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coupling or decoupling of vinyl to/ from platter

Dear all,

I'm puzzled by a number of claims about record clamps and mats. 

I own an old Rega Planar 3, and I was reading about the importance of coupling the record to the platter, to add effective mass to the record to reduce vibrations, slippage etc, and improve the solidity that the groove "image" presents to the stylus. 

I also read about the importance of de-coupling the vinyl from the platter to prevent the transmission of unwanted vibrations from the motor. Rega has a very dense platter made of glass with a fluffy felt mat on top. So, felt to decouple lp from platter, is that right? 

Then, I purchased a cork Music Hall mat, which has a dozen raised cork discs on the mat to BOTH "decouple" the lp from the platter and "grip" the lp.  Music Hall claims that clamps are unnecessary with this mat because coupling discs, etc. I also, without knowing this, purchased a Rega Michell record clamp. The clamp seems to do good things regardless of the mat, and of course evens out warped records a little bit. 

There needs to be, it would seem, a clear objective answer to all of  this from an engineering perspective. Coupling does x, and decoupling does y.  If you look at all the high-end turntables, they have massive platters and clamps. So coupled mass is good for flywheel effect and also  for presenting a solid "image" to the stylus? 

Either Rega and Pro-Ject are dead wrong with felt mats, and have been runaway successes in spite of this, or the felt is adapted to their setup: weak motor, relatively light but super-dense platter, and decoupling felt to manage the motor and rotational noise transmitted up the spindle, and to hell with coupling?  

I did some quick and tentative experiments with the Music Hall mat and clamp vs. Rega felt mat with clamp. I need to do more comparison. The results are different but hard to characterize. I'll post again with more comprehensive subjective tests. 

From an engineering perspective, which should be best, Rega clamp w felt, Music Hall mat by itself, or "screw the mods, Rega it great just the way it is, heretic!!!" ?

Let the games begin!

Paul

paulburnett

Showing 3 responses by atmasphere

atmasphere's avatar
atmasphere
12,012 posts
 
If a center weight is used that is very heavy, let’s say 2 lb. the lighter records will lift from the mat. This happens because the mat depression edge will act as fulcrum. This information tells us we should use a center weight tuned for the record thickness and weight. However this is impractical. Here is the solution: Use a center weight that weighs 8-12 oz . This weight will work with all but the lighter records. The alternative to a weight is the screw down clamp. These clamps have pluses and minuses. The plus is down force on the record can be controlled. The minus is if not designed properly (unfortunately most are not) spindle energy is coupled into the record. It takes very little intrusion of external energy to cloud the mechanical output of the stylus. (I wrote a paper on proper screw down clamp design about 25 years ago.)

George misses an important element of clamps, which is that to gain the most out of them quite often a spindle washer of some sort is used beneath the LP so that the clamp can dish the LP against the pad surface. This can also be used to reduce warp.

I generally don't use the clamping aspect of the clamp I use (Basis; one of the better clamps made), I just use it as a weight. Its easier.

Many years ago I saw a demonstration of how profoundly a mat can affect a turntable. A friend of mine who has been involved in several damping products over the years (Analog Survival Kit, the damping rings used by ARC, the Ultraresolution Technologies damping platform) spent some years developing a platter pad. He arrived at a fairly high degree of refinement with the mat. What local audiophiles found was that any turntable that could accept the mat (due to height and weight; it weighed about 3 pounds) sounded better and also sounded better than nearly any turntable that lacked the mat.

This seems to hold true to this day. IMO/IME most mats out there don't seem to do the job as ideally as I laid out in my opening post (IOW it was a statement of the engineering principle behind the mat, not actual execution). This is why you see so much variance in opinion about the topic.
atmasphere's avatar
atmasphere
12,012 posts
 
In my case, with a Classic 1, VPI recommends against a mat. To reiterate the original question, what's the theory?
I explained the job of the mat- it is ideally a damping device.

The problem is that when the stylus tracks a groove, the vinyl resonates a bit and so can talk back to the stylus. By damping the vinyl, the talk back is reduced and so the resulting signal has less distortion. You can tell this is happening; if the LP is being damped the stylus will be inaudible with the volume all the way down. If the LP is not damped, you will be able to hear the stylus tracing the groove from across the room.

I´m not excited in damping the vinyl, damping the platter (from motor noise) is completely another thing.
You can't damp motor noise with the platter pad. If the motor is not sufficiently decoupled (IOW its noise is apparent when the stylus is in the groove) then the turntable needs repair or replacement. It really is that simple. I agree that acrylic mats don't sound right- they are too hard and don't match the durometer of the vinyl so energy is reflected back into the LP and picked up by the stylus.
atmasphere's avatar
atmasphere
12,012 posts
 
If the LP has to be decoupled from the platter to get rid of motor noise, the turntable has a really serious problem and should be repaired.

I think we can discount that explanation.

A proper platter pad does several things at once, that is if its intended for best reproduction as opposed to DJ service:

damp the vinyl
damp the platter

To do this properly the pad has to be very nearly the same hardness as the vinyl, to avoid reflected energy. Yet it has to have damping properties.

You can see right away that bare glass or metal is too hard and won’t damp the LP properly. If the mat is really doing its job, the stylus tracking the groove will be relatively mechanically silent. The purpose of felt platter pads is to act like a clutch so the platter can rotate while the LP is held still. This is for DJ work; such pads can improve the sound if they are interfacing between the vinyl and the bare platter but other materials can work much better.
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