[H]ow do you describe something that isn’t there? How to you qualify and quantify an absence? With a subjective audio review, lacking a scientific test procedure, describing my impressions will only offer limited benefit to the prospective buyer. In the case of isolation devices, the more energy that is pumped into the system, the greater will be the effect. Poorly constructed rooms with poor acoustical properties will show a greater effect. With no signal, an isolation device has no effect, gradually coming into play as the signal levels rise. This is totally unlike power conditioners and special cables that are able to isolate the raw power, and hence the lowest level signals. Those devices, such as power conditioners, special cables, shielding products, have greater effect as the signal level decreases. The low level information is destroyed due to EMI/RFI and noise on the AC power lines. As AC levels increase, at the source, and also as you increase the volume, acoustical feedback becomes a bigger issue. At the worst, you can have cables and equipment that are both susceptible to EMI and RFI, while also being microphonic. In the end, a system full of equipment and cables like that wastes oodles of power reproducing distortion, acoustic feedback and noise that has nothing to do with the music.
A few products combat both electrical and acoustical noise: Purist Audio Design cables come to mind, as they do a magnificent job of addressing both issues. Certain capacitors are wound and packed in a way that they are shielded electrically and are critically damped. However, there will always be components that lack adequate isolation from the environment, meaning almost every piece of audio ever created. After you’ve addressed cables, AC power and even acoustical properties with room tuning devices, you still have big boxes that resonate, and big speakers that put tremendous energy into the room where these big boxes reside. Some manufacturers attempt to isolate the equipment from vibration, though not very successfully, and most don’t even try. That’s where isolation platforms come into play.
Until I used the Minus K product, my experience with “isolation platforms” were either the snake-oil variety where sharp pointy feet and thick acrylic magically make vibration disappear, with DIY racquetball devices, which work okay, or with the more ambitious air diaphragm isolation bases from Vibraplane.
Technology and Choices
The Minus K 100BM-1 weighs around 80 lbs, and is quite large at 24” x 23” x 9”. Moving it around is a bit challenging due to the combination of weight and size. I recommend you seek assistance when unpacking and moving. There are four red “shipping collars” that secure the top and prevent damage during shipment. They are held in place with allen head cap screws.
I find it difficult to describe the internal mechanisms of the Minus K isolation platforms. If you refer to the diagram and descriptions below, it says “vertical vibrations are isolated by the spring’s interaction with four pairs of flexures. The weight of the instrument compresses the pre-loaded spring, floating the isolator and aligning the flexures”. Without tearing it apart, I get the “feeling” that it acts like a combination of preloaded coil and leaf springs.
The resonance frequency in the vertical plane is .5 Hz, or less, and is achieved over the weight range of 60-100 lbs, while resonance frequency is load-dependent in the horizontal plane. This differing compliance reminds me of the Decca stereo cartridges. Optimal performance occurs when the platform is loaded with the “nominal” weight. So, if you have an amp that weighs 85 pounds, you will want to add an additional 15 pounds of ballast for best performance.
I found the main disadvantage of the Minus K, in contrast to the Vibraplane, is load leveling. The adjustable feet can be used to level the table, but I felt that it slightly compromised the performance. In my experience, you will get better performance by adjusting the position of the load, and ballast, to level the top plate. With some tube amps, this is a major PITA. I was able to use the BM-1 with the Raven integrated amp, but only with the front of the unit well forward; I had to get the output transformers and power transformer, located at the rear of the amp’s chassis, as close to the middle of the platform as possible. Think of it like this: Cars with 50/50 weight distribution perform better than cars that are nose-heavy. Perhaps I am mistaken, but I believe the Minus K performs best when you center the load and avoid any other “tricks” to level the table..
Once properly adjusted, the Minus K proved markedly superior to every other type of isolation device I’ve used or investigated. The load felt like it was held in suspension by a spherically shaped magnetic field. Regardless of the direction of deflection, the unit maintained its .5 Hz resonance. The various cables attached to the amp affected the performance. Further adventures in cable stiffness, as well as various isolation devices made specifically for cables, could bring further improvements in isolation. The price of the 100BM-1 is $4,150, and there are weight options up to 1,050 pounds in the BM-1 line, costing $6,580. An “automatic” version of the BM-1 series is available in the BA-1 series. The BM-8 series offers similar performance to the BM-1 series, but in much more compact dimensions, and would be appropriate for many source components. There is a wide range of products at the Minus K website. I wouldn’t be surprised if they were able to design a system to isolate an entire listening room.
1 – Vertical vibrations are isolated by the spring’s interaction with four pairs of flexures. The weight of the instrument compresses the pre-loaded spring, floating the isolator and aligning the flexures.
2 – A squeeze force from another spring, controlled by the knob O. is applied to the outside of the flexures via a screw. The “squeezed” flexures constitute a “negative stiffness mechanism” (NSM) that acts like the negative of a spring, reducing the stiffness of the system.
3 – Four beam-columns connecting an upper and lower column plate act as a horizontal spring to isolate the horizontal motion. The beam-columns are vertically very stiff, but bend slightly in response to horizontal vibration. The weight on the deflected beam-columns reduces the stiffness of the spring, making the system behave like a spring with an NSM.
4 – The crank moves the base of the spring up and down to compensate for changes in the weight of the payload and to keep the flexures in their straight aligned position. If you increase the weight on the spring (by swapping a lighter microscope for a heavier one, for example), its base must be raised by turning the crank clockwise.
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