aluminum platter vs Acrylic not what I thought

Atmasphere said:
A correct platter pad will have the same hardness as vinyl so that any vibration from the LP can be absorbed.

Aluminum and any metal for that matter is far too hard.

Acrylic is too, but obviously less so than metal. IME, the harder the material, the more issues with high frequencies.

To expand on Ralph’s comments, you’re trying to absorb and transmit spurious vibrations away from the stylus tracking the groove so they don’t reflect back into the cantilever/motor assembly of the cartridge. The key to this lies in matching the speed of sound of the two materials (platter surface and record).

Our plain-Jane, PVC platter did this extremely well (the “V” in PVC stands for vinyl), but the market was fascinated with flame polished acrylic. The PVC was as much of a technical success as it was a commercial flop. And so it goes …

Now, you can work with aluminum but it gets tricky. In an after-market context, you can try playing with mats, but it will be a long, bumpy road (or … you might get lucky on the first try).

The key in working with materials that don't interface well with records (from a vibration transmission perspective) is to do this in stages - starting with an ideal platter surface, and working through intermediate material transitions,. 

This is how we approached our composite (Gavia and Stelvio) platters.

Lew said:
For what it’s worth, I have not liked acrylic mats or platters in the past, but probably that’s just me. The only platter per se that meets your criterion pretty well, Ralph, is one of the several options offered by Thom Mackris on his Galibier turntables. It may be made of Delrin, but I cannot recall.

Top to bottom, the platters are carbon fiber, PVC (for the Gavia Platter) / Brass (Stelvio Platter - 14 Lbs. worth of it), and aluminum, along with damping chambers in the aluminum.

… Thom @ Galibier Design

Dear Thom, My apologies for my error in guessing at the materials used to make your Gavia platter. PVC not Delrin. PVC is probably better than Delrin for matching the energy transfer to that of vinyl, because it IS vinyl. Did you ever consider selling platters per se, for use with other brands of turntable? PVC platters could become quite popular as an aftermarket add-on to a VPI or SOTA, for example. I could foresee legal issues, however. But on the other hand, many aftermarket suppliers sell platter mats intended to replace OEM mats.

Hi Lew,

I have trouble keeping this straight sometimes ;-) The PVC layer (in the Gavia platter) and brass layer (Stelvio platter) are hidden under the carbon fiber top plate. The efficacy of a carbon top layer was surprising to me (first employing graphite, then carbon fiber).  Frankly, I never expected the brass layer to work, but obviously the staged material transition I referenced above works with these materials.

As I began to compose this reply, I started thinking about what it would take to do aftermarket platters - whether solid PVC or the more exotic ones.  I was seriously pondering some sort of shared-risk group venture with a few willing VPI owners.

Three key challenges would be (1) developing a reliable QC process. (2) Individually matching platters to bearings and (3) possible changes to the mass equation.

My current QC process involves everything that's released from the shop being auditioned with real music (after measurements are taken). Perhaps this isn't an issue - as long as the platter/bearing set meets the runout (eccentricity) specification.

With respect to bearing/platter matching, my current production flow begins with bearings that are initially oversized by a few thousandths (at the bearing platter interface). After the platters are complete, the bearings are individually trimmed to match each platter.

For an aftermarket scenario, I'd have to reverse this process by matching platters to existing bearings. I believe this is possible to do while leaving the platter on the end mill. Removing the platter before completion could add significant production time, but more importantly, it could compromise the runout - the eccentricity of the platter perimeter with respect to the platter/bearing center.

With respect to the mass equation, turntables (for how few parts they have) are complex, interactive beasts. For all I know, VPI has tuned their drive system to specific platter masses and a change to PVC or a composite platter could upset everything.

I think I just talked myself out of it.

... Thom @ Galibier Design

Just keep in mind folks, that when you’re ascribing characteristics to a material (whether it be acrylic, PVC or aluminum), making an apples to apples comparison is difficult to do if you’re comparing platters of identical dimensions.

You won’t know if the differences your hearing relate to the mass difference or the material’s sonic characteristics.

Higher mass may a sonic effect with respect to how the bearing’s thrust interface is loaded (for better or worse, and affecting energy transmission).

More importantly, higher mass will in general, smooth out the drive system’s speed characteristics, and you "hear" the effects of speed instability in ways that are much more subtle than pitch stability, wow and flutter, etc. At it’s finest level of granularity, micro levels of speed instability are experienced as IM distortion. 

Looking at it from the other side (improving these micro-instabilities), you'll get a cleaner rendering of high frequencies, as well as more extended highs and (surprising to many) a richer harmonic presentation in note fundamentals (think bowed double bass and cello).

If you have the time, wade through the first page of Moncrief’s review of the Rockport Sirius III on IAR.  Moncrief is a strange dude, but he lays out the situation fairly clearly in this regard (albeit in a very wordy manner). At a minimum, it will cure your insomnia ;-)

Having said all of this (in the VPI universe) you’re obviously left with the decision as to whether platter "A" works better for you than platter "B". I didn’t mean to imply otherwise. Just be careful about generalizing your conclusions with respect to materials.

Cheers,
Thom @ Galibier Design