LP mat/platter interface

I have been conducting some simple experiments and measurements on various mat/platter construction materials. My basic test is an impulse test of dropping a 15.5 mm diameter glass marble on the surface of a sacrificial LP, with a peripheral ring and centre weight in place, total mass ~ 2 kg. I measure the rebound as % of the fixed drop height (50 cm). Listed in order:

Carbon graphite (Boston Audio Mat 2) 5%
Cork/Rubber (DIY) 15%
Felt (Rega) 15%
Carbon Fibre/Stainless steel (TTW Ultimat) 40%
Carbon Fibre/Copper (TTW Ultimat) 45%
Hard Rubber (Thorens, Technics) 50%
Sorbothane (Platter Matter) 50%
Delrin/Stainless steel 80%

I am only really interested in the extremes, which are both surprising to me. The carbon/graphite rebound is like throwing play dough on the table. One bounce and it is rolling. The Delrin/SS is the bigger surprise. The platter is 11 kg with label recess machined into the Delrin.

One of these sounds clearly superior with LP replay, to all others, in my system. Here is my take on why.
The signal source is the record groove. The function of the groove is to move the stylus accurately for conversion to an electrical signal. To this end the groove motion should remain uninfluenced by extraneous inputs. In moving the stylus, the groove will be subject to reaction forces that push back into the groove. The impulse test indicates the extent to which the LP interface rejects (vertical) reaction forces.
The higher bounce examples indicates less energy into the LP, more returned to the dropped ball.
In the LP replay situation, this means more energy into the cartridge/tonearm/plinth.

I am prepared to be completely wrong in my analysis, however I am trying to make sense of what my ears are telling me.

The first plot is the carbon graphite mat, with the second bounce at approx 0.2 s after the initial impact. The main mode frequency is approximately 19 Hz.
The second plot is the Delrin/SS platter. The principal damped resonance is at 12 Hz and is most likely the tonearm/cartridge resonance.
It is quite apparent that the second impulse plot decays much more rapidly than the first although the first is at a lower amplitude which is not apparent because of the normalisation.
My ears/brain seems to prefer the rapid decay version.
 

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I wanted to test under close to normal playing conditions as possible. The tonearm/cartridge resonance which is always present, is easily recognised and can be discounted. If the timescale is expanded in the Delrin/SS, the next significant resonance at 750 Hz is observed superimposed on the damped 12 Hz. It similarly shows rapid damping. There are a small number of higher frequency modes also quickly damped.
For the same time scale, the carbon graphite has much greater proportion of high frequency artefacts, especially in the range of frequencies 1-4 kHz. They are not well damped.

The cartridge impulse response spectrum.
Delrin/SS first
Carbon graphite second
 

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The Delrin/SS spectrum is cleaner than shown above because I neglected to eliminate the interval prior to the impulse. Replotting the impulses with the same time scale and the corresponding spectra, the Delrin/SS is now much cleaner than the Carbon graphite with much faster decay of resonances. The significant Delrin/SS at 750 Hz can still be seen but it is highly damped. It is possibly a tonearm resonance.

Delrin/SS first
Carbon Graphite second
 

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The tests were with all the mats, including the carbon graphite mat, on the SP10R except the Delrin/SS on the Kaneta. I used the 10R to give all the mats an even chance since the 10R platter is very inert and any mat related artefacts should be easier to isolate.

Just out of curiosity I will run the impulse tests again with all the mats on the SP10 mk2 OEM platter on the Kaneta plinth and report back. The results might not be as clear because the bare SP10 mk2 OEM platter has a clearly audible resonance.
 
I repeated the impulse tests with the OEM Technics SP10 mk2 platter in place on the Kaneta style plinth. The felt mat did not fit the platter recess so was excluded. I also did an impulse test with only the bare platter in place. The rebound height was 20% of the drop height.
Interestingly the rubber and sorbothane mats increased the rebound and the Cork/rubber, Copper/carbon fibre, stainless steel/carbon fibre decreased the rebound.

The carbon graphite was very different.

Rebound heights:

Sorbothane 30%
Rubber 25%
Copper/Carbon Fibre 17.5%
Stainless steel/carbon fibre 15%
Cork/rubber 15%
Carbon graphite 2.5%

The plots and spectra of the rubber, cork/rubber, Cu/CF, SS/CF, Sorbothane, were all similar, so I chose to show the rubber mat data as typical. The carbon graphite was very different, with a slow rate of decay by comparison and a spray of hf energy between 1-5 kHz. The other mats were relatively clean above 1 kHz. The Kaneta Delrin/SS impulse response showed the cleanest spectrum and fastest decay but the initial amplitude is relatively large.

Shown below are the test set up and data for :
OEM platter, rubber mat
OEM platter, carbon graphite mat
Delrin/SS platter

Of course this experiment does not represent typical operating conditions but it is an attempt to tease out information that is consistent with what I hear.

The OEM plus carbon graphite and Delrin/SS are polar opposites on this test. One appears to readily absorb energy and convert it to high frequency random noise and the other rejects much of the energy and rapidly damps the remainder.
 

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@ Johnny

I have been making plinths for the mk2 since 1998. The SP10 mk2 is the longest I have ever had a component in my system. Turntables I have owned include Linn LP12, Garrard 401, Thorens TD 125 mk2 but the mk2 is the only DD turntable I have owned. From the first I saw the need for a superior plinth to exploit the SP10 mk2 abilities. I have tried almost all of the available DIY materials except Panzerholz. The resin/bentonite is my best and final material of choice. I have taken it to the practical limit, with the Kaneta style of removing the motor to a seperate plinth and all the electronics in a separate enclosure.

The OEM chassis is a resonant component, as I quickly discovered when dismantling the mk2 for service. I was never happy with the knowledge that the bare chassis makes a passable dinner gong. The brake assembly is another area where rigidity is compromised. It is not necessary in a domestic environment.

I was even less impressed that the SP10R chassis was not much improved. I think Technics missed an opportunity by keeping with the legacy format, despite an outstanding achievement in the new motor and controller. It could have been untouchable with a fresh new chassis design which addressed the things I have mentioned.

IMO the mk2 motor in Kaneta style plinth far outperforms the stock mk2. It is not a simple modification though and took me a couple of attempts to get a satisfactory result. Not for the faint hearted but a very worthwhile result is possible.
 
Bon
Thank You for your prompt reply to my enquiry.
I am very much a SP10 MKII Convert myself.
Fortunately supported with ironing out initial TT issues by a friend who has got the required skills.

The exchange Platters that are under discussion are of interest, and the reports on the changes be perceived are very positive.

I have heard P'holz as a Plinth in a 20mm and 32mm thickness.

I am using MU25 Plywood as a Plinth with Solid Tech 'Feet of Silence' as footers,
this has been a sufficient support, but to wrest a little bit more is a attractive endeavor.
 
Order of preference

One of these sounds clearly superior with LP replay, to all others, in my system. ....


My ears/brain seems to prefer the rapid decay version.

Bon,

Good test.

Just to be clear, was it the Delrin one you preferred to listen to? Which ones would be second and third in the listening test?

Do you like Delrin/Acetal on it's own, or is it best when backed with Stainless?

Thanks,

Hugh
 
Hi Hugh

The POM/Acetal was stacked to the SS because POM flat plate warps when machined. The SS also keeps COM low and provides inertia.

POM thickness has an impact on sound quality. I made myself 2 platters POM bonded to gunmetal, 1 has 10mm POM and the other has 20mm POM both bonded to identical gunmetal bases. The 20mm POM has far greater detail retrieval and lower noise floor than the 10mm. The 10mmPOM/GM out performs the stock platter and bare SS with OEM mat by a large margin.

Bon sent me another 20mm sheet of POM I will be bonding this to an OEM platter shortly. It will be interesting to compare this to the POM/Gunmetal and POM/SS.

On a side note I removed the damping material from the bottom of the platter I will be bonding the POM to and found that the damping material had hardened and the glue (contact cement) had failed in many spots, rendering the damping ineffective.
 
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Hugh,

The Acetal/SS was preferred by a long way in my system. It has been pointed out by Cats Squirrel that Delrin may have very different properties from POM/Acetal. I sourced my acetal/POM from China and it performed up to my expectations. I don't know if Delrin is more or less suitable for this application.

Just seen Warrjon's reply, who covered the comments I was going to make. This project is a joint effort between myself and Warrjon, who does all the machining. No further plans to use a SS substrate because of cost and difficulty of machining.

We are toying with ideas for a thicker (40mm) acetal/POM layer on gunmetal substrate.
 
Hello,
I have a made a turntable '' base '' with pom.
Pom material is very difficult to glue. I never try to glue it because i knew it takes a very special glue.
My base consist of two pom CNC machined plates connected together by nylon bolts. I made some M8 or M10 threaded holes in one of the plates. I wanted to use bolts that would have similar properties like the POM anyway no metal bolts.
Greetings, Eduard.
P.s as you can see i also got the arm base and the motor base machined from POM
 

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