Alternative platters ?

[MartinT]

Now we see I'm the accountant not the engineer. I thought that the higher the rotational speed, the smaller the corrections required by the servo's and the LESS they were influenced by high inertia problems. Clearly I've had a Shrodigers cat moment. What have I got wrong?

Most likely that particular speed with that particular momentum has just set the overcorrection in the servo into an oscillation where it's never happy. Remember: it's expecting a low mass, fast responding platter.

[Oldpinkman]

Remember: it's expecting a low mass, fast responding platter.

That'll be low inertia, fast responding, I take it

[Arkless Electronics]

AFAIK all DC motors when used in a turntable will indeed need feedback to keep a steady speed. The motors used in cassette decks usually had this built into the motor itself and had a speed set pot accessible via a small hole in the back of the motor...
An AC synchronous motors speed is determined by the frequency of the AC that feeds it and can therefore be pretty constant even with load, whereas a DC motors speed is dependent on the voltage feeding it (and therefore the current it draws) and the load on it, which will vary with stylus drag, ambient temperature effecting viscosity of lubricants etc etc.

[RobbieGong]

Most likely that particular speed with that particular momentum has just set the overcorrection in the servo into an oscillation where it's never happy. Remember: it's expecting a low mass, fast responding platter.

Hi Martin, So at what point with the techie is there a 'negative' effect. Are we talking from the moment there is even a slight increase in mass over the stock platter ?

[MartinT]

Hi Martin, So at what point with the techie is there a 'negative' effect. Are we talking from the moment there is even a slight increase in mass over the stock platter ?

Don't know, Robert. I can only report on what I've heard but I should think that there would be a gradually deleterious effect from the reference of a Technics platter/mat upwards in momentum. At what point you could hear the effect I cannot say.

[Clive]

AFAIK all DC motors when used in a turntable will indeed need feedback to keep a steady speed.

Trans-Fi manage it with their Salvation TT though it took a lot of testing and development to make it work. It the end it was quite simple but everying had to be just right, including the 9kg platter.

[sq225917]

Robbie, the compensation loop will be accurate within a given range of masses, one could probably safely assume +10% will make no difference. Beyond that i'd really be looking for some measured evidence or a serious piece of calculation based on a high degree of familiarity with the parts and the circuit values. Perhaps one of the designers of a heavy replacement mat can provide us with this information- surely they did the sums of took some measurements of speed stability with their new/old platters. Surely? ;-)

I didn't hear Andrew's deck sans step-up. It sounded very good with the step-up and based on the values I assume was still less than optimal with the 133ohms I had the Paradise loaded with. I suspect sub 50ohm is optional for that cartridge.

I don't think that Andrew choosing to remove the speed slider on his deck was the cause of the lack of 45rpm ability. From what I know of the 1210 electronics it just forms a voltage divider and I know two other people with technics who have removed the slider and neither of them struggle with 45rpm. perhaps someone else with an aftermarket platter and a strobe could chime in and clear this up. I don't know the internals of Andrew's deck so anything is 'possible'- but I figure unlikely.

As Arthur clearly mentions 'mass' per se is no measure or mark, but it's comfortable shorthand to use for discussing inertia, or more accurately moment of inertia, specifically an items ability to resists changes in its rotational speed and something that is defined by its mass, distribution thereof and rotational speed. I think we all know what we are talking about...

[synsei]

This debate has answered one question for me. I have considered pulling the trigger on a record puck for a while now as some partaking in this debate have argued that it is a better solution than a clamp. If high mass can cause speed instability on an SL12XX then I will be steering clear of pucks.

[MartinT]

No Dave - a record weight/puck adds damn nearly nothing to the rotational momentum of the platter. My Stillpoints weight is heavy, but doesn't affect start-up or speed stability one jot.

The only thing to be careful of is not to use a very heavy weight with the standard Technics bearing as it may wear prematurely. If you upgrade to a Mike New bearing, you need not worry about any real-world record weight.

[Gordon Steadman]

I told you to be careful with that band-saw

Why didn't I see that coming