- May 2013
Minus K 100BM-1 Vibration Isolation
By: Philip Holmes
How do you describe something that isnt 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 youve
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 dont even try. Thats 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 springs 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 amps 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 Ive 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 wouldnt be surprised if they
were able to design a system to isolate an entire listening
|1 Vertical vibrations are
isolated by the springs interaction with four
pairs of flexures. The weight of the instrument compresses
the pre-loaded spring, floating the isolator and aligning
||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.
|Vertical Load Adjustment Crank
Simple manual adjustment compensates for changes in
|Vertical Load Adjustment Indicator
Easily determine optimum setting using this simple visual
|Vertical Stiffness Adjustment Screw
Dial in Guaranteed 1/2 Hz or less Vertical natural frequency
using this simple adjustment.
The curve below shows the typical vertical
1/2 Hz performance of the BM-1. It offers 10-100 times better
performance than typical high-performance air tables.Minus
Like setting the bias in a tube amp with a bunch of parallel
pentodes, it takes a while to get the Minus K tweaked to
perfection. I had intended to use one of my turntables.
Both tables are heavy, weighing from 55 to 70 pounds, but
that wasnt quite enough for the platform I asked for,
namely the 100BM-1. I tried using the ballast I had at hand,
but the two turntables were hard to balance. Rather than
spend dollars buying ballast, and many days precisely balancing
the setup, just to tear it down, it was decided that a tube
amp would make a good test subject. All vacuum tube pieces
are prone to ringing and microphonics, and the Raven Audio
Reflection integrated amp that was here for review had a
phalanx of tubes, fourteen total, I believe.
The Raven wasnt overly sensitive to
ringing, but it could be easily excited by thumping and
tapping. Even here, I ran into the problem of cords. This
is such a finely balanced device that a stiff cable will
wag the dog. My overly stiff speaker cables,
and ridiculously stiff power cable required lots of changes
in positioningI would move the amp, move the cables
and move the isolation base. Then I would do it again. Then
I would do it a few more times. After much moving and adjusting,
the Minus K was centered front to back, side to side and
top to bottom. In a way, it reminded me of setting up classic
spring suspension turntables, like the LP12, AR XA, etc..
The reason people have migrated away from those classic
designs has more to do with the difficulty of setting them
up properly than performance issue.
For instance, when you changed the tonearm
on a spring suspended turntable, it would change the center
of gravity, the flatness of the setup, and the resonance
frequency of that corner of the turntable. In an ideal world,
those spring-suspended turntables would be designed to work
with a particular mass of tonearm, and provisions would
be to add/remove lead shot from the armboard to keep the
turntable level, and with a unified resonance frequency.
So, the Minus K reminds me of those finicky designs, though
it is even more sensitive to small adjustments because the
range of motion is not as progressive as with
springs, which gets progressively more difficult to deflect
as you pushed/pulled from the resting place of the spring.
This one feels more like you are in a liquid or magnetic
field, and you can move freely until you hit the travel
Its much easier to make an overkill
solid plinth. It also reminds me of what F1 teams goes through
to tune a new car. They spend many hours running a course,
checking telemetry, making adjustments, and then they do
it again, sometimes for weeks. If you really care about
your audio, youll spend weeks getting it right.
The Minus K doesnt have a sound. When
properly adjusted, the Minus K removes the smear that decreases
image depth and width, obliterates low-level signals, robs
dynamic power and creates listener fatigue. With most systems,
listening at high levels is the same as out-of-control listening.
With the Minus K, there is greater musical intensity, without
the frenetic qualities of an out-of-control system. Very
demanding orchestral recordings, like the Bernstein-Chicago
live recording of Shostakovichs Leningrad Symphony,
dont scream as much as they usually do. That particular
recording is far from great, but can sound horrible on some
equipment. The low brass tone and bite were much clearer.
Double bass and cello lines were both easier to follow,
while having a truer harmonic signature. Without the Minus
K, you can hear the microphonic tubes modulate the signal
from acoustic feedback, creating a peculiar flavor of intermodulation
distortion. Thats the best way I can describe what
the Minus K achieves: lower IM distortion. It mostly isolates
the electronics from the acoustics, lowering distortion
that behaves very much like intermodulation distortion.
Due to the materials, mass, and the large
dimensions of the 100BM-1, the skirt/apron could
be made to ring. The skirt is isolated from the load, but
there is the possibility that it could ring, creating a
source of noise. With the smaller units offered by Minus
K, I dont foresee as much of an issue. There are several
steps that could be taken to eliminate the ringing, and
all could be hidden from the listener.
Another issue for some potential buyers will
be the pedestrian looks. Its not ugly, but its
not attractive. It reminds me of the VPI 16.5 record cleaner.
It just is. It exists to perform its duties. Its not
a problem for me, but it will be for some who value aesthetics
as much as sound.
Bass impact and depth were other areas markedly
improved. After being freed from the acoustical echo, which
is significantly slower in arriving than the speedy electrical
signal, the leading edge of midrange and bass transients
were cleaner. I heard no tube ringing the way I often do
when hitting pause or mute in the
middle of a loud passage. Its an easy test: Play very
noisy music, crank it up, and then hit pause. Do you hear
any ringing, like a wine glass? If so, then you have a problem.
I have always used tube damping rings, but they have only
been partially successful. With the Minus K I still heard
a benefit with damping rings, and more than usual. The damping
rings push the resonance frequency lower, which allows the
Minus K to be more effective.
For What Purpose?
I originally intended an analog application, but wound up
testing tubes. Both operations need help. I can see a system
that employs several Minus Ks for turntables, tape decks,
other source components, preamps and amps, interconnected
with carefully chosen cables. In my experience, everything
is affected by vibration. The only application that doesnt
make sense is for the speakers themselves. Speakers need
to be affixed firmly to the floor for best resolution. Without
isolation devices, the only way to take vibration out of
the equation is moving sources and electronics to the other
side of a massively built wall. Ive heard one system
with a similar approach, and it makes an amazing difference.
On the other hand, the majority will either want to see
their expensive components, wont have enough space
to sacrifice, or wont have the money for such a radical
step. I dont know if five or six of the Minus K platforms
will be cheaper than redesigning, and rebuilding your listening
room, but they are the next best solution.
In the Minus K isolation platforms, I believe
audiophiles have a thoroughly designed instrument with a
reasonable price. While others will be easier to set up
and use, the Minus K will do a better job. If it werent
for the monumental hassle of constantly changing components,
and the demands of reviewing, the Minus K wouldve
been a no-brainer purchase. If you are happy with your components,
and want to take your system to the next level of performance,
you should give these isolation platforms an audition. Youll
be surprised by what you dont hear.
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