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Arm & Cart xyz

Avon said:
It’s pointless getting another AT cartridge until the problem with the armboard is resolved. Cobbers had the same problem and resolved it.

I know nothing of the LP12 either, but can you not dampen the board with say Blu Tack? That comes in sheets which could be pressed on. If it’s likely to cause discolouration, could it not be pressed on from the bottom? It’s weight will displace the suspension perhaps, but at least it would point you in the right direction.
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Hi Avon, I agree entirely. Good point.

Well funnily enough I've been mulling this very idea, having recently bought a sheet of (fairly heavy) bluetak stuff. The idea at the time was put some in the CW sagging sorbothane.. but bits got lodged in the thread, & a sod to undo CW (panic ensued). Idea shelved.

So I could try this with the whole sheet of blutack instead, I guess.


Many thanks, Capt
 
@a.palfreyman Hi there 'freyman. Grateful for your post & thoughts, & although technical, I can actually sort of understand the essence of it. So it's when a resonance 'matches' from one thing to another, that problems can occur, with the page1 pair of tuning forks analogy image in my mind. I've got a bit closer to understanding resonance & frequency xyz information (which I still find very difficult, on the whole) from your post here.

Yes that photo is what I have, albeit an offset SME cutout not the centred linn hole. My mdf armboard says 'export' on, & look to be an even earlier type too. Up until this thread I had no idea that an armboard's material had -any- significance to the functioning of the TT. None. I assumed carbon fancypants ones for eg, were just vain Lp12 owners willing to spend £alot on something that merely looked 'snazzier' than your photo eg basic mdf one above.

Food for thought. Many thanks chaps, Capt
 
I think I got the bending stiffness formula wrong, thinking it was instead spring constant. Anyway:
The Captain said:
So it's when a resonance 'matches' from one thing to another, that problems can occur
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Yes, that is correct. The mdf plank that the arm mounts on is flexible and when you bolt the arm to it, the plank will then have a natural resonant frequency, just like the cantilever at the opposite end of the arm. If these are the same or very close, one will 'feed' the other. So the plank or arm board picks up the sound from the speakers and vibrates, setting off the cart because the cantilever has the same natural resonance.
Think about a washing machine as it slows down from spin. At a particular drum speed, the whole shebang shudders and dances around. That's resonance, set off when a particular drum speed (equivalent to frequency) is achieved.
Edit: the SME cutout is even worse because you only have maybe 15mm of material either side of the hole which makes the end even floppier...
 
Here's a recent interesting post on mdf versus laminated armboard:
colasblue said:
... The difference between laminated and MDF is huge. I found this out when I accidentally stripped the screws on my MDF one and got a laminated replacement from the dealers pretty much when they first came out. I had an Asaka on at the time and if you'd done a blind dem and told me I'd been upgraded to a Troika I'd have believed it.

The timing of the release of the laminated board was fairly shortly before the Cirkus and I think a hell of a lot of people who rave on about the Cirkus upgrade were mostly describing the change from MDF to laminated armboard....
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Although the DIY balsa composite armboard was a significant improvement for the PT1/774 I was lucky enough to get hold of an original honeycomb aramid one later which is what I have been using for years now.
 
a.palfreyman said:
So the plank or arm board picks up the sound from the speakers and vibrates, setting off the cart because the cantilever has the same natural resonance.
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I’m not convinced the problem arises purely through feedback from the speakers. That would be volume dependent. I think it could be that the armboard is only selectively damping vibrations from the cartridge.
 
You could be right. Just trying to offer ideas. I don't own an LP12 and haven't used one, but have read plenty about them over the years. I was reading something yesterday, written by @Mr Tibbs about the arm board (can't find it now.) Apparently if the underside bulges around the screw holes it can cause problems with stability on the sub chassis.
 
If trying to do something on the cheap I would either be trying to isolate the baseplate from the armboard by some means of rigid spacing to get the minimum contact area or DIY an armboard from 9 or 12mm ply (less than a tenner for either).
 
cobbers said:
If trying to do something on the cheap I would either be trying to isolate the baseplate from the armboard by some means of rigid spacing to get the minimum contact area or DIY an armboard from 9 or 12mm ply (less than a tenner for either).
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Hi Cobster,

that's a credible idea: I could knock up a 9mm birch ply one ok, not as a permanent board but just to see if this has any effect. If it does.. then I'm on the right path & the £45 laminated one a feasable option (still alot of extra £ for me tho). If it does nowt, then I think I've done all I can/ & will have to finally conclude AT's just don't work in my set-up, even if we have no idea as to why that should be.

Quite a whole-wknd job though, sigh. Many thanks chaps. Capt
 
Avon said:
I’m not convinced the problem arises purely through feedback from the speakers. That would be volume dependent. I think it could be that the armboard is only selectively damping vibrations from the cartridge.
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True, this issue is not volume-related. I thought the armboard resonance thing though wasn't including the speakers, but an interaction only between the cantilever/ arm/ armboard.
 
Only other option I can think of is if you can add say 50g (2ounce in old money) to the back of the arm board, preferably behind the arm. That'll change the frequency of the 'plank'.
 
I remember when I first bought the 774 to fit a 11X900 i knocked up an armboard out of a chunk of pine shelving (the armboard it came with was an ally one for a Rega - it was good enough to convince me the 774 was a keeper).
 
a.palfreyman said:
Only other option I can think of is if you can add say 50g (2ounce in old money) to the back of the arm board, preferably behind the arm. That'll change the frequency of the 'plank'.
Click to expand...

Understood, I have a 150gm pack of bluetak.

Changing the frequency. Now although I can't understand how anything inert, which is making no sound, can be having a 'frequency' (I can only understand the most basic analogy, the two tuning forks both of which make sound; but anything more 'technical' & Im as lost as I am with words like 'credit', 'finance' or 'value' in the world of economics)..

..wouldn't the 'change' of frequency you mention here, just shift the problem "up a bit" ? IE the boom perhaps instead of happening around certain bass areas (I can't put a finger on any specific 'note' or even 'area of notes' the current issue affects mind you: it seems to happen fairly randomly, recording to recording, simply within the lower half of the audio spectrum) just happen in slightly different ones. IE the issue remains there.

Thanks Capt
 
Here's another analogy for you @The Captain. Think of the armboard as a being akin to a plank, something like a diving board. If a thin (read light) person bounces up and down on the diving board, it will vibrate more quickly than if a fat bloke (like me) were to bounce up and down on it. Your arm is essentially the 'thing' on the plank (armboard) because it is mounted to the armboard behind where that is screwed to the sub-chassis. So to change how fast it (armboard) bounces, you have two options. Make a stiffer armboard as suggested by @cobbers as this will vibrate faster, or add weight to it, which will make it vibrate slower. Either way, all we are trying to do is see if changing the way the armboard reacts stops the 'boom'. Adding weight is easier/simpler than making and fitting a new armboard.
 
Akin to when I had a Systemdek - but I had a second one at one time which someone had loaded with kg upon kg of damping material it just killed the life in the music.
 
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