Wanted an(other) arm

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vinylnvalves
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Re: Wanted an(other) arm

Post by vinylnvalves » Thu May 21, 2020 11:48 am

Technically a very sound decision. I had an MG1 arm a few years back - it sounded okay at the time on the Cranfield Rock but wasn’t really suited to a suspended turntable. Wish I kept it as would have been interesting to hear it on the hyperspace. Are you going to run it off the aquarium pump directly or via a recuperator (tank). I know someone who put a pressure switch inline which was via some logic connected to a solenoid which lifted the arm in case of bearing air starvation, to protect the stylus.

Ant
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Re: Wanted an(other) arm

Post by Ant » Thu May 21, 2020 12:03 pm

Well I was debating about using a tank to smooth out the pulses, the builder says that he doesn't use a tank with it and instead uses a 10m length of air line which is long enough and has enough give in the tube walls to negate the pulsing from the pump.
I'll go with that initially as it eliminates a potential source of pressure loss. Last thing I want is an air leak and a busted stylus.
I can always add a tank later if need be
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JamesD
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Re: Wanted an(other) arm

Post by JamesD » Thu May 21, 2020 12:26 pm

Different and interesting :D

Looking forward to reading about your adventures with it!

Ant
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Re: Wanted an(other) arm

Post by Ant » Thu May 21, 2020 1:36 pm

We shall see.......
I learned a few things when I used the transfi arm, first and foremost that the deck itsself needs to be perfectly, and I do mean perfectly level in both planes before even attempting to set anything else.
I couldn't get the jbe level enough as the feet weren't adjustable at the time so I had to rely on shims.

This time however, the deck will be built around the eds1000 motor and this arm rather than bodging the arm onto an existing deck so I can address that.

Second, applying bias by angling the air track very slightly upwards towards the platter spindle so that the carriage tends to try to travel back towards the outside of the record is not the best idea, as it puts the azimuth slightly off and alters the vta as it gets closer to the centre of the record. So I'll not be doing that, it's also impossible to tell how much you are putting on as there isn't any way to measure it accurately
I might rig up a falling weight if bias becomes an issue

Thirdly, dressing the wiring needs particular attention paid to it so that it doesn't hang up and stop the carriage traversing

Next, the air pressure needs to be tweaked gradually to set it up. Being ham-fisted with this bit gets you nowhere

And lastly, to have patience. Which I don't have much of....
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JamesD
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Re: Wanted an(other) arm

Post by JamesD » Thu May 21, 2020 3:17 pm

Just remembered that I have an EDS1000 in the store too! Now I'm really looking forward to reading about your build :D

James.

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shane
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Re: Wanted an(other) arm

Post by shane » Thu May 21, 2020 3:47 pm

Isn’t one of the benefits of a parallel tracker the fact that it doesn’t need bias? I’ve not used one myself, but I’d have thought that if the stylus is at a true tangent to the groove, there should be no inward force to counteract.

Ant
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Re: Wanted an(other) arm

Post by Ant » Thu May 21, 2020 4:24 pm

Yep you're right but there will be a small amount of Inertia from the carriage once it gets going, not much, but it may need addressing. I couldn't say with this one as the carriage is different to the transfi arm
The pl l800 linear tracker I had used the lateral force of the groove and some built in play in the mechanism to activate micro switches, which in turn activated the motor in the sled to make it track across the record. It had about a degree of intentional movement in the wand either left or right. This is what activated the microswitches to speed up or slow down the motor to allow it to react to the record. Off centre pressings were interesting to watch, it didn't like them much. Its control circuit basically tried to keep the arm wand at 90 degrees to the rails it travelled on, thus is sort of 'crabbed' across the record in a series of small shallow arcs rather than travelling in a purely linear manner
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shane
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Re: Wanted an(other) arm

Post by shane » Thu May 21, 2020 5:42 pm

Ok, my take on the mass of tangential arms is this. Shoot me down if I’m wrong, as I’m just theorising in the back of my head now.

As I see it, lateral inertia shouldn’t be too much of a problem if you look at it from the resonance point of view. The resonance of a conventional arm/cartridge assembly is usually roughly the same vertically and horizontally, and is ideally about 10Hz to put it below the audio band but above the sort of frequencies where it would be excited by warps. Below resonance the cantilever/arm assembly is effectively rigid so as the arm rides over warps the cantilever isn’t significantly deflected. Horizontal deflections are mostly caused by eccentricity and so are generally around the 0.3Hz area, ie about 30 times lower than the highest warp frequencies. This means that the horizontal effective mass of the arm can be up to 30 times greater than the vertical effective mass before you start running into problems with big lateral stylus deflections. The horizontal effective mass is of course the same as the total mass of the arm and the carriage because there is no rotation involved.
A greater problem is making sure the actual arm itself has sufficient effective mass to keep the vertical resonance low enough. This is why they quite often have relatively small counterweights quite a long way behind the vertical pivot.


Does that make sense?

Ant
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Re: Wanted an(other) arm

Post by Ant » Thu May 21, 2020 5:56 pm

I'm sorry I haven't a clue :shock:
I've never thought about it
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Ant
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Re: Wanted an(other) arm

Post by Ant » Thu May 21, 2020 6:09 pm

Thinking on, that makes sense as it is essentially rigid in the lateral plane, only the air cushion the carriage rides on is lossy
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