Planet - September 2012
Minus K's David Platus Takes the Stand On Behalf
of His "Negative Stiffness" Isolation Technology
by Michael Fremer
While effective isolation from both
air and ground borne vibrational energy is important
throughout the audio playback chain, it is essential
for vinyl playback. It can be built into a turntable
in the form of spring or "O" ring suspensions
but current thinking downplays that in favor of separate
isolation stands rather than incorporating it into the
Why? Spinning platters, no matter how carefully machined
and/or balanced spinning upon a spring-suspended platform,
will inevitably set the platform in micro-motion. If
the motor is isolated by being placed upon a sub-platform
for example, the motion of the sprung platform will
vary the distance between the belt and motor, causing
micro speed variations. This can often lead to a pleasant,
warmish sound at best, though it is an undesirable coloration,
or at worst to sonic mush.
Stabilizing such platforms with damping troughs or built
in pneumatic stabilizers can help and some older designs
such as Oracle's Delphi have benefited greatly with
the addition of silicon troughs beneath each spring
tower. It's easy enough to run the latest Delphi with
and without the damping troughs engaged and the differences
are not subtle.
For these and other reasons, many manufacturers are
opting to design and build non-suspended turntables,
which gives end-users the option to add an aftermarket
isolation platform. These can be relatively inexpensive
but reasonably effective ones like the Gingko Audio
Cloud that utilizes squash ball-like devices sandwiched
between two polymer platforms or more sophisticated
ones like those from Halcyonics and the Vibraplane from
Sounds of Silence Audio that uses sophisticated pneumatic
air suspension devices incorporated within a very heavy
metal platform. This technology is based upon devices
manufactured by Newport Corporation and is more typically
used in electron microscopy stands. It is also what
was used in the no long manufactured $80,000 Rockport
System III Sirius turntable.
The Halcyonics and Vibraplane platforms are active systems.
Roughtly speaking, the Halcyonics platform uses an elastomer
based suspension and piezo-electric elements that react to vibrations
entering the system. The Vibraplane comes in two models: one
is passive and requires end users to occasionally refill the
air bladders with a pump. The active version requires an outboard
electric compressor that activates when the system pressure
goes below a certain preset level. There's a more recent addition
to the isolation platform "community" invented and
marketed by Minus K that, like Newport, is headquartered in California.
Unlike the others though, the Minus K is completely passive.
That is, it does not depend upon either electrical or pneumatic
devices to provide isolation, nor is it reactive. Another completely
passive platform is manufactured by Harmonic Resolution Systems.
The designer, Michael Latvis is a vibrational engineer who in
his spare time designs helicopter rotor damping systems among
other things for the military. I use the HRS racks for my components
and for reviewing turntables. It's what Brinkmann offers as
an option for its turntables and it works very well.
Of course there are many, many isolation stands manufactured
specifically for audio, some of which have been designed by
competent engineers, but others are just Gyro-Gearloose contraptions
based upon fanciful but hardly scientifically credible notions.
Like the Newport-based technology, Minus K's platforms, which
are available in a wide range of sizes and weight capable models,
are mostly used in the medical, scientific research and electron-microscopy
I was made aware of these Minus K devices by Mark Doehmann, the
former Continuum Audio Labs designer, responsible for conceptualizing
the design and overseeing the manufacture of the Continuum Audio
Labs Caliburn turntable, Cobra tonearm and Castellon stand that
I've used as a reference for the past seven years. It's hard
to believe it's been that long but that's how long it's been!
In all that time the turntable has given me zero problems. It's
been 100% reliable, which is the least one should expect from
a turntable that ended up costing $150,000 dollars!
Ridiculous? You should hear it some time. Audiophiles used to
at shows where Continuum displayed, but those were often marred
by poor associated gear. However, CD-Rs made from the 'table
never fail to amaze people, though of course live play back
is even better.
I was fortunate to buy the review sample early on when the projected
retail was around $90,000 and since mine was an early, failed
experiment in chrome plating magnesium that resulted in a cosmetically
unacceptable finish, I was able to get it for the cost of a
nice car. It took me a few years to pay off the loan, but it's
my favorite material possession and something for which I have
no regrets buying. I've yet to hear another turntable that can
match its sonic performance.
So, to make a short story long, the Castellon stand incorporates
a reasonably effective magnetic repulsion platform that "floats"
at a very low frequency. The horizontal part of the system incorporates
a thin diameter rubber inner tube that runs around the periphery
of the platform. It requires pumping up a bit every few weeks.
Overall the system is very effective because you hear the need
for air well before you see that the platform has settled somewhat.
Internal company politics were such that replacing the magnetic
isolation stand with one by Minus K would void the turntable's
warranty. Yet a few Caliburn owners (of which I think there
are at least one hundred or perhaps more) did make the switch,
including one in California whose turntable was located on a
springy second floor and suffered the dreaded and all too familiar
to vinyl enthusiasts foot fall induced arm jiggling and skipping,
despite the Caliburn's very effective magnetic isolation system.
This owner and a few others told me that the addition of the
Minus K technology took an already amazing turntable to a whole
new sonic performance level and that it also totally eliminated
foot-fall based stylus-skipping issues, though that's not a
problem in my concrete slab floor located listening room.
I didn't wish to get involved in the company politics but since
my warranty has expired, when one Caliburn owner bought a Minus K
and then chose not to go through with the modification (for
reasons that had nothing to do with its claimed performance)
and offered it to me for his cost, I gave in and bought it.
The Minus K modification for my turntable involved removing the
original magnetic isolation system and dropping the 'raw' guts
into the Castellon stand so that outwardly nothing appears to
have changed, though the mechanism does hang below the platform,
which the original did not do.
The sonic difference was "as advertised" to me by
one of the Continuum owners: (among other sonic benefits) the
blackness of the backdrop, which already was noticeably superior
to any turntable I'd heard got even blacker and took the term
"silence" to new levels. Another Caliburn owner was
so impressed he bought a self-contained unit and put it under
his DAC. He says it made an equally profound difference.
So, to get the scoop on Minus K, I contacted the inventor David
Platus and what follows is a transcription of our discussion.
It would be very helpful for you to click here technology, copy
the link and open in another browser window before reading further.
Michael Fremer: This technology was explained to me in simple
terms as isolation created with "bending springs."
Is that was causes the effective isolation?
David Platus: Well, we use something called "negative stiffness."
One example is the old oil cans where you squirt by pressing
the bottom? The bottom of that can is basically a negative stiffness
mechanism. You can't keep it centered. It's either popped out
or it's popped in. Basically it acts as the reverse, as a negative
of a spring.
After coming up with this idea, I was one of the early casualities
of the economic slowdown in defense and aerospace spending in
California, so I had time to review what was going on with air
isolators and I found out that they wouldn't exactly work for
super-low vibrations. So I came up with the concept and the
Air Force didn't want to develop it, so I got laid off from
the company for which I was working, so I said "I'm going
to develop this thing." I guess it was kind of naïve
MF: So this was the mid '90s?
DP: This was 1988 and the company didn't want to develop it
so they ended up giving it to me, you know, the initial germ
of the idea and kind of an added severance. They said "we
wish you luck." And that's basically what happened. I did
some consulting after that but devoted four or five years almost
full time working with an associate coming up with the patents
and building prototypes. Finally we attracted some capital and
started the company and incorporated in 1993. It was just me
and a couple of people part time for a few years trying to figure
out what to do with this thing.
Fortunately, the technology moved in our favor, moving towards
everything small: nano. Nanotechnology. So we started selling
finally, making improvements along the way and that attracted
some more good people.
MF: Now can this technology be made really small?
DP: The smallest things we ever made were 6" diameter and
4" tall and these were going into an ultra high vacuum
chamber for science work where they literally move atoms around
and build new structures. That's what nano technology is all
MF: And your background is in?
DP: Well I have a varied background. I started in chemical process
engineering, went to U.C.L.A. where they emphasize a generalized
curriculum. The Dean at the time said engineers end up in different
fields and we want to give you the fundamentals. So it was like
chemical processing was a major. And then I got into the nuclear
field. I went to a special school where I learned to design
nuclear reactors. I worked in that field for a very short time,
then got into working on a nuclear airplane engine and then
into the structural mechanical aspects.
MF: Is it true that you won a contract to develop the lunar
landing module suspension system?
DP: No I came up with an idea, which they used, to support the
crew couch in the Apollo command module. Basically kind of a
fancy shock absorber in the very limited space between the couch
that the three astronauts rode in when they went orbited the
moon. And if they had to abort the flight shortly after lift-off
and the winds are blowing inland the capsule could land really
hard and injure or kill the astronauts. So they were looking
for the best possible energy absorber-especially after that
tragic fire where three of the astronauts were killed on the
launch pad. And they ended up with my invention for that.
MF: That must be satisfying.
DP: Very! That was early in my career. I was just a young guy
out of school.
MF: I want to continue with this, but I want to give you a website
to look at because there's this whole thing happening in the
audio world with Quantum Physics and Quantum Mechanics. Weird
stuff like little tiny stick-on things that supposedly have
enormous sonic effects.
DP: Yes (laughs)! One of my associates showed me something like
that: little beads you hang on cables. Crazy! This strikes me
as very humorous. Let's go to the Minus K.com website and click
on the technology toolbar on top and then "negative stiffness
isolators how they work".
See that figure 1? That's a pretty good representation of how
our vertical isolator works. Now imagine the 3 balls connected
by two beams. Take the spring away. So now picture the three
balls aligned. And you are squeezing that from the sides. That
center ball is in a state of unstable equilibrium. It wants
to pop up or pop down. That's like the middle of the base of
the oil can. If you let it be displaced up just a little bit,
it takes a force to hold it in equilibrium. Now for small deflections
if you double the displacement, it's going to take double the
force. And if you triple the displacement it's going to take
triple the force.
So there's a force displacement relationship, which is really
what the stiffness of a spring is. See, to deflect a spring
upward you have to pull it upward. It takes a force to hold
it. If you pull it up a little bit it takes a small force, if
you double the deflection, twice the force, if you triple the
deflection three times the force.
So you have a "spring constant" or spring rate, or
spring stiffness. But this device is the negative of a spring,
because if the displacement is up, the force is down. If the
displacement is down to hold the negative stiffness device in
equilibrium, the force is up. Mathematically, the negative stiffness
part of this design is the reverse of a spring. In the actual
isolators we use flexure. We don't want any friction or "stiction"
or anything like that.
So now when you combine that with a spring as in our animated
illustration, the spring supports the weight load, whether it's
a few ounces or ten thousand pounds. But now, with the right
design of that negative stiffness mechanism, you literally take
the stiffness out of the spring. Now you have something that
supports the weight load that's got the stiffness of a slinky.
Very low stiffness. You can actually make the stiffness go to
That's how it works. It's very simple in principle. Another
way of looking at it is, if you took away the squeeze force
(the "p" load in the animation)? And you just pushed
down on the spring and deflected it unit displacement with whatever
force it takes to deflect it unit displacement, that's my definition
of the spring rate or the spring stiffness: pounds to deflect
it an inch.
But now, when you have this mechanism helping you, it's like
a servo-assist or something. You push down a little but, but
now it's pushing for you. So you don't have to push as hard.
That's almost an easier concept. So now, if you don't have to
push down so hard to deflect that thing it's got a lower spring
rate, lower stiffness. And that's what you want in an isolator.
You want something that supports a weight load with a very low
natural frequency because when the table or the ground or whatever
is shaking underneath, you've de-tuned the system.
And you want a very low natural frequency. And the lower the
frequency, the better. That's what the Air Force was looking
for. They were going to build an active system around a super
super low natural frequency system-something that's almost unheard
MF: And they were going to use air?
DP: No. They were going to start with this passive isolator,
which we were going to provide using my concept and then they
were going to build an active system around it. All of these
active or electronic cancellation systems use a spring of some
kind. It could be an elastomer or something that they build
the active system around and the Air Force was going to build
an active system with a super, super low frequency passive system
and that's where I conceived of the idea.
MF: I see. And how does this maintain its level?
DP: Okay, so this is only the vertical. Now go down (on the
website) to the next figure 2: and again, that shows a planar
example of a payload sitting on what we call a beam column.
Just a flexible rod. Now on the right side drawing, the rod
is pushed over to the right. And it's got a weight load on it.
See, there's something called an "inflection point"
where there's no curvature right at halfway up the beam column?
Because the curvature changes. There's no bending moment. You
know what a bending moment is? A bending moment is just like
a torque that causes the beam to bend.
So basically you can analyze this as little cantilever beams.
Like a little flagpole coming up from the bottom and one coming
down from the top. So it's legitimate to show this thing as
a little cantilever beam and it's got a horizontal force on
it. This is the way we isolate horizontally.
So, if you didn't have any weight load (showing the weight load
and the payload on that little column?) it acts just like a
horizontal spring. It takes a horizontal force to deflect it
and if you want to deflect it twice as much, twice the force.
But see, now when you put a weight load on it the weight load
acting on the deflected beam causes additional bending.
MF: Yes I can see that.
DP: Especially at the root. The maximum added moment is right
at the root. So when you put weight on it, it makes it easier
to bend the beam. In effect, it makes the beam act like a spring
and a negative stiffness mechanism. And the more weight you
put on, the more negative stiffness you get.
MF: So the better.
DP: And the better until you reach a point where it goes totally
unstable. That's called "the critical buckling load."
So we operate just below "the critical buckling load."
So it will always come back, but it's like the slinky again.
It's got a super-low stiffness. And that's what we do for horizontal.
And so now, we combine that in a compact unit that's got the
vertical and the horizontal and that's show in Figure 3. That's
a pretty good representation of all of our isolators.
MF: So what I've got under my turntable is within the stand
so I can look in and see all of these components. In your case
it's a little different because there are two springs instead
of one, but basically there's a spring and series of these compressed
horizontal flexures. But see, they're compressed like the beam
column. They're compressed beyond their "critical buckling
load." So they not only take out their own stiffness, but
they take out the stiffness of that vertical coil spring that's
supporting the weight load.
So now the vertical motion isolator is
the coil spring
is analogous to the coil spring in figure 1. And those compressed
flexures, compressed again with a spring, see that screw over
to the left (on Figure 3) that says "Vertical Negative-Stiffness
Flexures"? And they look like lines. All they are, are
sheet metal flexures. And there's a set of them that goes to
the center, just like that center ball (in Figure 1) and then
they go to the left to a rigid support and they go to the right
to a support that can move in and out that's got a spring in
it and that screw goes right through from the left support to
the right support and as you tighten that screw you put more
vertical negative stiffness into those flexures. You can literally
turn that screw and watch the frequency of your system change.
MF: So you manufacture for different essential load ranges?
DP: Yes. Every standard product we manufacture gives you the
dimensions, the range of weight loads and so on. And now if
you go to that center hub? That center block? Upper spring support
(Figure 3)? There's a support shaft that goes from the top of
the spring down to that dark gray charcoal thing at the bottom,
the Lower Column Plate. See, that plate moves up and down with
the vertical motion isolator. So now we attach a set of these
beam columns to that lower column plate and it goes up and then
we attach an upper column plate. Now we've got horizontal motion
just like I showed you in Figure 2.
A weight load, loading these columns and you load them near
the nominal capacity of the isolator and you get that very low
horizontal natural frequency.
MF: So the key is to make sure that the structure itself is
DP: Yes. When you get very low natural frequency you have to
level the frame otherwise there will be a little component of
gravity that pulls the thing laterally into a stop. So that's
basically the whole isolator.
So that's why we call these things "negative stiffness
isolators". By using negative stiffness we get these very
low natural frequencies. Lower than any practical thing on the
MF: And people can buy this directly from you?
DP: Yes. If you click on "products" it lists all of
them. The big SM-1s we use as floor platforms and then you go
down to the benchtops and so on and then you click on pricing
and specs you see how big they are, what the weight ranges are,
everything. So yes, people buy these things directly from all
around the world.
MF: Okay. I think I understand it now. I can't believe I didn't
think of it myself!
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