A unipivot made from scrap

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Ant
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#181 Re: A unipivot made from scrap

Post by Ant »

The thinking behind using carbon fibre for the wand this time comes from the previous iteration.

The wand on that was stainless and hardwood, and was relatively heavy. I had to compromise on the pivot height with it and have it set higher than the top of the arm wand, so the pivot point was quite a bit higher than the stylus, a'la the mayware f4.
Thats also the reason that the bearing housing was made from steel on it, so that the mass was centred around the pivot to compensate for the high pivot point and actually raise the centre of gravity up higher, closer to the pivot.

With this one i wanted to get the pivot lower, so the less weight there is in the arm wand and the less weight above the pivot in the bearing housing the better, hence a lightweight arm wand. as the pivot is 4mm lower than the arm tube, it needs to be. Even so, the majority of the mass without any of the weights is higher than the pivot point making it unstable.

So, to make it stable while having the pivot fairly close in height to where the stylus sits (about 12mm above it so abit higher than a rega), the counterweight stub is centred on the same plane as the pivot so the heaviest bit is at a neutral point, so doesnt affect the lateral balance. The lateral weights are on the same plane as the stylus, and the ring weight, which is only 12g is at the lowest point of the structure.


The ring weight i added to the bottom of the housing tips the assembly into being stable, putting the centre of gravity about 4mm lower than the pivot. Or it would if the mass was arranged in a linear manner, which it isn't so in practice its probably lower but working out where exactly it is will need further brain power.

The parts above the pivot weigh in at about 86g (half the counterweight, half the stub and half the bias arm are included), parts below weigh in at about 104g, same again for the c/w, stub and bias arm as these are all in line with the pivot point, so half the weight above and half below assuming there arent any heavier spots inside.
The actual weights themselves (lateral balance, their stubs and the ring) come in at 30.64, so they are just enough to make it stable, and sufficiently light so that it returns to stable laterally nice and quickly if its upset.

A heavier wand meant with the earlier ones that it took longer to re stabilise when upset.

Anyway, thats the thinking, in practice, we shall see.
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Ant
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#182 Re: A unipivot made from scrap

Post by Ant »

The silicone tubing and the wiring arrived today, so a 100mm piece has been shoved up the arm tube to damp it, and the arm wand and pillar epoxied together.
Needs to be left for 16 hours to achieve its full strength, so ill leave it overnight before doing anything else

With the tubing in the wand to damp it, the effective mass has gone up to 8.8g, putting it between the project 9cc and an rb series arm. About what i wanted, ideally it would have been around 9.5 to be smack in the middle of the range i was after, but as is its about right for any of my carts.

I could have put in a longer bit of tube, but didnt want to add too much mass above the pivot

The vm series carts are quoted at 100hz, so is the 33 and the dynavector, interestingly all at 10x10-6 dyne so all will be somewhere around 15 to 17 ish at 10hz. Quite why the japanese manufacturers do this im not sure, but with this in mind, the vinyl engine resonance calculator puts all of them in the green zone.
Not a 103 though....... :mrgreen:
It would definitely be interesting to get hold of a grado prestige black 2 same as mi dads to compare with his 3009 though
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#183 Re: A unipivot made from scrap

Post by IslandPink »

I'm still working through the previous email - it's obvious that you gave it a lot of thought !
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#184 Re: A unipivot made from scrap

Post by Ant »

Got it up and running

ImageCx mk5 unipivot by anthony cresswell, on Flickr

Bias guide outrigger could have done with being longer, there's not much clearance between the guide and the left hand lateral balance when the arm is in the clip.

The clip needs redesigning to have a bit more to rest the arm on when its not clipped in

The wiring needs some sort of little clip on the top of the bearing pin to route it through, as it is it, the hole for it to come out of the wand is about 10m forward of the pivot so it tries to push the arm over as it rotates which alters the azimuth. if its held above the bearing pin it stops it from doing it.

Setup of the cart was simple, vta was simple, these are pretty standard methods of adjustment, setting the azimuth was an arse until I realised it was the wiring that was changing it as it moved. Bias is via a 2.5g nut with some string through, I couldn't be bothered to make a weight today, and I figured the weight would want some adjustment so best to use something to get it where I wanted it and then make that weight to the eventual figure.

A pretty small list of fixes
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ed
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#185 Re: A unipivot made from scrap

Post by ed »

Awesome Ant...

You made me remember something I thought about some time ago...Is there any likelihood we'll see some sort of RF link between the cart and the pre-amp...in order to do away with all the clutzy wiring.
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#186 Re: A unipivot made from scrap

Post by Ant »

fixed (bodged) 2 of the issues

ImageCx unipivot bodge by anthony cresswell, on Flickr

Simple fix to the bias guide problem, just put a longer string on the bias weight and swung the guide out further. Sorted.

second fix which is a bodge, just got a little bit of 4mm I/D air line tubing and cut a slit in it. It sits over the bearing pin which is m4, the wiring is held in place by the rubber and it goes straight up above the centre point. Sorted.

not sure if a little rf sender would work with such small signal levels ?
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simon
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#187 Re: A unipivot made from scrap

Post by simon »

This is all well and good Ant, but we all want to know it sounds :-D
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#188 Re: A unipivot made from scrap

Post by Ant »

Dont really know yet, it was not particularly impressive yesterday with its issues, the changing azimuth being the one needing particular attention. Wonky azimuth isnt going to do the records or the stylus any good so i will reserve judgement until ive fixed that properly.

I also need to see how much bias is being applied by the 2.5g weight, i can use kogens tables to work that out. It looks ok, but the numbers need looking at to be sure.

I need to make a permenant part that can be threaded so it screws down over the pivot pin and holds the wiring in the centre. I havent listened to it since bodging the bit of air line tube onto it to see if it worked

We shall see
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Nick
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#189 Re: A unipivot made from scrap

Post by Nick »

ed wrote: Thu Mar 25, 2021 3:40 pm Awesome Ant...

You made me remember something I thought about some time ago...Is there any likelihood we'll see some sort of RF link between the cart and the pre-amp...in order to do away with all the clutzy wiring.
Put me down as £5 on no.
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ed
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#190 Re: A unipivot made from scrap

Post by ed »

Nick wrote: Fri Mar 26, 2021 9:00 am
ed wrote: Thu Mar 25, 2021 3:40 pm Awesome Ant...

You made me remember something I thought about some time ago...Is there any likelihood we'll see some sort of RF link between the cart and the pre-amp...in order to do away with all the clutzy wiring.
Put me down as £5 on no.
it's almost worth having a go then......

but seriously, I was tricked into the thought by remembering the beeb documentary about attaching gps transceivers to bees in order to track their migration paths.
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vinylnvalves
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#191 Re: A unipivot made from scrap

Post by vinylnvalves »

You could always get rid of the clutzy wiring with the Keith Monk approach of a mercury bath.. adds a little bit of damping too as an added bonus. Unfortunately what was acceptable in the 1970’s from a COSH perspective isn’t acceptable now. The wiring looks fine to me
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#192 Re: A unipivot made from scrap

Post by Nick »

vinylnvalves wrote: Fri Mar 26, 2021 11:02 am You could always get rid of the clutzy wiring with the Keith Monk approach of a mercury bath.. adds a little bit of damping too as an added bonus. Unfortunately what was acceptable in the 1970’s from a COSH perspective isn’t acceptable now. The wiring looks fine to me
You could use Galinstan instead.
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vinylnvalves
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#193 Re: A unipivot made from scrap

Post by vinylnvalves »

Nick wrote: Fri Mar 26, 2021 11:08 am
vinylnvalves wrote: Fri Mar 26, 2021 11:02 am You could always get rid of the clutzy wiring with the Keith Monk approach of a mercury bath.. adds a little bit of damping too as an added bonus. Unfortunately what was acceptable in the 1970’s from a COSH perspective isn’t acceptable now. The wiring looks fine to me
You could use Galinstan instead.
Yes - go down Boots and buy a dozen thermometers. Would need to be careful on what materials it comes into contact with, eats through aluminium nicely from recollection.
Ant
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#194 Re: A unipivot made from scrap

Post by Ant »

Ok, so with the bodges in place I have a few observations of it.

This is running into a project phono box se which im not entirely familiar with, but is necessary because of where it has to be set up. Cart is the spare dynavector dv20x2h

It's is very even up and down, it doesn't seem to be emphasising the top end, midrange or the bottom end, although I can hear a little bit of colouration in the mid. But this is compared with the air bearing arm which has none. At all. So wether someone else picks it up will be interesting.

It seems very clean and ordered, especially when things get complicated it seems to tease things apart. Not necessarily clinical, but heading in that direction.
Bass has some really good texture, and voices seem to be nuanced.
It seems to be tight and fast and clean.

Strangely, it seems to make the dv really quiet in the groove, pops clicks and scratches seem quieter that they do when its on the air bearing arm. Not sure why.

Handling wise, I took the lessons from the earlier iterations wrt where and how the bias guide is located to make it very stable. How it's set up means that for it to rock in either direction it has to pull the bias weight up, so when you cue it, it immediately centres itself once you let go of the fingerlift and doesnt rock at all. It doesn't do that little shimmy that other unipivots I've used do when they have a little wobble before settling down when the are cued.

So pretty natural sounding, a little bit of 'character' in the mid if im being charitable, and very stable.
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#195 Re: A unipivot made from scrap

Post by simon »

On the face of it Ant that sounds very good :-). But you don't seem overly impressed??? Just the way it comes over to me.
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