Rerurn to Romy the Cat's Site

My first post here, although I have read some of the discussions on this site over the years.

I wanted to share my experience with my Micro Seiki RX-5000 and resonance control of its heavy platter. I have seen posts here before about the bronze platter ringing. Romy suggested that a simple hard rubber mat would be the answer. I tend to agree, but I found another approach that I do not see explicitly written about here. Forgive me if my search failed to turn up this topic already addressed.

The concern is, of course, is that these resonances will color the mechanical pickup of groove modulations by the cartridge. The stylus, which is forced into motion by the music being played imparts opposing forces back into the vinyl and then into the platter. If the platter “answers back” with ringing at specific musical pitches and not at others, the cartridge will pick up these resonances too. You may not hear anything obviously amiss, since all transducers impart resonances, but they are there. Perhaps the best test is to play well-recorded piano which runs up and down the scales. Notes exciting resonances may sound slightly louder, with more smearing and sustain. When resonances are vanquished, the music flows with more ease and naturalness, all music, not only piano. Of course resonance control is valid for more than just platters - tonearms, cartridges, speakers and rooms matter too.

I have heard an argument that the forces imparted into the platter by the stylus must be so weak that they can’t possibly excite the heavy platter. Large heavy objects are moved by tiny forces. The motion is just correspondingly tiny. It’s all about relative proportionality. True, the excitations and resonant responses are very small, but then the cartridge is designed to pick up tiny groove modulations that can be measured in the Angstroms. And we follow that transducer with huge amounts of preamp gain. So, as weak as the excitations are, a phonograph system is equally sensitive in picking up their responses.

If you gently rap a turntable platter (even with a record on top) you can get a sense of its resonant properties. A sharp rap (perhaps with a plastic-tipped object like the handle of a screw driver) can impart a pretty good approximation of an impulse excitation into the object being tested. This impulse (an approximation of the "Dirac delta" function) excites the object equally at all frequencies and therefore excites resonances without having to perform a sine-wave sweep as excitation. With a microphone or an accelerometer and an FFT spectrum analyzer, you can see discrete frequencies representing resonances at the various modes of vibration.

The original gun metal bronze 5000 platter exhibits a primary bell-like resonance at 855 Hz. I’m guessing that this mode is due to the comparatively thin horizontal deck (just under the groove area) acting as a spring and the heavy peripheral ring as the mass. There are other modes too, but they are down much farther in level. The next most significant resonance is a damped one at 2258 Hz. There is nothing unusual here. All platters will exhibit resonances; it’s just the physics of things.

Micro Seiki offered a “copper” mat called the CU-180, which some of you may know about. This 1.8kg device is very popular with the Panasonic SP-10 crowd, but it can be useful with heavyweights like the RX-5000 too. Suspended by itself with something like a screwdriver shaft through its hole so that it is free to move (like a cymbal), it exhibits numerous resonances, at 51, 132, 312, 549, 844, 1182 and 1569 Hz and several spread higher in frequency. Think of it as a flat and thick cymbal. It might not seem promising, but it is.

The magic comes when you place a CU-180 on top of the RX-5000’s gunmetal platter. The pairing is completely devoid of readily measurable resonances. Together they comprise a constrained layer mechanism. The CU-180 must be perfectly flat so that is sits on the 5000 platter with no gaps or rocking. Never drop or bend a CU-180 because you’ll never get it perfectly flat again!

The improvement can be demonstrated easily. If you rap on the bare platter, you’ll clearly hear the pitch of the 855 Hz mode. When you rap on a suspended CU-180, you’ll hear the multiple pitches combining into a sound roughly reminiscent of a cymbal. Now combine the two pieces, and rap on the copper mat. A dead thud. It sounds like knocking on a concrete wall. The two devices have constrained each other from exhibiting their natural resonances.

Next, remove the CU-180, and then place a rubber mat of your choice back onto the patter. Rap again, and you’ll hear the muted, but still quite audible, 855 Hz mode. I have used a Fulton Kinetic Barrier mat, a very thick and well damped rubbery mat. It does reduce the ringing (shortening its duration), but the CU-180 quenches it completely.

Lately I have been playing with a sandwich. First the CU-180 is placed on the platter, then the Fulton mat (or a Micro Seiki leather mat) is placed on top of the CU-180 (minding the VTA adjustments). I have also experimented with the record sitting directly on the CU-180, and weighted down by the 1kg Micro Seiki ST-10 gun metal disc stabilizer . This weight forces the LP to sit on the copper mat’s surface tightly, with no air gaps that could cause buzzing or unchecked resonances. I have not yet formed hard conclusions about which configuration I prefer sound-wise, but I’m leaning toward the CU-180 without any compliant mat at all, in terms of pace and rhythm. One risk of any rubber mat or other compliant mat is that its top surface can shift in the shear mode relative to its bottom surface (without actually sliding) as stylus drag changes with groove modulation. Thus speed is thus slightly modulated by groove modulation, more like in a light platter table. I believe that if the record is held tightly and directly against the heavy platter surface with its high inertial momentum, the speed is less perturbed. More listening needed.

So, I can recommend that other 5000 owners experiment with the CU-180. As always, though, YMMV.

*******************************  

Chapter Two: Recently I had machinist Mirko Djordjevik make me a new 5000 platter out of 316L stainless steel. As most of you know, the platters on the last mega tables from Micro Seiki were made from stainless steel. The exterior dimensions and appearance are the same as the original 5000, but I had him bevel the inside edge of the peripheral ring to reduce the cantilevering compliance, and also we thickened the horizontal deck (just below the groove area). Mass went from about 16kg in the gunmetal platter to about 22.4kg in the new stainless platter. That’s roughly going from 36 to 49 pounds. We both believe that the spindle bearing can handle the higher mass. The new platter is a beautiful thing to behold, and tarnishing will no longer be a concern. The only resonance I could pick up was a well-damped one at 1667 Hz, which is about one octave higher than in the original platter, but lower in magnitude with little sustain. With the CU-180 on top, even that resonance was squelched entirely, and once again the sandwich is quite quiet. I have yet to spend time auditioning the new platter. > >

Rerurn to Romy the Cat's Site