PT mini-anni instructions

[tim_bissell]

I've posted my instructions for building a PT mini-anni on the web; quite a lot of work!

http://web.me.com/tim_bissell/PTMA

Not completely finished yet, but some people might find them interesting.

-- Tim

[rolleye]

Let me be first to congratulate you Tim, an astounding piece of work....easy to read and follow and even an idiot like me could follow it....great stuff

[abril]

And for those of you not confident putting a PSU together ,my K1823 PSU recently went pop
finished up buying this to replace it
http://www.amazon.co.uk/gp/product/B000NJ3TKG

one hefty bit of kit.

PS
great job

[tim_bissell]

Thanks Guys!

I forgot to mention that you can click on most pictures for a bigger version - some open in-place, most open in a new window (I will fix it over time).

Also, the diagrams are done in Google SketchUp; I'll put the model into the SketchUp warehouse some time, and will make the files of individual parts available some time.

I picked up a proper Thurlby Thandar bench PSU from eB*y for about the cost of parts for the PSU described - bigger and uglier, but the digital voltmeter and ammeter are quite cool!

-- Tim

[ProfTournesol]

What we are doing here (if I understand it properly) seems to be mass loading the plinth (with MDF) and therefore changing the rigidity and resonant properties of the plinth. Arthur used MDF for commercial reasons (cost, easy to machine) but I imagine that we could also use alternatives like acrylic, even stone if it could be machined at an affordable cost. It would seem then that whilst using end-grain balsa would be easy as it could even be cut with a knife and hand carved, it would certainly increase the rigidity of the plinth but not necessarily add much mass. I say not necessarily because this also depends on whether you use any feet under the PT. I've added an end-grain balsa baseboard with some considerable improvement in sound however I think that this is because the turntable now sits directly on my rather heavy isolation table and this effectively increases the mass of the plinth as it is now coupled. When I've added isolation feet (decoupling it from the stand) the benefit is removed. Therefore it also makes sense to me to couple the turntable to your isolation stand by removing any feet and sitting the baseboard directly on a high mass table or stand.

Also using contemporary ideas of constrained layer damping, I wonder whether even adding a layer of rubber sheeting or even aluminium or acrylic between the MDF etc would be an advantage?

[tim_bissell]

Sounds interesting; rather than 4 x 15mm MDF, you could interleave 3 x 5mm acrylic / delrin / something and 3 x 15mm MDF.
I have my PT on sorbothane feet as it is shelf mounted; I have suspended wooden floors everywhere, so my turntables are shelf-mounted, so high mass is not an option for me... I'll have to try without the feet and see what the effect is.
I think with the extra weight of the achromat and 180g vinyl, the front left suspension point of my PT Vector is reaching the limits of adjustability; what is needed is a modified subchassis shape with the suspension points moved to gain more leverage at the front and rebalance the spring tensions. This means a new plinth, and would be a chance to try out these new ideas. If I can track down someone who can profile and cut a nice piece of timber or other material to make a nice box (I like a traditional shape; not so keen on oil-rigs)

-- Tim

[SimonLogan]

Outstanding job Tim; well done and thanks for all your effort in writing this up for others.

Simon

[ProfTournesol]

Tim I found this page on a constrained layer damping plinth for a Thorens. It would be good to do something like this for a PT.

[natty_dredd]

Wonder what a box filled with sand and lead shot with 'floating' platforms for the suspension would be like.. Although I suppose the rear mounting would be stuck where it is without a completely new design.

There's that heavy lead lined asphelt stuff too for roofing, if they still make it, that could be a layer.

[tim_bissell]

Hi Prof - I can't see the link in your post - have I failed an intelligence test

Natty,

My simplistic view of t/ts is that we have two units, and two sources of vibration; in PT terms:

Subchassis; motor and bearing

Plinth; traffic, neighbours

We need vibration sinks for both these units. For the subchassis, we reduce the vibration sources as much as possible, and (as in the PT Anni and Vector) use the bulk of the subchassis material (balsa) as a vibration absorber.

We can't control external sources of vibration, so we should build stands/shelves/plinths to absorb or reflect away as much as possible.

The connection between the sub chassis and the plinth should allow as little vibration as possible to travel from plinth to sub chassis (most likely in either direction) and should be damped to avoid feedback loops.

I guess if you want to go the sand/lead shot route, then a rigid t/t like a Vector or Saffire would be the best 'sub chassis' to go on top. I wonder what Arthur's 'Kinetic Cradle' was going to be?


-- Tim

[ProfTournesol]

oops! Here's the link.
http://www.theanalogdept.com/150_plinth_r2.htm

[natty_dredd]

What is the kinetic cradle thing? A special platform for the Saphire etc?

I did mean a kind of sand box for the suspension to sit on rather than mounting the subchassis directly on it.

I consider the deck to be just the subchassis really and the rest is no more than part of the stand with springs for isolation.

So the plinth needs to provide a way to use up energy at the frequencies that can still pass through the springs. Either use up or reflect or translate to another frequency that won't pass through the springs.

A very massive plinth would probably reduce the amplitude of any vibration at low frequencies that might pass through, just via conservation of momentum. And a massive plinth with some in-built lossy or decoupling may also mean that higher frequencies can't pass all the way to the springs either.

[ProfTournesol]

I've been inspired to think about this ever since I went to a talk last year given by Mark Doehmann, the designer behind the Continuum Caliburn turntable. What constitutes the plinth depends on whether you couple or decouple: for instance using a balsa baseboard sitting directly on my heavy support table couples the masses together, effectively creating a more massive plinth. As an experiment I added some HRS decoupling pucks under the baseboard, isolating the turntable from the support table and decreasing the mass, the sound was noticeably worse, less bass, less precise imaging.

From this clumsy experiment, I'd suspect that a PT can benefit from increasing the mass of the plinth, adding it inside the box is one way to do it, coupling the existing plinth to a heavy support stand or platform is another. Of course there are issues like footfall and vibration isolation to consider, but probably wall mounting something like a slate shelf would be the best type of platform for the PT as long as it is coupled to it, not isolated by rubber feet.

The sub chassis and platter are low mass and so won't store much energy, the idea should be to keep the energy out of the sub-chassis

[rolleye]

i think you're right here Prof, AK always said PT's sounded better without the baseboard and rubber feet and with the plinth directly coupled to a 'heavy' support.