Vibraplane vs. Minus k for Turntables

Hi Larry,

I have a Minus-k BM-8 system that supports my TNT Mk.? (upgrades of various vintages). Performance is extremely impressive and I especially like the maintenance-free aspect of the isolator. Although I have no experience with the Vibraplane per se, I have professional experience with similar isolation systems for electro-optical instrumentation and atomic-force microscopy (I'm a biophysicist). You can expect significantly better performance from the Minus-k BM-8 than is available with passive air isolation due to the fact that the BM-8's resonant frequency of 0.5 Hz is at least a factor of 4 lower than that of the Vibraplane. What this translates to is 99% isolation at 5 Hz vertical frequency versus 50-75% for the Vibraplane (depending on load and air pressure). To get better performance than this using an air table requires some sort of active isolation, which is not worth the trouble or expense in my opinion.

I'd be happy to tell you more offline, so feel free to send email to me directly.

I'm not sure what's meant by "100% effective." Whether f0 = 2 Hz is enough depends on specifics. In my case, I live in an 80-year-old house with suspended wood floors and there are plenty of ambient noise sources near 5 Hz, e.g., washing machine, outside traffic, etc. As for not hearing the difference, one can experiment with the BM-8's resonant frequency by manually adjusting the suspension's stiffness with an Allen wrench. I could hear a significant change in the sonic character of LP reproduction - mostly tighter bass and improved image stability at lower f0s. I haven't yet brought home an accelerometer/vibration analyzer to correlate the isolator's performance with acoustic power spectra, but probably will do this eventually.

The BM-8 gives vertical transmissibility values that are comparable to Halcyonics' benchtop product (the Micro) in the range >=5 Hz. However, the BM-8's horizontal isolation is significantly less effective than the Halcyonics product. As Tbg points out, there are significant differences between the Minus-k technology, which involves a resonant system, and an active electro-mechanical vibration-suppression system, such as that used by Halcyonics, Herzan, etc.

In most cases, laboratories opt for passive isolation over active. There are two main reasons for this: cost and complexity. Active systems have the edge in applications where settling time is critical - basically these systems behave like overdamped oscillators over a wide frequency band. A downside of active systems is that the cost is proportional to the number of degrees of freedom because of the number of transducers required. Also, feedback loops in such systems become complex; for example, small angular displacements of a payload can generate significant horizontal accelerations. Settling time shouldn't be much of an issue in analog playback as long as the resonant frequency of an isolator is well below that of the tonearm/cartridge combination. Also, for most audio applications, vertical isolation is likely to be much more important than horizontal.