An Interesting Thought in Turntable Design

[MylesBAstor]

The relationship between plinth surface area and acoustic feedback.

A corollary is how dead is the plinth at audio frequencies. Not some knuckle test.

How does that all related to turntable design and sonics?

[Albert Porter]

The relationship between plinth surface area and acoustic feedback.

A corollary is how dead is the plinth at audio frequencies. Not some knuckle test.

How does that all related to turntable design and sonics?

In my opinion the turntable drive system and turntable suspension play a role in this. I use very heavy, solid plinth weighing more than 100 pounds and have tried several methods of isolation. In my room the Vibraplane was a perfect fit.

I must say, I think the flexibility of the floor (suspended ?) and proximity to speakers plus room pressure at various SPL and frequency response all playing a role too. I've had turntables go completely unstable using expensive high end racks that were supposed to provide mega isolation.

My current and best result is a rather short TIG welded 6061 aluminum turntable stand with 5/8" stainless spikes. On top is a Panzerholz platform plus Vibraplane and Stillpoints Ultra 5 under the turntable. With these 5 layers of isolation I can cue the cartridge on to an LP, turn the preamp volume to position equal to about 90 DB at 15 feet and jump up and down beside the turntable and zero sound from the speakers.

I think that's a good test, bass feedback is the most likely issue and my 16" field coil woofers are capable of deep bass.

[Guest]

45 pounds (with controller)
An alternative to mass loading.

Vewy small plinth
Vewy vewy vewy kwiet
Coincidence?

Double-skin carbon-fiber construction with polymer damping compound between inner and outer.

[1morerecord2clean]

I have probably asked before, but is that a Dynavector arm?

I'm not totally certain but I believe it may be the 507mkII Alvin Lloyd used for a Positive Feedback review of his Monaco table.

http://www.positive-feedback.com/Issue25/monaco.htm

[MylesBAstor]

There aren't a huge number of plinth-less or very small plinth footprint tables around.

Couple come to mind.

Of course Tim's GPA Monaco
Michell
SME
VPI Avenger series
Brinkmann
Simon Yorke
Kuzma
AMG
Wilson-Benesch
Spotheim
Project

Well maybe a few more than I thought.

Interestingly in some cases, more the entry level models for different company's offerings.

[Bill Hart]

Interesting thought, Myles. But even the plinthless table has to come in contact with a supporting surface- and there's the rub. Mass is fine, but what material does the table sit on? And what is that resting on? And how does that supporting structure interact with the floor (or wall)?

[Metamatic]

Plenty of small plinth tables depending on your criteria - on top of Myles list

TW Acustik
Transrotor
Avid
Air Force III
EAR
Triangle Arts
Pluto
Oracle
to name a few more

[1morerecord2clean]

The TW does have a small plinth which is removed from the platter.The platter sits on a sub-platter which is on top of the plinth.

[Metamatic]

I know. I own a TW AC-3 😊

[MylesBAstor]

Interesting thought, Myles. But even the plinthless table has to come in contact with a supporting surface- and there's the rub. Mass is fine, but what material does the table sit on? And what is that resting on? And how does that supporting structure interact with the floor (or wall)?

Perfectly valid points but we can and should talk about three ways energy is generated within or enters the turntable system from the outside environment:

1. Playing the LP
2. Acoustic feedback picked up by the plinth
3. Feedback transmitted through the supporting medium

(Outside of table being in separate room.)

So it would seem beneficial to try address at least one of the three pathways. And maybe the most important (?) and least controllable? I remember years ago using some of Lloyd Walker's weights on the plinth of my table and when you found the right resonant spot, they did make a difference. Conversely, if you use a vibraplane, Herzan, etc. you still have to deal with acoustic feedback (of course where the table is sitting in the soundfield and at a null or peak in the room can affect the sound!). Also, one reason like the SRA say over a Herzan is that the SRA will deal with energy developed within the table as well as outside environment. In a predictable manner; or at least as predictable as can be.

So it would seem a small footprint and stiff structure would be very beneficial to a turntable.

For #2, I'd include acoustic feedback picked by any part of the table, arm and cartridge, attached cables, etc. Everything has a resonance frequency potential.

[Joe Pittman]

Way back in time I had some extreme feedback issues with suspended floor and the use of Entec subwoofers ( 3 10" drivers with servo control) which had awesome bass power. My solution was not to improve my LP12 but to suspend the table from the ceiling in an adjacent room. It solved the problem completely by removing the source of feedback rather designing a solution to deal with the floor and acoustic energy.

It's not practical with my current dedicated room layout to move the table to an adjacent room, but I have thought about it. My current Roksan Xerxes has excellent isolation from floor based vibration and I suspect it has excellent feedback resistance to air born acoustic feedback as I have played some music at insane levels without issues.

I have often thought about designing an enclosure for the turntable to eliminate acoustic feedback as there is no better solution than to prevent the source of feedback itself.

[PeterA]

Here is a photograph of a friend's Technics SP10 mk3 in one of Albert Porter's plinths and one of my SME Model 30/12A. Both are on my Vibraplane with an enlarged steel ballast plate to preload the Vibraplane and provide space to isolate the motor controllers.

Though both turntables are very heavy, the respective approaches differ. The SME is an open structure which combines suspension, mass and drainage paths. The turntable proper is 110 lbs. The ballast plate is 136 lbs, and the Vibraplane is 150 pounds. That is a lot of mass. The platter and arm board are fixed on the top plinth (shelf) and isolated from the motor vibrations by the four suspension towers and rubber drive belt. Vibrations from the cartridge drain down the arm tube into the massive arm board and top shelf. Vibrations from the motor and perhaps airborne vibrations hitting the structure drain down through the four feet which have steel balls and into the massive ballast plate and Vibraplane. So, at least in theory, there are paths for the vibrations to move away from the platter surface and cartridge. The platter surface is scrolled for contact with the LP and is made of a damped material similar to vinyl. The steel platter, arm board and top and bottom shelves are also damped.

I have two lolli columns under the front legs of my rack from the floor down to the basement ledge. There is no floor bounce at the equipment rack.

[MylesBAstor]

Here is a photograph of a friend's Technics SP10 mk3 in one of Albert Porter's plinths and one of my SME Model 30/12A. Both are on my Vibraplane with an enlarged steel ballast plate to preload the Vibraplane and provide space to isolate the motor controllers.

Though both turntables are very heavy, the respective approaches differ. The SME is an open structure which combines suspension, mass and drainage paths. The turntable proper is 110 lbs. The ballast plate is 136 lbs, and the Vibraplane is 150 pounds. That is a lot of mass. The platter and arm board are fixed on the top plinth (shelf) and isolated from the motor vibrations by the four suspension towers and rubber drive belt. Vibrations from the cartridge drain down the arm tube into the massive arm board and top shelf. Vibrations from the motor and perhaps airborne vibrations hitting the structure drain down through the four feet which have steel balls and into the massive ballast plate and Vibraplane. So, at least in theory, there are paths for the vibrations to move away from the platter surface and cartridge. The platter surface is scrolled for contact with the LP and is made of a damped material similar to vinyl. The steel platter, arm board and top and bottom shelves are also damped.

I have two lolli columns under the front legs of my rack from the floor down to the basement ledge. There is no floor bounce at the equipment rack.

I understand that you are mass loading the Vibraplane but have you tried any other platform under the SME? Again that platform has to perform double duty: 1) prevent energy from reaching the table; and 2) Drain energy away from the table.

[PeterA]

I did try a Townsend Seismic Sink under my SME Model 10. It did help, so I then got a Vibraplane for that table and then bought my Model 30. I think the Vibraplane addresses ground borne vibrations, but I don't see how it can effect air borne vibrations generated by the speakers. Energy generated from the table itself is drained down via the feet with ball bearings into the massive steel support of the ballast plate 136lbs + Vibraplane chassis 150lbs on which it is placed. I don't think the Vibraplane counteracts vibrations entering it from above the way the active Herzan is purported to do. For that vibrational energy, the 286 lbs of steel act as a mass sink. At least, this is my understanding.

[MylesBAstor]

Something culled from the Wave Kinetics website.
Active Isolation

Active isolation systems attempt to cancel out vibration by vibrating a platform 180 degrees out of phase with a measured vibration stimulus. A typical system has three main parts; a passive base, a measurement unit, and 6 axis voice coils/ rotational motors. The passive base attempts to limit the transmission of higher frequency vibration (>200 Hz) through passive means such as air bladders, springs or viscoelastic material.  The “active” components are usually only active below 200 Hz. The measurement unit accurately, and hopefully quickly measures the actual vibration of the platform on which a component rests. The measuring unit sends this information to the 6 axis voice coils / motors. The voice coils then vibrate 180 degrees out of phase with the measured vibration. That is, if the platform is moving one direction, they move the opposite way to cancel out the motion.  The measuring unit continuously senses the motion of the platform and thus forms a feedback loop with the voice coils / rotational motors. Generally, active isolation systems do a good job at isolating a component from vibrations in the structure it is sitting on. They are used extensively in the electron microscope industry and work particularly well with relatively large amplitude vibration sources such as footfall. They have two significant drawbacks however. First, they do less well at dealing with airborne vibration (such as loudspeaker output) and machine generated vibration (such as motors/transport mechanisms. The reason is the sensor that detects the vibration is in the platform, not in the component. Typically the amplitude of the vibration in a component case is reduced by the time it reaches a sensor leading to erroneous inputs (or no input) to the motors. Second, latency causes unwanted voice coil / rotational motor movement.

The system feedback is deigned to operate quickly, but by definition, the system can’t respond instantaneously. Thus the voice coils will still be moving even after the original vibration has stopped. This ring-down effect slows down the settling time of the system.

Pros:

• Attenuates even very low frequency (1-2 Hz) component vibration while providing a stable base.

Cons:

• High cost, large size, extreme complexity (pneumatics, etc).
• Measurements take place in the platform – not the component, therefore ineffective at attenuating vibration that does not reach the sensor.
• Latency and use of servos is source of noise and ring down effect. Voice coils still moving after vibration has stopped.