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Thread: tangential arm design

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    Default tangential arm design

    I'm slowly getting all the bits together to test proof of principle for a tangential arm I have designed. It is based on the premise that a 'normal' arm (although with straight head shell) can be nudged along a linear track without upsetting the reproduction from a cartridge. I have made a moveable arm pod and shown that it is possible to move a Rega 250/cart along side a turntable, so changing the bearing/pivot distance, without any detrimental effects being heard. [Ultimately there will be problems as the geometry changes].

    The arm pod is nudged along its track by another pod moved by a stepper motor and lead screw. Position (and degree of nudging) is determined by a laser beam and photo-detector, all run with a simple controller (not a microprocessor device).

    I was wondering if J7 had contemplated such a device?

    Here's my idea:


    Last edited by cat's squirrel; 30th November 2016 at 15:09.
    cheers.....Bryan

    http://qualia.webs.com/

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    Good luck with that Bryan. What's the next step?

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    well, I've ordered loads of stuff, so the first thing is a simple prof of principle, by getting the stepper motor chugging along, and nudging the arm pod. As the arm (a highly modified Lenco L75) is allowed to wander in the vertical and horizontal, exact positioning is not necessary, as long as the nudger keeps nudging! I think that getting the controller to work is important, too. I hope I haven't complicated the whole thing.
    cheers.....Bryan

    http://qualia.webs.com/

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    Is your design much different from the Japanese-made linear tts of the 70s?

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    I hope mine will work!

    The main difference is that I wanted to separate (de-couple) the stepper motor drive from the arm itself, so that any vibrations from the motor would not be felt by the arm. Essentially, the arm sits on a movable pod which slides on a dual rail. The drive pod moves on another rail and lead screw, driven by the stepper motor, and the pod just nudges the arm pod along.

    If my calculations are correct, and all the parameters such as step angle and screw pitch are correct, the driven pod will nod the arm pod 10 microns every 0.8 seconds. I hope this will be 'silent' enough.

    The position sensing is not new, and was patented by Pierre Clement in 1968, I have read. All I have done is 'modernised' it a bit with a laser beam instead of a light bulb.
    cheers.....Bryan

    http://qualia.webs.com/

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    I'd suggest Bryan that you don't want any 'nudging' whatsoever, I'd think that a constant moving solution is more what's needed, any nudging will show up in sounds or measurements, stepper motor would still be best for this as should be easier to control at a slower constant speed, especially given that there's not much mass to move.

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    Ps given that records have different groove spacing, how will you overcome the different speeds needed between records and when gradually decreasing circles, perhaps that's why most decent sounding tangential arms are dragged through the groove rather than servo driven.

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    As in conventional tts, isn't it the spinning record with its continuous grooves that moves the arm along?

    See, you've lost me technically already lol.

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    maybe nudge is not the right word. The stepper motor I have chosen has 200 steps per revolution. The pitch of the lead screw is 2mm, single start. It is estimated that each step will progress the pod by 10 microns, and for a 20 minute record covering about 80mm of track, this will mean 6.6 steps per second from the stepper motor, hopefully below the resonance frequency of the arm/cart.

    I see the stepper motor pod nudging the arm pod continuously, rather than discrete steps, although in reality, it will be discrete steps, albeit very small ones. The exact position of the arm/cart over the track is monitored by a laser beam generated by laser diode on the arm pod, and detected by a photocell on the nudge pod. A small mask on the arm will cut off the laser light when perfectly aligned, and send a signal to the controller to advance the pod when misaligned. This should happen about 6 or 7 times a second. This way, the cart/stylus should always be at a perfect tangent to the track of the record, regardless of track widths.

    By doing it this way, the poor cantilever is not called upon to drag all that mass across the record, as normal arm designs do. And, of course, there is no need for bias compensation.
    cheers.....Bryan

    http://qualia.webs.com/

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    getting a bit nearer to construction. It is now in Mkii form, although only in design stage, but most of the bits have been assembled. Control will now be with a Raspberry Pi Zero computer, and the arm will not use a Lenco arm, but my own duopivot design. The turntable will be based on a Lenco GL72, which is a light platter design, but may be modified to accept the heavy platter and bearing, now with ferrofluid bearing lubrication and bearing clamp.

    All this (arm, control and turntable) have to pass 'proof of principle' first, before serious assembly.
    Last edited by cat's squirrel; 29th November 2016 at 19:27.
    cheers.....Bryan

    http://qualia.webs.com/

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    I am pretty sure that during the heyday of tangential tracking arms there was some measurements that showed that due to the way they moved the overall tracking error was more than a conventional arm,but if you can get one to track smoothly you are on to a winner,most of the older ones seemed to go in steps from being slightly behind the record to being slightly ahead due to slight overshoot to behind to ahead again this seemed to be the norm,bit like the older servo controlled dd turntables that never seemed to run at the correct speed,always slightly both sides of correct,one thing you have in favour in your design is arm length,due its length any errors will be smaller,especially compared to designs like the revox,where basically the arm length was the length of the cartridge + a little bit!
    Last edited by steve195527; 30th November 2016 at 01:13.

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    Mmmmm. . . That sounds a bit like a broken watch. If it's stopped it tells the right time twice a day, whereas one that is running slow or fast......

    It's never crossed my mind previously about getting a parallel tracker to be set up 100% correctly!!
    You can only be young once, but you can be immature for ever.....

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    thanks Steve,

    I can understand what you have mentioned, about being just behind or just in front. That must have been a feedback issue coupled with a speed to adjust issue. That's why I decided to use a Raspberry Pi (GHz region) and lasers, and a stepper motor, which, if my maths is correct, can be incremented, linearly, in 10 micron steps.

    The long arm reduces error, but is more for a simpler design. The important difference in my design from others is that the stepper motor is separated from the arm, hopefully lessening vibrations from the motor. I have had some experience with stepper motors and their programming, so I hope that will help in sorting out feedback timings.

    As for time, Wullie, as it's measurement is derived from human intervention, no-one knows the real time, it has to be defined, and even two atomic clocks don't agree on the exact time, whatever that is! Any audio design has to be good enough, and no more, IMHO.
    cheers.....Bryan

    http://qualia.webs.com/

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    How are you getting on with this Bryan?

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    Quote Originally Posted by TIU View Post
    How are you getting on with this Bryan?
    not at this moment, I had to put it away for Xmas! I have changed my ideas on the arm, which will now be a duopivot design, but most of the rest remains the same. Control electronics will use the Raspberry Pi or similar, and maybe two or three lasers.

    This is not the same as most linear trackers, where a firmly fixed arm moves on a linear track, and pushed along it by the poor old stylus/cantilever. Mine could be said to use a moving arm board, something I have tried using conventional geometry, and it seemed to work well. The duopivot is similar to the Conqueror Tonearm, and I have a quote for the same bearings as it uses! Just need a Lottery win!
    cheers.....Bryan

    http://qualia.webs.com/

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    Don't we all, lol.

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    Just buy this Bryan - the Bergmann Audio 'Sindre'. Only 9k second hand.

    bergmannsindreaspect.jpg

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    not sure I could justify the price! And there is nothing like doing it yerself, and getting it to work well.

    That turntable doesn't have the aesthetics I look for, and would, in truth, better the rest of my system by some way! However, I hope to finish the high resolution power amplifiers (even ordered the pcb's,) the super tweeters and bass bins/class D amplifiers (300 watt each).

    I remember the first computer I built (by soldering in the chips, etc.) and then using it to control stepper motors which controlled two wavelength changers on a piece of lab. gear. Nothing like the feeling of 'I did that'.
    cheers.....Bryan

    http://qualia.webs.com/

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