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Old 21st January 2003, 11:54 AM   #1
EC8010 is offline EC8010  United Kingdom
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Default Unipivot pick-up arm

Other pick-up arms have been shown, so I thought I ought to share mine.

The unipivot is a cup of PTFE bearing on a Biro pen tip. (Lowest possible friction, and it won't rattle).

All adjustments are non-interactive, and can be locked off after adjustment. For initial setting up on a plinth, the arm length is adjustable (by about 5mm) using a screw, and then locked off by the countersunk screw visible on the top of the bearing housing.

The bearing and bias compensation are at stylus height to maximise rocking stability and minimise interaction. The centre of gravity is fractionally below bearing height to maximise the frequency of rocking (which makes it easier to damp).

Damping is different in the three degrees of freedom. Minimum damping up and down, and maximum damping for rotation and rocking.

The headshell was made from a block of 1" square section in order to make a tapered stub to go into the arm tube. My only regret here is that I made it before the Naim Aro, and didn't think to leave a side brace...

I made it because I wanted an SME V, but that cost 1000 (at the time), and I decided that for 1000 I could easily buy a small lathe, build the arm, and still have a lathe afterwards.

The arm tube is wired with fine copper litz wire in two twisted pairs, which is then wired to twisted pair silver in overall screen.

I'm very pleased with it.

PS I've just discovered that I've posted this in the wrong forum. How do I move it?
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Old 21st January 2003, 12:18 PM   #2
peterr is offline peterr  Netherlands
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Great looking tube amp!
Seriously, the arm looks great. Is it as good as the SME you didn't get?

To get this thread moved I guess you best mail a moderator.
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Old 21st January 2003, 12:48 PM   #3
EC8010 is offline EC8010  United Kingdom
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Default Unipivot vs SME V?

Who knows how it compares to an SME V? But the sound is good enough to have justified the time and expense (I've made lemons too). It's better than my previous unipivot, it's easy to set up, and it's even reasonably tolerant of sideways thumps on the plinth.
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Old 21st January 2003, 05:18 PM   #4
dhaen is offline dhaen  Europe
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Hi EC8010,

In case no one else did it, I've emailed Dave (Planet10), who moderates this forum, with a request to move the thread to "analogue".

I must say I'm impressed with the arm.

Cheers,
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Old 21st January 2003, 05:54 PM   #5
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Nice arm. Do you have drawings you would be willing to share?

dave

PS thread now in analogue
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Old 21st January 2003, 06:27 PM   #6
EC8010 is offline EC8010  United Kingdom
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Default Drawings

Thank you Planet 10 for moving the thread. I will be careful not to repeat the mistake.

No, I don't have drawings, but I could have. The only problem is that they would be in AutoSketch (baby sister to AutoCad) format. I'm quite willing to share, though.
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Old 21st January 2003, 06:59 PM   #7
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Default UNIPIVOT.

Hi,

Yummm,nice looking arm.

Have you tried heatshrink tubing on the armtube to tame (and lower) it's Q?

Cheers,
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Old 21st January 2003, 07:20 PM   #8
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Default Re: Drawings

Quote:
Originally posted by EC8010
No, I don't have drawings, but I could have. The only problem is that they would be in AutoSketch (baby sister to AutoCad) format. I'm quite willing to share, though.
if you can't i can convert that into gis or pdfs so they can be shared.

dave
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Old 21st January 2003, 07:27 PM   #9
EC8010 is offline EC8010  United Kingdom
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Default Re: UNIPIVOT.

Quote:
Originally posted by fdegrove


Have you tried heatshrink tubing on the armtube to tame (and lower) it's Q?

Cheers,
Not on this one (it just doesn't seem to need it), but I have on previous arms. Surprisingly, you don't need very much damping at all to tame them. Insulating tape (much lighter) is perfectly effective, and it doesn't even need to cover the entire tube. I found that a single layer ring, then a gap of equal distance, then another ring was quite sufficient.

It's possible that the reason this arm doesn't need damping is that the tube is not abruptly terminated at either end. The headshell stub has a conical hole so that the wall thickness tapers from <0.25mm to 3mm over a distance of about 25mm, and, as you can see, the bearing housing is also tapered.

Just for laughs, I've attached a picture of my very first unipivot. Sadly, this was taken by a Kodak Instamatic pointing into the light, without a flash. And if you think the quality is poor, you should have seen what it looked like before I tweaked it. The high-tech Garrard suspension can clearly be seen together with the Acrylic arm board. The honey damping isn't visible. (By the way, honey crystallizes after time.) The cartridge is a Shure M95ED.

The bits of metal lying front of the turntable later became a rather more sophisticated arm. The die-cast aluminium box is a mono valve pre-amplifier with passive RIAA. Lurking at the back is a Quad II.
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Old 21st January 2003, 07:35 PM   #10
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Default HONEY?YOU'RE KIDDING.

Hi,

Well,yes.
The better the arm the less damping it requires.

I always found the better unipivots to sound far more dynamic and live sounding.

Currently I use a couple of early Air Tangents which are,in a way, unipivots too.

The reason for the arm not requiring extra damping lies in the fact that a single point of energy drain is a much better defined path than any multiple bearing one.

The point and cup arrangement also makes it hard for any straneous energy to run back to the arm/stylus.

Cheers,
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