A unipivot made from scrap

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IslandPink
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#256 Re: A unipivot made from scrap

Post by IslandPink »

It would look cool, but given the HF bias of the energy coming out of a cartridge ( pre de-emphasis ) , and glass not being damped, it seems unlikely to be successful to me.
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izzy wizzy
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#257 Re: A unipivot made from scrap

Post by izzy wizzy »

Ant wrote: Sun May 30, 2021 1:32 pm A diversion, found some glass drinking straws while i was looking for the arm board insert materials. 300mm long.
I bought them because they were cheap, anyone know of an arm with a glass arm tube? Ever been done? Or just a silly idea.... :mrgreen:

I shouldnt be allowed out....
I made an arm with a glass tube but that was 40 years ago. It was a unipivot. I did better later with gimbal designs but that was my first forays into DIY armery. You're way more experienced than I was back then. It has some good properties like stiffness but relative to other materials vs weight?

Re the arm rest. The SME 3009 arm rest with its wire flip over lock looks doable and doesn't need flexi materials.
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IslandPink
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#258 Re: A unipivot made from scrap

Post by IslandPink »

While I was out for a walk on the hills it occurred to me that a perspex tube, a bit bigger than a typical metal one, might be an interesting choice, given its very good HF damping properties.
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#259 Re: A unipivot made from scrap

Post by Ant »

Well the deck and arm has been with our simon for a couple of weeks, i needed a second opinion on it, so asked simon to take it and put it in a completely different system to see if i was being overly critical, or the balance is off as i think it is.

It is.

and simons feedback on it has been very helpful, firstly in confirming that i am hearing what i think i am, and that in the context of a completely different system the characteristics are the same

And secondly that the potential for better if i can rework the balance of its sound is there.

So thanks go to simon, an orange juice at the next meet up is on me

Onwards!
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simon
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#260 Re: A unipivot made from scrap

Post by simon »

It's a really interesting arm I must say. I had to turn the bass amps down quite a bit and turn the tweeter amps up a smidge to get the balance right, but once I did the sound snapped in to place. The bass is immense, fantastic grip and control, and really deep. Overall I felt it perhaps tended towards being analytical rather than emotional to use some arguably meaningless words. Perhaps like the difference between SE and PP - neither is right or wrong.

I don't have heaps of experience with lots of tonearm, and of course the whole TT is different from what I'm used to, but this is perhaps the best arm I've heard, balance aside. It really sounded that good.
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#261 Re: A unipivot made from scrap

Post by Ant »

Up to revision 7 now

ImageCx unipivot rev:7 by anthony cresswell, on Flickr

Changes are brass for aluminium for the counterweight stub and cartridge mounting, and no damping at all in the arm tube.

ImageCx unipivot rev:7 by anthony cresswell, on Flickr

The bearing housing is the original one from the first iteration with the external wiring, so is the arm wand, the brass lateral weights are off the second iteration.
The arm pillar is brass too this time, and the outrigger assembly is also off the first one. No need to re make parts that are interchangeable.

ImageCx unipivot rev:7 by anthony cresswell, on Flickr

The bearing itself is abit 'better' than before too, its a little sapphire V jewel instead of being cut directly into the top of the pillar. There is no movement in it at all this time, the other version could be provoked into moving if you pulled on the arm wand. The pin is diamond ground to 50 degrees, I have a tool sharpener with a diamond grinding wheel which is perfect for forming the tip of the pin using a little jig I made to set the angle.

The pillar is much better than the last one, I've bored the end to 11.5mm to fit another Cardas mdin socket, had to get a little teeny boring bar to do it. Much fun was had scrounging up bits of flat stuff to shim it to the correct cutting height....... I'll get some steel and mill a shim to the correct height for it before i next use it. Much better than drilling it to 12mm and sleeving the socket up to size. The slot for the wiring has been milled on the new machine rather than being bodged in with the old machine

ImageCx unipivot rev:7 by anthony cresswell, on Flickr

Next job is to make a better arm rest than the last one which was marginal at best.

onwards!
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Ant
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#262 Re: A unipivot made from scrap

Post by Ant »

Oops
Over did it with the extra weight from the brass, ended up with a calculated effective mass of nearly 25g....

Balls. Already glued the cart carrier bit into the tube... bit of brute force will be required to get it back out
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Paul Barker
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#263 Re: A unipivot made from scrap

Post by Paul Barker »

Punch out with a drift through middle of arm. Sharp shocks will free it not the maximum force.

Else heat up the wand and spray co2 on the headshell. Pipe freezer, disposable can, is Co2.

If neither cracks the seal alone, combine.

In my job I have to get everything undone I can’t fail. These principles work and transfer to youre situation.

Biggest mistake people who fail make is just using strength. The skill is a secret missed by the shallow thinking stubborn Asses I encounter daily.

Pearls before swine.
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Ray P
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#264 Re: A unipivot made from scrap

Post by Ray P »

I would be wary of applying heat to the carbon tube you've used for the wand and anyway, IIRC, carbon fibres shrink when they're heated.

Has the glue cured - I assume it's epoxy so it may still be soft enough to pull out the brass headshell??
Sorry, I couldn't resist!
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#265 Re: A unipivot made from scrap

Post by Max N »

Maybe set the whole arm up in the mill and machine a bit of weight off the brass carrier in situ?
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#266 Re: A unipivot made from scrap

Post by Ant »

It was abit soft still ray, it hadn't fully cured, so i used some of pauls pipe freeze spray left over from when i fitted the rodi filter for the fish tank (good shout paul, forgot all about that) to get it to shrink abit, gripped it in the vice then twisted the tube to crack it off.

Took some off the stub that goes into the tube then drilled out the middle of it.

Now have a calculated effective mass of 12.6g which is rather better than it was
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#267 Re: A unipivot made from scrap

Post by Ant »

Finished the initial build of this version, version 7 I think

ImageCx unipivot rev:7 by anthony cresswell, on Flickr

It's on test at the moment, initial thoughts are actually much more encouraging than I'd hoped.
The glaring problem of the top end seeming recessed seems to have been sorted, although our Simons observations on the tilted up bass end are abit more difficult to fathom.

The bass seems 'better', not that there is more of it than I can remember, but more structured and tighter. Wether Simon would have to turn the bass amps down on his system I don't know, but my feeling is that the rest of the range is more in balance with it. I can't quite tell if the balance is completely right at that end as my big fanes start to roll off at about 45hz, and behave more like hugegreatenormous standmounts than Simons multi way system.

So, changes made to this version.......

From the top, the cartridge mount is brass instead of aluminium, and is epoxied in place rather than being a tight interference fit.

The fingerlift is about half the length it was as I wanted less of the brass rod to be excited and vibrate along with whatever the cart is doing. Not so easy to use, but we shall see how much of a pain in the arse it becomes

It has been assembled in a jig I made for it this time instead of by eye, so everything is much better aligned

The counterweight stub is now also brass, as a result, the wand assembly is marginally heavier than it was

The ring weight in the base of the bearing housing is a couple of grams heavier to compensate for the marginally heavier wand and the affect on the centre of gravity

The bearing pillar is now much better made and is brass again, otherwise its design is the same as it was

The bearing cup itself is now a teeny little sapphire jewel glued into the top of the pillar rather than being machined into the pillar

There is no damping at all in the wand anymore

The arm rest is much more useable, if a bit clunky looking, that can be sorted whenever

The bias guide has been reprofiled to make the string path better with less friction on it

So quite a few changes

ImageCx unipivot rev:7 by anthony cresswell, on Flickr

I do need to make a heavier counterweight as the current one is 115g and is hanging of the end of the stub, probably brass again as I don't want to end up with something substantially bigger. The calculated effective mass with an 8g cartridge is 12.6g, this will probably come down abit with a heavier counterweight closer in to the pivot point, I'd estimate it would come down to somewhere close to 11g.

The deck is taking a trip over to mi dads tomorrow, so we shall see what he thinks of it, and hopefully over to Simons again if he has the time to give it a listen. It was most instructive last time it went and definitely helped

Onwards!
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#268 Re: A unipivot made from scrap

Post by simon »

Looking forward to trying the latest iteration ant :-)
Ant wrote: Sat Jul 10, 2021 6:30 pm The glaring problem of the top end seeming recessed seems to have been sorted, although our Simons observations on the tilted up bass end are abit more difficult to fathom.
Mmmm, I'm thinking about this...
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#269 Re: A unipivot made from scrap

Post by Ant »

Its more difficult to fathom because i cant recreate the conditions in your system simon rather than understanding why
Problems with using my atypical system to develop something :mrgreen:
P'raps i should buy some commercial speakers at some point
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#270 Re: A unipivot made from scrap

Post by simon »

Well, I suppose it might not be your arm that's bass heavy, it might be mine that's bass light. That would skew things. I'll investigate...
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