Tonearm For Lenco L-70

Variant 1 of Magnetically Damped, Filament Suspended Tonearm or "The Home Depot Tonearm - v 1"

A Schroder-like tonearm, built by a clueless woodworker.

I have put in about 40 hours so far in designing and fabricating my first attempt at a tonearm based on the suspension principles patented by Frank Schroder.  Since I work primarily in wood, the majority of the resulting parts are executed in wood or lucite, and the metal and other parts are so far limited to off-the-shelf bits of hardware from:

 Home Depot -

 (5/8 inch joining nut -[11 tpi] for the main tonearm body segment

and 2.5 inch 5/8 bolt for the counter weight stub and shaft

1/4-20 threaded inserts and bolts

or Lee Valley -

1/2 inch Neodymium magnets

1/2 inch steel magnet cups

a small scrap of African blackwod

a 2-foot length of cocobolo

and Sport Check -

20 pound test fishing leaders

the main support structure and fixed magnet support were fabricated from a turning square of California Claro Walnut burl, while the top and bottom support plates of the moving assembly were constructed of 1/4 inch tough lucite left over from the bottom layer of the plinth.  The main tonearm wand was made up of laminated cocobolo and ebony (with two tonearm wiring channels inside the arm), turned to allow the arm to be threaded into the joining nut at the main body joint.

The tonearm is still incomplete - no wiring, headshell mount, or azimuthal balance system.  However, the main construct is well balanced before cartridge mounting, and exhibits many of the properties touted for the Schroder.  Effective mass with the cartridge mounting plate will be about 20 gm.  This could be reduced to 17-18 gm if I was to rework the counterweight and mounting stub using a more conventional counterweight concentrating more mass nearer to the suspension centre of the tonearm. (I might have to beg some help with the metal components from my brother, who is into metal lathes and milling machines and other high tech stuff.).

The challenges of working this type of design into the low profile deck and platter of the Lenco L-70 were severe, and I learned a lot about the precision and finesse that must be inherent in the real thing. It looks simple, but there are some critical and subtle parameters involved, some of which I have yet to encounter or deal with, but which are covered in general in Frank Schroder's FAQ on his tonearms.. For an intermediate wood turner, it was a big challenge working to the exacting tolerances required. (Lots of trial and error).

At any rate, the first pictures of the tonearm at first flight (ie. finally getting it suspended) are shown below

I have set it up so that the integral arm lift of the Lenco works with the arm, and I think it actually looks rather good for a DIY tonearm.

More to come ...

Update 1 - the 1/4 inch lucite top and bottom plates were not stiff enough, and when the magnet spacing reached about .5 mm, the plates bent enough for the magnets to make contact.  To get closer spacing and keep the structure stable, I had to add a stiffening plate between the top and bottom lucite plates, as shown below:

The next step involved making the cartridge mounting plate out of lucite with a single embedded stud of 1.5 mm cartridge mounting bolt centred over the stylus mounting point, with standard 1/2 inch mounting holes drilled 7/16 th of an inch behind. The design places the stylus along the mid line of the arm.

The mounted cartridge was too heavy for the 5/8 th inch bolt to counterbalance, and the axial imbalance of the cartridge on the mounting plate caused the tonearm to tilt slightly.  The solution was a proper counter weight, this time using a very dense wood called African Blackwood which is sold in small irregular chunks in a junk box at my local Lee Valley store.  All of the disassembled pieces of the tonearm are show below:

The assembled tonearm mounted on the Lenco now looks like this:

Just waiting for the tonearm wires and cartridge mounting clips. I have done some preliminary setup and tests without the wires to set up the VTA, the VTF, and the tonearm azimuth.  There is some variation in the tracking force across the disk width, heavier at the outside edge and lighter at the centre, but the azimuth seems to be constant and in proper balance.  With my AR balance and weights I can only weigh within .5 gm, and the variation appears to be about half of that.- i.e tilt to the balance moving across the width.  This will require some experimentation to see if I can tweak this effect out of the arm, or minimize it.

The tweaking required was more significant than I thought at first, as it was necessary to remove the bottom magnet support and make it thinner to fit with the low profile of the Lenco platter, and still allow the tonearm to track absolutely flat and parallel with the stylus on the record.  Getting everything about the magnet assembly parallel in tracking position was several hours of re-work.  In the process, the counterweight proved to be too large in diameter in the new lower position and it to had to be reduced in size, and then this caused the azimuth to change .... several iterations to get everything back to working order.  I finished off by drilling the tonearm wire exit holes, just past the arm lift, and threaded manget wire through the arm to act as a puller for the tonearm wires.

The Cardas tonearm wire and clips finally arrived 8 days after I ordered them, and I got the wires and clips installed and pulled through.  I installed all of the colour-coded shrink  tubing on the cartridge tags and really should have cut these in half.  (oh well maybe next time).  The cartridge wires were then routed through the arm and into an Altoids tin that I used as a small chassis for the phono  jacks.  Some concern had been expressed on the Lenco forum about capacitance of the tonearm leads.  Measured capacitance from the cartridge tags to the phono jacks and through 3 ft of interconnect to the preamp is about 150 pF.   More tweaking as the arm azimuth became unbalanced again.

I went through some serious levelling and counterweight balancing and tonearm wire routing trials to get everything back to working position again.

There was some hum pickup in the phono preamp, but it did not seem to matter whether the turntable was plugged in or not (after a lot of debugging). It turned out to be from a tape monitor connection to my USB sound card (through 6 ft of interconnect). Disconnecting the soundcard solved the problem, and the turntable and tonearm and cartridge are quiet. Hum and ground loops are weird.

Finally - time for a test flight - and it worked amazingly enough. 

Great bass, very smooth overall, and clean treble.  No tracking troubles (it managed all of the tracking test bands of the SR-12 test record at 1.5 gm with no problems at all), but the flimsy kids play table I have the turntable on temporarily while I work on the shelf is touchy with the springy wood floors in my office.  I have spent far too many hours working on this instead of listening to music, and hopefully I can get back on the listening rather than doing side of things soon.