make your own g88

[Vinyl-ant]

The knife edge is at 90 degrees to the arm tube but the headshell is at an angle. the arm goes straight up and down over warps, this motion describes an arc as it moves. Because the angle of the headshell is not the same as the angle of the knife edge, the vertical angle of the stylus alters at the same rate as the arc the vertical movement describes. so the azimuth changes. If a record is flat, it doesn't alter as the arm is not travelling up or down, if it is warped, it alters depending how bad the warp is. The bigger the vertical swing, the bigger the change.
This is why most arms have the vertical bearings set at an angle relative to the arm tube. That angle is the same as the offset angle of the headshell.
Altering the pivot height changes the rate of the angle change. If it is too high or low, I. E the arm tube is not square with the record, it will make the azimuth alter more for the same vertical travel

[scotty38]

Thanks Ant, "vertical bearings set at a relative angle" sounds like the bit I was missing......

[Vinyl-ant]

It's interesting to look at different types of arms because they all have different sets of compromises.

J shaped arms behave one way eg the 3009, s shaped arms without offset bearings like the robot arm on my jvc behave differently to that, s shaped but with offset bearings like the acos gst1 more like a straight arm, and straight arms with offset bearings like my audiomods different again.

Then there is the AT1100 which has its headshell offset to the right of the arm tube which puts the stylus on the centreline of the arm tube. And has offset bearings, and also puts the pivot point on the same plane as the stylus with the vertical bearings being set lower than the arm tube. The audiomods doesnt offset the headshell so the stylus is to the left of the centreline, which puts it more towards j shaped arm behavior, and the bearings are higher than the stylus so the vta alters more than the at1100 over warps and doesn't counteract warp wow.
Not all straight arms work to the same set of compromises either.

Another to put in the mix is a single point bearing arm like the mayware which pivots in whatever plane it wants so is yet another different barrel of monkeys.

This trades off the stability of the gimbal arms for the theoretical ability to not affect azimuth because it is not tied to a fixed movement range. The instability of not fixing the vertical plane movement however will affect azimuth. So building in enough stability is key to getting a unipivot right. The usual way to do this is to have all the weight lower than the pivot point so the lost stability is regained. But that then makes it much more susceptible to warp wow. Very tricksy to design out. That's what the moerch unipivot tries to do by having a thin light arm tube high up and a low counterweight, with the pivot point close to the record surface plane. This then makes it difficult to mount on a deck unless the platter is tall.
That's what I was working on with the arm I was building. I figure that centralising a fairly large mass in the bearing housing so the centre of gravity is just a teensy bit below the pivot might allow me to find a better set of compromises. Possibly.

[scotty38]

Very interesting but you couldn't help making it more complicated could you

[Vinyl-ant]

Yeah but it's interestingly complicated.

[savvypaul]

Yeah but it's interestingly complicated.

It's very simple, ime.

I hand you the arm and cartridge and you ring me when it's ready

[Latteman]

^

[scotty38]

[Vinyl-ant]

Well, this deck has gone to a new home straight away again, guess I'll have to make another one
My supply of donor lencos is rapidly running out....

[Latteman]

Hi Ant- looking at the 2 plinths you have made they are not as deep as the one housing my 59- why do they have to be so deep- vibrations?