials and ended up with a constrained layer design. eventually on the Mk3 he discovered that (in his opinion) mounting the Mk3 with the casework still in place compromised the performance due to the high torque of the Mk3 motor. the original casework allowed 'flex' and a slight smearing of the sound. eliminating the case and mounting the Mk3 'nude' inside the plinth performed better.
not every plinth designer necessarily agreed with Dobbins on that issue.
in any case; it should not be surprising that 30+ year old casework could be improved upon.
