1. Things you need to know about the new ‘Conversations’ PM system:

    a) DO NOT REPLY TO THE NOTIFICATION EMAIL! I get them, not the intended recipient. I get a lot of them and I do not want them! It is just a notification, log into the site and reply from there.

    b) To delete old conversations use the ‘Leave conversation’ option. This is just delete by another name.
    Dismiss Notice

How I redesigned the PU7

Discussion in 'd.i.y.' started by Luca, May 12, 2021.

  1. Luca

    Luca pfm Member

    Part. 1

    (I apologize for my bad english but it isn’t my mother language).

    In the 2005, I looked for information, on the net, about a rare tonearm the Syrinx PU3B. I own one since the 1990. A kind forumer replied me on vinyl engine. He was J7. We had an intense exchange of e.mails and found to have the same purpose: to design a toneam. I sent a drawing to him and he asked my help for the design because he needed a skilled drawer. After more than two years of work, J7 sent me the final result of the research: it was the PU7 "the product of the year award winner of HFW". :)

    With a lot of enthusiasm, I mounted it on one Sondek of mine for a first listening test. It didn't sound badly but I feeled that something was missing. So I compared it with the Syrinx PU3B. I own two identical Sondeks and for a first test used two AT95e. What was missing comes out at the fist listening test with evidence. The PU3B had a vivider, richer and defined sound expecially on the mid-high frequencies. I definy that defeat in that PU7 as "a lack of energy". Energy wasted for an undeterminated reason. I tried every kind of set up, several cartridges but the sound didn’t improve. I spent a lot of time in the two following years to solve that defeat ...

    I guessed it was due to some mistakes in the design as :

    discontinuity in structural parts of the arm;

    the choice of materials;

    shapes in the component parts;

    an excess of damping somewhere in the arm;

    or other ...

    The part 2 will follow soon so pls don’t reply before the post ending.
  2. myles

    myles Intentionally left blank

    Really interesting Luca, and good on you for sharing with us.
    Luca and Chops54 like this.
  3. stevec67

    stevec67 pfm Member

    Thanks Luca, I'm sure there are a few of us interested in this.
    Luca likes this.
  4. rbrown

    rbrown I think therefore I think I am

    Thanks Luca - I saw your posts on the LP12 Tonearm thread, so really interested in what you've done.
    Luca likes this.
  5. Luca

    Luca pfm Member

    Part 2

    A premise - I know two ways to control the resonances in a tonearm: (a very cheap and easy method) using damping materials as foam, silicon, oils or other liquids; (a more sophisticated and expensive method) wisely distributing the masses and carefully tapering the shapes.

    The PU3B is a masterpiece of this latter way. It is totally devoid of those damping materials but it sounds extremally controlled.

    It has every structural parts smartly tapered and the masses are wisely distribuited. This is the solution I prefer and adopted.

    My redesign started from the structural parts.

    The Pillar

    The PU7 has a pillar composed of two pieces of different material: a cylinder of aluminum and a piece of brass tube kept togheter by grub screws. All very cheap and of easy making. To avoid marks on the aluminum pillar a rubber pad is placed on the tip of the collar grub screws. It is better to remove those pads because they are dampers and give a weak and unstable tightening. I thought that a jointless piece would have been much better; the stainless steel has better mechanical properties than the aluminum, more mass and the screws don't engrave it. The aluminum is better if you want a metal of easier working.

    This is the raw and unpolished pillar I designed in the 2008. It is machined from a single bullet of stainless steel and is accurately internally tapered.

    [​IMG]pillar by Luca B, on Flickr

    The part 2 will resume soon so pls don’t reply before the post ending.
  6. Luca

    Luca pfm Member

    The Housing Bearings Bell (HBB)

    I saw handreds od tubolar HBB: shorter, longer, differently turned, of many colors, every one good as the others ... That was a the prove that a strong solution had not yet been found.

    I understood that a tubolar bell hasn’t an ideal shape. The surface of a solid of rotation (with a center of rotation) has a better profile than the surface of a solid of revolution (with a line of revolution).


    A solid of revolution destibuites mass around a line and not around a center (the center of the gimbal). A part of its mass would be where it doesn’t need. It'd give an uselessly heavier and voluminose HBB.

    A solid of rotation has a surface perfect for the purpose with a parabolic profile (the best tapering possible). This gives more thickness and mechanical resistance around the bearings and zero mass where it would be useless.

    In my HBB the parabolic profile is part of the central section of a perfect sphere. It is compact and not resonant. Not being uselessly huge, can be made of better metals than the aluminum.

    [​IMG]01_SFERA 3D fronte by Luca B, on Flickr

    I still have many things to show you. Please wait before you intervening.

    P.S.: my drawings are covered by patent. No one has the right to use them without my permission.
    Darren L, myles and JimmyB like this.
  7. awkwardbydesign

    awkwardbydesign Officially Awesome

    In the UK patented designs CAN be used for personal use without permission. So DIYers do have the right to use them.
  8. Luca

    Luca pfm Member

    Thanks for the info. The DIYers are welcome.
    I had excess parts made for my needs and am thinking of sending an arm or parts to someone of trusted expertise for a demonstration. I gave an arm to a friend, collector of high end equipment, who is amazed by the performance of the arm.
    Vinny likes this.
  9. Vinny

    Vinny pfm Member

    Levels of accuracy and finish are going to be WAY beyond 99.9% of DIYers, not least where there are no dimensions. :)

    I suspect that details of patent law are probably universal - a patent is infringed only when something that is patented, is sold/traded. (At best drawings might be able to be covered by copyright, not patent, FWIW.)
    Last edited: May 14, 2021
    Luca likes this.
  10. Luca

    Luca pfm Member

    I did a comparison by images among HBBs.

    [​IMG]HBBs comparison by Luca B, on Flickr

    Isn't this an evolution?
    Alan Chung, myles and Vinny like this.
  11. sq225917

    sq225917 Bit of this, bit of that

    Keep it coming Luca.... ;)
    Luca and myles like this.
  12. divedeepdog

    divedeepdog pfm Member

    :cool: Didn’t know there was pu3b either, is this the one known as Ipsissima?
  13. Luca

    Luca pfm Member

    The Tube Holder and the Rear Stud.

    I tried two kinds of tube holder: one with a screwed in metallic rear stud; another integrated. This latter is good for a 9 inches arm and the arms I built so far have it. The mass is adjusted by a metal sleeve. The vertical effective mass of the tonearm I will show you is 12 grams.

    The Counterweight

    I designed a counterweight composed of two parts: a core made of tungsten pressed into a stainless steel cap. The counterweight is decoupled by 2 o-rings fixed symmetrically around the barycenter. This solution allows to take the barycenter forward closer to the gimbal.

    [​IMG]Tube Holder and Counterweight by Luca B, on Flickr
  14. Luca

    Luca pfm Member

    I bought that tonearm ex-demo and boxed. Unfortunately the sleeve around the box was missing. For this reason, I'm not sure if was ipsissima ... I assume so.
  15. Luca

    Luca pfm Member

    I'm building the enclosure for my power amplifier. So I was busy last weekend. ;)
  16. Vinny

    Vinny pfm Member

    Not wishing to steal your thunder Luca, but one or two at least will be curious about some of this, even outside the earlier more technical bits.

    Barycenter, for anyone interested, is centre of gravity/centre of mass.
    Using tungsten allows for greater mass within the same size/geometry constraints, which allows the CBW to be nearer the bearing/gimbal, which reduces effective mass of the arm in total.
    Luca likes this.
  17. Bob McC

    Bob McC Living the life of Riley

    I’m waiting to see if he made a cigar shaped arm tube like the PU3 had and the PU7 doesn’t.
  18. Luca

    Luca pfm Member

    The tube I use isn't cigar shaped as the PU3 and PU3B but isn't foam damped as the PU3.
    Foam dumping absorbs energy and since energy = information in an analogic system the dumping by foam is always harmful.
  19. Luca

    Luca pfm Member

    The Arm Platform.

    The arm platform was shaped to keep the anti skating force along the best angle possible vertically and hozontally. The anti skating device is interchangeable and a small recess in a corner is its seat. I can choose if to mount it gravitational or magnetic.

    An imperfection in the PU7 is that the vertical angle isn’t correct. A too big pulley diameter gives a decomposition of the AS force which isn’t ideal.

    The Armtube and the Shell

    After considering various more or less exotic materials, I used a cheap aluminum tube. It is internally tapered close to the shell and

    is totally foam damping free. I put only a ring inside the tube close to the shell.

    I think that a replaceable shell is a good thing because doesn't exist that perfect for every cartridge.

    I hand made some shells with an easy and usual shape trying to find the one good. Then I drew pieces achievable with a CNC machine. I chose the one that best suited my favorite cartridges. It weighs 5.6 grams and is tapered to improve its stiffness.

    [​IMG]DSCN1986rr by Luca B, on Flickr

    [​IMG]04-DSCN1990 by Luca B, on Flickr

    [​IMG]DSCN1930tt by Luca B, on Flickr

    [​IMG]DSCN1975R by Luca B, on Flickr

    [​IMG]DSCN1965R by Luca B, on Flickr
  20. JimmyB

    JimmyB pfm Member

    Well that's just GORGEOUS!!!

    Love the cartridge choice too.
    Luca likes this.

Share This Page


  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice