©2001 Jeremy Epstein. No reproduction, retransmission or web distibution without prior permission.
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teres turntable |
Since building my Ladegaard arm and widely touting it on the internet, I have been asked a few questions on my implementation of the arm in emails from interested DIY'ers.
It seems like a FAQ is in order. Here are some questions I have answered so far:
What air pump did you use?
For the air supply to the arm I used a RENA 400 aquarium pump, which is rated as "80-160 gallons." This pump is adjustable and has two air outlets. I use both outlets turned up to full pressure. The pump is not as quiet as I would like so I built it into a cavity in a nearby wall, which renders it nearly silent.
Do you need a smoothing tank?
A smoothing tank is necesssary (at least, with the RENA pump, it is) and I made mine from a 2' length of 4" PVC plumbing pipe, capped at both ends, with two inlets for the pump feeds and one outlet for the supply to the arm.
What did you use for the air lines?
I used 1/4" white plastic tubing for all air lines, with 1/4" brass compression fittings for all connections. These fittings and this tubing can commonly be found in a kit for installing icemakers in your refrigerator at the hardware store. I did not find the softer, clear tubing sold for aquarium use suitable - it appears that the softer tubing wastes some air pressure as heat when it flexes, in any case, the pressure seemed to be slightly less with this tubing somehow.
What is the arm tube made of?
I used .24" carbon fiber tubing for the arm tube, available at a kite-builder's site somewhere on the web. It was dirt cheap, something like 3.00 US$.
How is the headshell implemented?
The headshell is a small pice of aluminum bar stock cut into a "T" shape. Two holes are drilled in the crossbar of the "T" for the cartridge mounting bolts, and a cylindrical recess is filed along the upright of the "T" to mate with the arm tube. It is attached to the arm tube with epoxy.
What tonearm wiring did you use?
My first tonearm wiring was Kynar-insulated 30 AWG copper, available at Radio Shack in small spools. I twisted each channel pair and then wrapped the channels seprately around the arm wand in opposing spiral wraps, held in place by small pieces of tape. Near the knife-edge pivot, there is a strain relief. from the strain relief, the wires arch up to another strain relief, in a "gantry" which is attached to the turntable base. At this point, the twisted pairs converge and are shielded inside a braid covering. The Kynar wire continues unbroken to the RCA jacks at the preamp end of the cable. Wire dress is VERY important: the wires must not exert too much side force on the arm or you WILL hear distortion and perhaps have tracking problems and skipping. Also, by locating the first strain relief right near the vertical pivot point, the wires have very little leverage to affect the VTF.
More recently, I tried using thin silver medical probe wire. This is 40AWG wire with a Teflon coating available from A-M Systems (http://www.a-msystems.com/physiology/products/wire/coatedsilverwire.asp) It is difficult to work with but it completely eliminates the problem of side force on the traveler so it was worth the effort.
How did you create the air outlet holes?
I made the air outlet holes in a strip of Mylar tape which is dimensionally stable so far. It has no adhesive, so it is held in place on the back of the air manifold by a "window frame" of PVC electrician's tape.
How did you make the counterweight?
The counterweight is a small lead casting. I drilled a flat-bottomed hole in a wooden plank with a Forstner bit and pored molten lead into the hole. I used lead wall anchors for the material and a propane torch to melt it. (It was much, much easier than I thought it would be.) I drilled a 1/4" hole through the lead casting and this is a press-fit on the .24" carbon-fiber tube. Downforce adjustment is made by sliding the weight along the arm a little bit at a time.
What alignment procedure do you use for setup?
Level the turntable before you begin. (See below.)
To set up the arm geometry, I first made a template from heavy cardboard which consists of a 1/4" hole (for the spindle) bisected by a radial groove (made with an X-acto knife.) Then I drew some lines perpendicular to the radial groove which are tangent to the record groove.
The first adjustment is overhang. Set the stylus in the groove near the outer edge of the platter. Fix the platter in position temporarily using a piece of tape. Now cue the arm over to the inner portion of the platter. If the arm extends past the groove, it is too short. Adjust the length and repeat until the stylus is in the groove at both ends of its travel. When the overhang is adjusted correctly, adjust the cartridge body to be parallel to the groove tangents you have drawn. If the stylus is not exactly parallel to the cartridge body, make any necesssary correction so that it will be parallel to the groove tangent. Now, adjust the azimuth - I use a mirror and set the stylus down on it to see this alignment. Finally, recheck overhang and make any necessary adjustments. After the geometry is set up, adjust the downforce using the counterweight and a stylus pressure gauge.
Did you run into any problems?
I have run into a few problems in getting the arm working properly.
First, be sure there are no air leaks anywhere in the system. A great way to check for leaks is the technique gas line installers use: make a solution of soapy water and liberally wash the areas you want to check with the solution. Any leak will immediately reveal itself by blowing a bubble.
I found that Ladegaard's system of using double-sided tape did not work well with the foam tape I was using (perhaps he used a different type of tape?) I took small pieces of double-stick tape to locate the front and back of the manifold together, then ran a bead of silicone bathtub caulk around the edge to seal it. (I wanted to do it in a way that I could take apart should I have problems.) In retrospect, since I have not had to open the manifold since getting a proper seal in place, I would use something more permanent and rigid like epoxy.
Likewise, ensure that you are not leaking any air in the pump/smoothing tank/supply hose system. The compression fittings need to be cranked down pretty tight, for example.
Mechanical alignment of the manifold is very important: it must be exactly parallel with the record's surface. Any slant of these two relative to each other will cause major headaches.
As mentioned above, tonearm wire dress is very important. You can check whether the tonearm wires are exerting a side force after levelling the turntable, by lowering the stylus onto a mirror. If the arm sldes in either direction, the wires are pushing the arm to the side, and you must make sure that this is fixed. I have had the best success by making the gantry very tall and by having the tonearm wires arch almost straight down to the tonearm wand.
The way I built the arm wand assembly, it is very lightweight, which was necessary because I was building it to match a very high-compliance (soft suspension) cartridge (a cheap Grado.) If you are using a low-compliance (stiff suspension) cartridge, you might find that the arm's moving mass is too low. I have also tried a Denon DL103D and for this cartridge I built another arm wand assembly. To make it heavier I may wrap around the tonearm leads with shielding foil, killing two birds with one stone, and/or fill the interior of the arm wand tube with something like blu-tak. Also note that adjusting the mass of the counterweight will have an effect on the vertical moving mass (moment of inertia.) A lower counterweight mass results in a larger moving mass, because of the following relationship.
For a given balance condition, mass (m) x distance from the pivot (d) is a constant. However, moment of inertia varies as the square of distance from the pivot. So, a weight that is half as heavy (.5m) will balance at twice the distance form the pivot (2d). The original moment will be md2, whereas the smaller weight will work out to (.5m) * 4d2 = 2md2, or twice the original moment of inertia.
Finally, take your time and double check the set up procedure outlined above. Any tracking problem, skipping, etc, is probably attributable to errors in geometry, insufficient pump pressure, or pull from the tonearm leads. Adjustment and set up are as critical for a linear tracker as they are for a pivoted arm. However, with a linear tracker, you can get it right! It can be made perfect for every point on a record's surface, and I hear a huge improvement which I attribute to this. No longer does the "focus" change across the album side: the last track is as well-balanced and symmetrically dynamic as the first. If you hear any asymmetry (one channel may have less "life" than the other, for example) double check azimuth and tangency. Old records from the early days of stereo with simple hard-left, hard-right, and dead-center instrument placement are excellent diagnostic tools for making this assessment - all three locations should be equally dynamic and the center image should be dead center, stable, and well focused with little spread to left and right.
Any tweaks?
I have found that there can be a bit of a zingy sound, a HF resonance I guess, which can be softened by applying damping material such a Blu-Tak to the surface of the knife edge bearing, just above where it touches the carrier. This has to be very thin at the bottom so it does not touch the carrier when the arm pivots back and forth.
I think there are possible benefits to be obtained by damping within the arm tube. With the Grado I needed to keep the arm's moving mass very low so this sort of experiment was not really possible. Now I am using the Denon DL103D cartridge which is much lower compliance and so permits a higher moving mass, so I plan on trying this out.
Revised 2/28/2002 (Added info on silver tonearm wire)