So what about the plinth?

[Lost Monkey]

Please! Tell us more!

Couldn't really have told us less...

 

[waterjetguy]

I'm happy to, if anyone is interested in listening, but that hasn't been the case.
I've tried to do so several times, very politely and constructively, on this subject and in this particular forum. Not one poster has commented except to argue that their previous, completely erroneous post was, in fact, correct.

It seems that most audio fans are more interested in parading a tiny bit of information as knowledge rather than in actually learning how things work.
(Which is unfortunate, since, for example, better understanding of the mechanics of vibration would make it easier to make successful improvements to your TT, without as much trial and error.)

Reminds of a quote that someone (Chicks, maybe) had on their signature for a while: The world of hifi is bizarre, full of dangerous amateurs. Or words to that effect

 

[Andyman]

Please! Tell us more!

Here's something he schooled me in in another thread that is probably appropriate here...

The turntable base's job is to sit there quietly. It shouldn't ring (like a tuning fork), so that vibration from either the support structure (shelf) or the air will cause it to resonate. Airborne vibration will not be a problem for all but the lightest bases, but structurally borne vibration can be an issue.

The mass (weight, in a gravity field) of the base acts as an attenuator (NOT a damper) for structurally transmitted vibration. That's Newton's 2nd Law, F=m*a, at wok. The structural vibration is a forcing function, and that force can accelerate the mass of the turntable base by an amount inversely proportional to that mass. If the base is heavier, it can't be vibrated as much.

Thus, heavier is better, just like intuition tells you.

The TYPE of material determines the amount of internal damping that a base of given mass will exhibit. All materials have internal damping. Even a tuning fork doesn't ring forever, even in a vacuum, (in which case it doesn't RING, it just vibrates). If the base is quite massive, then by Newton's 2nd Law we know that it will be vibrated much less than a light one. Therefore there would be less vibration to contend with, which means that its internal damping is not as important as for a light base. If the base is heavy, then only a small amount of damping would be necessary, say maybe some relatively thin (.125-.25" rubber, say) damping material under each foot.

So, if your turntable base is going to be heavy, the internal damping of the base material is not so important. If it will be light, then you'd want to have a lot of internal damping. That can be accomplished by using materials with high internal damping (particle board, acrylic) or by building the damping into it.
Steel or aluminum laminated with rubber or acrylic are examples of building damping into a structure. (Steel and aluminum both have low internal damping)

If it were my turntable, I'd use whatever type of wood I liked the most, because I'd be looking at it for a long time. I'd add weight to it, probably a steel plate in the bottom. Be sure that whatever weight you add is firmly attached to the base, so that they are forced to act as one piece. Unless you're using something loose like sand or steel shot, then that's not possible, but it's ok because the movement between the particles dissipates energy and adds damping as well as weight.

Set your pretty, massive turntable on some compliant feet, and get ready for some high fidelity bliss!:thmbsp:

(Note that the vibration that I refer to above is low amplitude, like the way a shelf vibrates when the music is turned up loud. It's not the high amplitude, short duration type that engineers refer to as shock. That is what you get with heavy footfall on a bouncy floor, or someone banging into the audio rack. You need isolators with long displacement capability for that. Like spring isolators with long strokes (1/4" stroke would be reasonable for a turntable isolator)

Seems that mass is easiest way to quell vibration.

 

[waterjetguy]

Mass is one easy way to limit vibration. It costs more to make and ship a heavy turntable, though, so makers work hard to get the job done with less material.

And beware of jumping to conclusions. I didn't say it was easiest, I only explained how it worked.

 

[Andyman]

And beware of jumping to conclusions. I didn't say it was easiest, I only explained how it worked.

Sorry. It just seemed that way to me given the obvious choices. Increasing mass via the metal plate you suggested, or another means just seemed simpler than rebuilding a whole new plinth out of a more highly damped material.

 

[waterjetguy]

Sorry. It just seemed that way to me given the obvious choices. Increasing mass via the metal plate you suggested, or another means just seemed simpler than rebuilding a whole new plinth out of a more highly damped material.

Right, for that particular situation increasing the mass is probably the easiest.

Especially if you like the look of your existing plinth!

Just be sure to fasten it well.

 

[Lost Monkey]

If it were my turntable, I'd use whatever type of wood I liked the most, because I'd be looking at it for a long time. I'd add weight to it, probably a steel plate in the bottom. Be sure that whatever weight you add is firmly attached to the base, so that they are forced to act as one piece. Unless you're using something loose like sand or steel shot, then that's not possible, but it's ok because the movement between the particles dissipates energy and adds damping as well as weight.

Set your pretty, massive turntable on some compliant feet, and get ready for some high fidelity bliss!

Thanks for this!

 

[DougMac]

Having spent the better part of a decade of my engineering career as a vibration analyst, I find this type of discussion truly amusing.

Now I know how the EE guys feel when the subject turns to cables....

Based on your professional experience and interest in audio, I'd like to hear your opinions on how big a factor the turntable base really affects the sound quality. I know it will be hard to generalize.

Some feel turntable manufacturers use lightweight plinths because of cost of manufacture and shipping, implying they may do something different if not so constrained. Perhaps they aren't trying to fix a problem that doesn't exist to any meaningful degree.

Maybe it's just me, but instead of spending money and engineering to try to prevent motor noise from being transmitted to the tonearm, it would be wiser to spend the same resources to develop a more quiet motor. Good base design and good transport design would seem optimum.

I understand your heartburn over non-informed opinions and I apologize for any contribution to your discomfort I might have made.

 

[waterjetguy]

Based on your professional experience and interest in audio, I'd like to hear your opinions on how big a factor the turntable base really affects the sound quality. I know it will be hard to generalize.

Some feel turntable manufacturers use lightweight plinths because of cost of manufacture and shipping, implying they may do something different if not so constrained. Perhaps they aren't trying to fix a problem that doesn't exist to any meaningful degree.

Maybe it's just me, but instead of spending money and engineering to try to prevent motor noise from being transmitted to the tonearm, it would be wiser to spend the same resources to develop a more quiet motor. Good base design and good transport design would seem optimum.

I understand your heartburn over non-informed opinions and I apologize for any contribution to your discomfort I might have made.

Well, since we're generalizing:

A less massive base/plinth will transmit more vibration, assuming equal damping.

Any mechanical vibration transmitted to the tonearm or to the platter is bad: it will either mask low level sonic information or be heard as noise/hum/howl.

A less massive base with appropriate damping can perform (transmit as little vibration as) as well as a more massive one.

As is true with most things, the devil is in the details. It takes a lot of r & d to work out a lightweight tt base that will be sonically "dark" (extremely quiet).
Keep in mind that we're dealing with the audible bandwidth here: 20 Hz to 20,000 Hz. That's 3 decades of bandwidth that have to kept entirely free of transmitted vibration in order to allow accurate sound reproduction.

Note the use of the word ALLOW. That's all a plinth can do, is to allow the rest of the TT system to do its job in peace.

So, to answer your first question, a plinth should have absolutely no effect on the sound of a turntable. It should only provide a silent backdrop so that the platter/tonearm/cartridge can do their jobs. If it is not doing its job well, then any transmitted vibration will show up as feedback (howl), hum, or loss of low level detail. Or all of the above. Motor vibration will likely be heard as hum, if its level is high enough.

Another complication is that there are several possible sources of vibration, and that they'll often vary from one installation to another. Airborne and structure borne vibration vary with placement of speakers relative to the 'table, the structure of the flooring and support shelf, etc. That makes it challenging to design a system that will effectively filter out all of the vibration from the environment and leave a truly quiet platform that our platter and tonearm to function on.

Regarding the motor, belt drive is a great isolator itself. A thin belt will not be able to effectively transmit vibration to the relatively heavy platter. The platter's mass helps it in this way, in addition to the flywheel effect which helps stabilize speed. Now, the motor mounts should isolate any vibration from the plinth. Obviously, a well balanced motor will generate less vibration to start with, and good electrical design will minimize "cogging". Note that a lot of the really expensive TT's use standalone motors to isolate them from the base, and that those tend be housed in massive assemblies. The extra mass means that any vibration from the motor will generate much smaller amplitude vibration to be transmitted to the support structure (see Newton's 2nd Law discussion in my quoted post). So, if you have a very massive plinth, that will tend to knock down motor vibration as well as vibration from external sources. I'd guess that a 12dB reduction in motor noise might be possible just from having that heavier plinth. But you'd have to measure it to be sure. Vibration transmission is a very tricky thing to predict, especially at such low levels. (As compared to, say, that of an aircraft propeller spinning at 1,000 rpm through a rigid support structure.)

And lastly, I don't have a problem with un-informed opinions. It's just when they're accompanied by closed-mindedness that I lose my patience...

 

[gusten]

One can find rumble measurements at Miller Audio Research. A lot of TTs have been measured during the last years.
In nearly all cases the hum is the most dominent problem, if we don´t take into account the hump arm/cartridge resonance. I cannot see a corrolation between heavy TTs and light TTs, except in maybe one or two cases, (it might be there but it´s not easily obvious)
The difference is surprisingly small but in a couple of cases.

The measurements I have done also show that the problem is the hum, from motor vibrations and transformer vibrations.
gusten

 

[Stacker45]

Well, since we're generalizing:

A less massive base/plinth will transmit more vibration, assuming equal damping.

Any mechanical vibration transmitted to the tonearm or to the platter is bad: it will either mask low level sonic information or be heard as noise/hum/howl.

A less massive base with appropriate damping can perform (transmit as little vibration as) as well as a more massive one.

As is true with most things, the devil is in the details. It takes a lot of r & d to work out a lightweight tt base that will be sonically "dark" (extremely quiet).
Keep in mind that we're dealing with the audible bandwidth here: 20 Hz to 20,000 Hz. That's 3 decades of bandwidth that have to kept entirely free of transmitted vibration in order to allow accurate sound reproduction.

Note the use of the word ALLOW. That's all a plinth can do, is to allow the rest of the TT system to do its job in peace.

So, to answer your first question, a plinth should have absolutely no effect on the sound of a turntable. It should only provide a silent backdrop so that the platter/tonearm/cartridge can do their jobs. If it is not doing its job well, then any transmitted vibration will show up as feedback (howl), hum, or loss of low level detail. Or all of the above. Motor vibration will likely be heard as hum, if its level is high enough.

Another complication is that there are several possible sources of vibration, and that they'll often vary from one installation to another. Airborne and structure borne vibration vary with placement of speakers relative to the 'table, the structure of the flooring and support shelf, etc. That makes it challenging to design a system that will effectively filter out all of the vibration from the environment and leave a truly quiet platform that our platter and tonearm to function on.

Regarding the motor, belt drive is a great isolator itself. A thin belt will not be able to effectively transmit vibration to the relatively heavy platter. The platter's mass helps it in this way, in addition to the flywheel effect which helps stabilize speed. Now, the motor mounts should isolate any vibration from the plinth. Obviously, a well balanced motor will generate less vibration to start with, and good electrical design will minimize "cogging". Note that a lot of the really expensive TT's use standalone motors to isolate them from the base, and that those tend be housed in massive assemblies. The extra mass means that any vibration from the motor will generate much smaller amplitude vibration to be transmitted to the support structure (see Newton's 2nd Law discussion in my quoted post). So, if you have a very massive plinth, that will tend to knock down motor vibration as well as vibration from external sources. I'd guess that a 12dB reduction in motor noise might be possible just from having that heavier plinth. But you'd have to measure it to be sure. Vibration transmission is a very tricky thing to predict, especially at such low levels. (As compared to, say, that of an aircraft propeller spinning at 1,000 rpm through a rigid support structure.)

And lastly, I don't have a problem with un-informed opinions. It's just when they're accompanied by closed-mindedness that I lose my patience...

You obviously know a lot more about this subject than i do, but then again, i'm a longshoreman, not a vibration engineer, now, i just want to make sure i didn't make an a$$ of myself in stating that all else being equal, a heavyer plinth will do a better job at dissipating vibrations.

 

[Bigerik]

Great info!
We should separate this into its own thread and make a sticky out of it.

Btw, have you had a look at the latest Regas? Still using the same lightweight plinth, but now they have rigidly coupled together the arm and bearing. Any thoughts on that?

 

[gusten]

Great info!
We should separate this into its own thread and make a sticky out of it.

Btw, have you had a look at the latest Regas? Still using the same lightweight plinth, but now they have rigidly coupled together the arm and bearing. Any thoughts on that?

It´s lightweight, but it´s very different as I see it. Now it´s a constrained layer design with two aluminium sheets and a foamed PE with high losses inbetween.
gusten

 

[WmAx]

I just wanted to give an example of the massive difference that can be had with improved design, not just increasing mass. The following graph is a CDM-170SE loudspeaker that I modified for someone. The OEM line is the stock form with accelerometer fastened to the center of the side panel. The MOD line is also attached to the very same point. Both measurements were made with identical driving voltage. The MOD version is a little over 2x the OEM mass, as a consequence of the new cabinet internal additions, but the overall amplitude of structure vibration is far less than what mass alone can accomplish, with up to 30dB attenuation in some cases. The modified cabinet used an added internal 2nd layer wall of 1/2" thick with a thin sheet of viscoelastic highly lossy material placed between the original wall and new internal layer wall. Some bracing was added of course; but this is the overall result of an example using something other than mere added mass, and how it can have radical improvements.

 

[waterjetguy]

Note that I meant to quote Gusten's post regarding rumble being due primarily to motor hum:

Are those tests taken under actual usage conditions?

Namely, are there speakers nearby being played at high SPL's?

Is there any foot traffic over the flooring?

Or are the tests merely being performed on a turntable that is effectively playing in a vacuum?

If so, then motor hum and platter bearing noise would be the only significant sources of rumble.

 

[waterjetguy]

I just wanted to give an example of the massive difference that can be had with improved design, not just increasing mass. The following graph is a CDM-170SE loudspeaker that I modified for someone. The OEM line is the stock form with accelerometer fastened to the center of the side panel. The MOD line is also attached to the very same point. Both measurements were made with identical driving voltage. The MOD version is a little over 2x the OEM mass, as a consequence of the new cabinet internal additions, but the overall amplitude of structure vibration is far less than what mass alone can accomplish, with up to 30dB attenuation in some cases. The modified cabinet used an added internal 2nd layer wall of 1/2" thick with a thin sheet of viscoelastic highly lossy material placed between the original wall and new internal layer wall. Some bracing was added of course; but this is the overall result of an example using something other than mere added mass, and how it can have radical improvements.

While loudspeaker cabinet and TT plinth design are apples and oranges, note that I have strongly alluded to the importance of design vs mass. I pointed out that TT makers work HARD to achieve minimal vibration in a lightwieght plinth.

For example, the new Rega design was certainly the result of a LOT of serious r & d. Its primary structure, which provides the support for the platter and the tonearm, apparently uses constrained-layer damping to minimize resonance and kill extraneous vibration. It requires a thorough understanding of the both the structure itself, as well as the nature of the applied vibration, to successfully implement. Dummies need not apply.

That is a lot more difficult than it is to make the 'table 4X as massive.

 

[waterjetguy]

Great info!
We should separate this into its own thread and make a sticky out of it.

Btw, have you had a look at the latest Regas? Still using the same lightweight plinth, but now they have rigidly coupled together the arm and bearing. Any thoughts on that?

Glad you like it, Big Guy. If I have the slightest inking that it is appreciated, I will happily share knowledge all the damn day....

Ideally, any plinth should rigidly locate both the arm and the platter bearing, and that entire ass'y should be isolated, by hook or by crook, from extraneous vibration. And we should be getting the idea by now that that goal is not nearly as simple as it may sound, and that there are a lot of ways to skin that particular cat. Apologies to the cat lovers, it's just an expression!

Regarding the new Rega design: nice work boys! It is something fresh without striving to be so, and it looks entirely functional. I think Dieter Rams would approve.

 

[Bigerik]

Forum mods, What say you? I think we have enough good info here for a sticky. Thoughts?

 

[Bigerik]

Glad you like it, Big Guy. If I have the slightest inking that it is appreciated, I will happily share knowledge all the damn day....

Ideally, any plinth should rigidly locate both the arm and the platter bearing, and that entire ass'y should be isolated, by hook or by crook, from extraneous vibration. And we should be getting the idea by now that that goal is not nearly as simple as it may sound, and that there are a lot of ways to skin that particular cat. Apologies to the cat lovers, it's just an expression!

Regarding the new Rega design: nice work boys! It is something fresh without striving to be so, and it looks entirely functional. I think Dieter Rams would approve.

It is very nice work if you ask me. And it looks like the RP6 and RP8 are even better.

I love threads with really good science behind them. I really do enjoy and appreciate learning new stuff on AK. Or in this case, getting a good understanding why something I like works as will as it does.

 

[waterjetguy]

It is very nice work if you ask me. And it looks like the RP6 and RP8 are even better.

I love threads with really good science behind them. I really do enjoy and appreciate learning new stuff on AK. Or in this case, getting a good understanding why something I like works as will as it does.

Very interesting point that you make about how well a turntable works at reproducing sound. It's pretty amazing that they work as well as they do. Considering that all of that beautiful music is generated by a squiggle carved into a piece of vinyl, which must be rotated at a constant speed, which then drives a tiny stylus on the end of a cantilever with a magnet or a coil on its other end so that it can in turn generate an electrical signal that is an exact analogue of the recorded sound. It is truly amazing that it works as well as it does. If you gave a group of engineers a design brief tasking them with the development of such a system, one that was as accurate from 20-20KHz as the current top-of-the-line turntable systems, they would likely throw in the towel. There's around a hundred years of development in turntable technology, and that is nothing to sneeze at. It really is an amazing mechanical/electrical device.