401 bits

301, 401, plinths and assorted idler stuff
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Greg
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#31

Post by Greg »

Hi Ant,

You can work it out from this. See page 13. The rest is useful anyway.

http://www.arrakis.es/~igapop/Garrard.htm

Regards,

Greg
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#32

Post by cressy »

cheers for the link greg theres allsorts in there.

so abit more info on the plinth i'll be building, it'll be 4 layers, top 10mm perspex, layer 2 another sheet of 10mm perspex, 3rd layer, bloody great rubberwood butchers block and bottom 30mm slate kept after i sold off the lenco. the idea is that the perspex will absorb certain frequencies and transfer the rest, (plus look nice) the butchers block absorb a different set of frequencies and transfer the rest and the slate will absorb and disperse the rest as it sits flat to the shelf its on so will disperse into the walls of the alcove. wether it works is another matter............ having done the slate thing before i decided against it simply because of the time it took to do the lenco one and the time it took to get it right once built. tbh it was awful when it was first done and a right pig to cut and finish and tweak. and soon to have 3 kids, time is a precious commodity :roll: i.e, i havent got any!

the arm will not be connected to the layers, it will be on an acrylic and slate tower (still have the offcuts from the lenco plinth) that sits on the slate bit but is decoupled by a layer of rubber, and as it will overhang that base by around 30mm, a single spike will ground it into the shelf. that way, any vibration from the motor will have to get all way through the slate to the shelf to get back up the arm pillar, plus the arm can still use the mass rather than the layers of the slate to kill any untoward vibration.

if my reasoning is correct anyway.........................

idea behind that is so that the plinth deals with the decks vibrations, and the arm tower deals with the arm separately.......hopefully
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Greg
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#33

Post by Greg »

Sounds interesting. I look forward to the updates.
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Nick
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#34

Post by Nick »

the arm will not be connected to the layers, it will be on an acrylic and slate tower (still have the offcuts from the lenco plinth) that sits on the slate bit but is decoupled by a layer of rubber, and as it will overhang that base by around 30mm, a single spike will ground it into the shelf. that way, any vibration from the motor will have to get all way through the slate to the shelf to get back up the arm pillar, plus the arm can still use the mass rather than the layers of the slate to kill any untoward vibration.
Why would it have to go all the way up and down? if the platter/motor vibrates relative to the arm the cartridge will see it, doesnt have to travel anywhere.

I know we all talk about energy transfer and stuff, but all we hear from the cartridge is based on how the stylus moves. Isn't it?
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#35

Post by cressy »

i think that the cart will pick up any vibrations that get through the stylus, so if those can be sunk away, and the arm itsself isnt subject to vibrations transmitted through the plinth into it, that things could sound errrrrr 'clearer' so to speak.
when i was faffing with the idler i made out of the lp12 and lenco, the motor wasnt sunk to anything, it vibrated and ruined the whole thing as it transmitted this into the platter and the cart picked it up. that was an extreme case, and i shelved the whole thing. having had time to mull it over, i reason that if that vibration can be directed elsewhere into the plinth instead it'd be quieter. one thing i noticed with the last incarnation of the lenco plinth is that the deeper into the slate the spikes from the top plinth went into the bottom one the quieter it got.
i think it was sort of transmitting vibrations from the top plinth straight across the top layer of the slate of the bottom plinth and up the leg nearest to the arm base. so to avoid that i thought if the only path for motor vibrations to get up into the arm was under all the layers there'd be a better chance of it being dissipated. if that makes any sense
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#36

Post by simon »

I might be able to help you with the template Ant. I'll get back to you.
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pre65
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#37

Post by pre65 »

Good luck with your plinth Ant !

I have a feeling that several layers of BBP or just the slate on it's own might be preferable !

After the recent discussions prompted by Bunsen (where is he ?) I might be tempted to try the lightweight sandwich type plinth if I was to build another one.
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#38

Post by shane »

Nick wrote:but all we hear from the cartridge is based on how the stylus moves. Isn't it?
No, it's based on relative movement between the stylus and the cartridge body. This means that motor vibration can result in output from the cartridge in two ways: firstly, vibration fed into the platter will move the stylus relative to the cartridge body, and secondly vibration reaching the arm will move the body relative to the stylus. The combination of the two will obviously have complex interactions due to the differing path lengths and the vibrational modes of the bits in between, and I suppose there could be instances where the two cancel out, but I think Ant is right to treat the two as separate problems to be tackled.
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#39

Post by cressy »

it certainly a thorny problem to look at, to keep the cart relatively still and the stylus wiggling. its been in the back of my head for a while, ever since i took the lp12 lenco idler up to simons a few years ago and it got comprehensively destroyed by both simons 401 and that jvc direct drive. the lash up lenco i brought to owston a few years ago was an idea i had to use as many different materials as possible in a plinth. simply because i thought at the time that differing material densities and properties could keep unwanted things out of the way of the stylus. a very crude way to go about it, and not very successful tbh, but one of the ideas i had that i thought was worth persuing.
dr bunsen honeydew had a valid point with the idea of the torlyte plinth, the idea to very quickly sink vibrations away from important areas into something else has got my grey matter working again. but i suppose it also depends what it is sunk into. using something that doesnt store energy so to speak to transfer it away and into some other object or material would work in principle, but the energy has to go somewhere, so where?

of course this is probably all bollocks, but i suppose its better to take an idea and run with it rather than dismissing it straight away without having a good think first :lol:
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#40

Post by Nick »

shane wrote:
Nick wrote:but all we hear from the cartridge is based on how the stylus moves. Isn't it?
No, it's based on relative movement between the stylus and the cartridge body. This means that motor vibration can result in output from the cartridge in two ways: firstly, vibration fed into the platter will move the stylus relative to the cartridge body, and secondly vibration reaching the arm will move the body relative to the stylus. The combination of the two will obviously have complex interactions due to the differing path lengths and the vibrational modes of the bits in between, and I suppose there could be instances where the two cancel out, but I think Ant is right to treat the two as separate problems to be tackled.
Err, yes, maybe I should be more explicit in future, when I said the stylus moving I did of course mean relative to the body, and not to some absolute inertial frame of reference. Thats was the point I was trying to (and clearly failed) make. Motion of the arm and platter in itself is not important, assuming one end of the cantilever is attached and referenced to the arm, and the other the platter, what matters is how the two systems move in relation to each other.

Just to be overly picky, I guess what actually matters is how the coils move relative to the magnetic field lines in the generator.

What I believed Ant was describing was the platter attached to a support that allowed the platter to move relative to the base, and the arm attached to another such system. The goal being that vibration in one system was isolated from the other. I was trying to point out that (at least as I see it) given it was the relative motions of the two systems that mattered, it was possible that instead of isolating the two, maybe it could be better to allow the two to be tightly coupled, and then attempting to damp vibration of the single system.

Though, I wonder how much acceleration of the entire system would be required for the inertia (and thus associated damped resonant system) of the free parts of the cartridge to contribute to the output?
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#41

Post by cressy »

ive edited this post as what id put before didnt explain what i wanted to say, but i think ive grasped it this time.......................

i think i see what you mean nick, so very simply, if everything is tightly coupled and the lot (arm, cart motor unit) vibrates to the left then the stylus will see the initial acceleration to the left, and when the vibration changes direction it will 'see' the inertia for a split second as it changes direction and swings to the right. so it should only do something relative to the vibration of the unit for a miniscule amount of time and this shouldnt be heard.

wheras what im describing, if the 2 can move freely, and the motor unit vibrates left theres nothing to say the arm is doing the same. it could be vibrating the other way, or up or down and causing either less or more movement at the stylus. which on a perfect deck would only be moving as it traced the groove.

of course perfect isnt possible and there are so many factors to consider that are so far beyond my meagre capabilities that my brain hurts :roll:

so, i think i'll follow nicks advice and tightly couple it to the base first
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Nick
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#42

Post by Nick »

Yep, that is the point I was trying to make, that is not to say that its true, but just something to consider.
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#43

Post by Nick »

i think i see what you mean nick, so very simply, if everything is tightly coupled and the lot (arm, cart motor unit) vibrates to the left then the stylus will see the initial acceleration to the left, and when the vibration changes direction it will 'see' the inertia for a split second as it changes direction and swings to the right. so it should only do something relative to the vibration of the unit for a miniscule amount of time and this shouldnt be heard.
Well, it will be the inertia of the moving part of the arm (or at least the moment of inertia, I should have paid more attention in those applied maths lectures about simple harmonic motion :-)) so more than just the stylus.
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#44

Post by shane »

Assuming that we're talking about motor vibration here, which I would hazard a guess would be from 50Hz upwards, I wouldn't have thought that the turntable/arm assembly would be moving as a solid entity with no relative movement between the various parts. At that sort of frequency, it's more likely to be shaking like a jelly.

It would be interesting to put a turntable into one of those hologram vibration observation devices that Rank and Celestion used to use to study vibration modes in speakers, but I don't suppose anyone's got one knocking around!
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#45

Post by Mike H »

Who was it suggested listen with screwdriver against lug-ole?


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