A Copernican View of the Turntable System

Ct0517,
A pedestal is nothing more than a 'plinth' :^)

The issue of having isolation between the arm bearing and table bearing can be illustrated, in extremis, by imagining a jack-in-the-box with an immovable armpod next to it. After jack has popped out of the box and does not bounce anymore, he is perfectly stable and unmoving at the end of spring. As long as he is perfectly still with absolutely zero resonance, the relationship between your tonearm pivot and jack's nose is always the same, whether Jack is encased in concrete (a plinth) or perched on the end of his spring (on top of a 'platform' which moves in relationship to the world around it. As soon as there is any resonance in the system however, jack will move with regard to the tonearm pivot (and therefore the stylus), and may do so in slightly unpredictable ways. In any case, the slightest movement will cause a kind of intermodulation.

If you fix Jack to the platform your armpod is resting on in order to keep the relationship between his nose and the armpod-mounted arm pivot perfectly constant, you have effectively plinthed him.

If Jack has some kind of internal resonance, the method of fixing him to the board may have more or less resonance. This is the difference between a bad plinth and a good one, but the inherent goal of a plinth is to keep the relationship between the arm's pivot/bearing and the motor bearing as stable as possible. If one removes the plinth which surrounds the motor, and mounts it naked on the same table as the armpod, one has removed an intermediate coupling material (the one which connected the frame of the motor to the bottom of the footers, but one has not changed the concept/goal, one has simply changed the method of execution.

If, however, you stick your TT motor on an isolation base which is not the same base that your arm bearing is resting on, you are effectively changing the distortion relationship. You may find that a substandard plinth will resonate unpleasantly because of a resonant frequency of the material which is in the audible range. If you change this frequency to a 3-5Hz frequency (the goal of most isolation systems), you have exchanged the plinth resonance-induced distortion for one which is like an off-center 45 record.

Oops. Writing on an iphone is a sure-fire way to make lots of typos. So please ignore those.

One can add to the last point that it is not necessarily a BAD thing to exchange one set of undesirable resonances for another set of undesirable resonances. It could be that one likes the flavor of the second better. But for people who are trying to get rid of ALL distortion, introducing an isolation layer (distortion transformer) between two points which are supposed to be absolutely fixed together with zero distortion between them is in some way 'giving up.' And while it may be better than it was 'before', it makes one wonder about the state of the 'before' and almost certainly can be improved upon.

That said, it is not a difficult thing to do, and I will try at some point. I can think that it might even be an improvement on some of my stock plinths (because some of them were not top-notch). I can probably figure out an armpod of some sort relatively easily (it could be an arm attached to an current plinth and the motor outside it, either spiked (rigidly coupled) or isolated (on magnetic levitation footers) with regard to the surface below.

T_bone -
I think you are wasting your time on this. This thread goes round in circles.
If I look at Halcro's system it astonishes me that for all the discussion on turntable set up I see the following issues in his system -
Turntables located on a large resonant shelf behind the speakers where there is massive feedback. There is effectively a bass trap where his turntables sit.
Gear stacked on top of each other rattling away ( yes electrical components resonate when on ).
He has a TV spewing out RF inches away from turntables and sensitive phono stages.
He has a massive glass coffee table bouncing off hi frequencies in front of the listening seat.
I also note that the speakers have huge lips around the edges which will cause a tunneling effect and restrict soundstage.
He may like a horribly discoloured sound who knows, but for me strapping a cartridge on with a rubber band might well sound better in this suboptimal set up.

Dear Raul: I feel that your posted of 9-11-11 was a direct response to my posts. I would like to make two points in this regard.

First: Being critical of a given approach or "road" as you put it, does not mean that one is against it. I doubt that the purpose of this thread or Audiogon in general is to host forums where people spend there time complementing each other's system. (BTW you have a very impressive system). Being critical challenges these approaches and helps them evolve. While receiving complements is pleasent (I enjoy receiving them as much as anyone) they will only lead to status quo or possibly shinnier and more expensive systems.

Second: you suggest that unless we try a given approach we cannot comment on it. Once again, then why participate on A'gon. I thought the point was to gain from the other participants' experiences and mutually advance. Moreover, I do not have the means to try all that I would like to.

With this said, Raul and the others, please explain the following. With the TT mounted on a suspension system above a mounting surface and the armpod rigidly seated on this same surface how is the distance that must remain fixed between the pivot point of the TT and the tonearm fixed?

The trade off, is between the variations in this distance, most likely in scale of 1/10 to 1 mm vs the variations caused by the micro vibrations from a bearing of a shaft turning at 33 or 45 rpm with a scale in likely 1/1000 to 1/100 of mm. My instinct would lead me to believe that the first problem is far more problematic then the second.

In general, I try to avoid commenting on things that I have no hands-on experience with, or at the very least, have never discussed with another designer who has had hands-on experiences with the topic in question.

This thread is no exception to my policies. I have tried and listened to independent arm-pods on multiple occasions, spanning from the early 1980s to earlier this year.

Any dissent that I may have voiced for independent armpods or other design particulars espoused in this thread, is based on a combination of personal experience (including listening), engineering training, and study and analyses of other turntable manufacturer's design efforts.

Everything that I have learned and experienced about turntable design (particularly regarding DD) suggests that the plinth should have very high moment of inertia but with as few structural or cavity resonances as possible, and as small diaphragmatic area as possible, while the tonearm mount should be non-resonant and completely rigid in relation to the platter so that there is no possibility for relative movement between the LP and tonearm pivot. The reason is that if any relative movement occurs between the LP and tonearm pivot, the cartridge has no way of distinguishing whether the relative motion comes from noise or vibration in the environment, or is part of the LP groove.

This should be accompanied by full isolation from structure-borne noise and vibrations from the environment, and preferably full isolation from air-borne noise and disturbances from the environment.

Some of the posts in this thread very clearly suggest that the poster isn't hearing distortions that should be quite measurable and audible, or that the sound of their system is deemed to be preferable with those distortions present.

cheers, jonathan carr (hugging a pneumatic isolation platform - grin)

BTW, if you place equipment between the speakers, you create an acoustic problem for the speakers, not only a vibration problem for the turntable, CD transport or electronics. IME, if you must place equipment between your speakers, it should present as small of an acoustic profile to the speakers as possible, and should be placed as far away from the speaker's radiating pattern as possible.