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Well, I see that I did not bring over my project description from the Facebook page so I'd better start putting it in here. Please consider this note as a spaceholder for the article.

In general, I hoped to save money in the long run by buying a platter-and-bearing set and a linear air-bearing tonearm and putting together a turntable with performance representing a product that cost much more. I don't know if I achieved that, and I should put a better cartridge in it to find out! However, I'll add my long, drawn-out story hear in time.

_________________
- Guy

Introduction

As a member of a DIY club, I was inspired by other members’ projects to present my own project. Other club members had built turntables, and I thought I’d try. However, I didn’t have access to a machine shop, so I had to find other ways to get crucial parts, namely a bearing-and-platter assembly and a respectable tonearm. Internet searches turned up a manufacturer that provided platter-and-bearing kits, and another manufacture that produced nice-looking tangent-tracking tonearms. This article describes how I built a wooden plinth, a motor enclosure, and an air-supply system to combine those two subassemblies into a turntable that performs better than a commercial turntable that costs the same as my investment in the project.


Platter

An Internet search yielded a company called Choir Audio (http://www.choiraudio.com) in Georgia. John Parker at Choir Audio designs and markets his own turntables. As part of his custom manufacturing service, he offers a platter and bearing kit called the CAK 1.1 Audiophile Turntable kit (see Figure 2). This consists of a high-carbon-steel bearing assembly with ceramic ball bearing, Delrin thrust plate, and stainless-steel main spindle topped off with a 1”-thick platter made of 6061 aluminum billet. I exchanged e-mails with John, who freely offered advice and opinions that help make my project a success: for example, we discussed other platter materials (such as Delrin), drive systems, grounding, platter mats, etc. By the way, this platter was designed to use a mat – do not run it without one! The kit cost $685 including shipping.

John believes that strength, weight, and rigidity are important to turntable building, and this was reflected in the fact that his bearing required a 1¼“ mounting hole, top-plate thickness between 1¼“ and 1¾“, and 3” of clearance below the upper mounting surface for clearance. This was incorporated in the plinth design.


Tonearm

I could have bought a new or used commercial tonearm, but I’ve always been intrigued by tangent-tracking arrangements. Another Internet search revealed Ada Lin’s web page (Advanced Analog Audio Lab, http://www.adanalog.com), which offers an air-bearing, tangent-tracking tonearm that he makes and sells for $659 including shipping (see Figure 3). It had a low-mass slide-arm tube weighing only 25 grams without cartridge and counter weight, and a core-arm tube of carbon fiber bonded to precision-machined 7075 aluminum. It was leveled by three pointed setscrews, and had 1.4” of vertical tracking angle (VTA) adjustment monitored by a digital readout. The air-bearing portion required a low-pressure air source (minimum 2 psi at 15 liters per minute), which added another level to the project, and is covered in the construction description.


Copyright 2012 Guy W. Riffle. All rights reserved.

_________________
- Guy

Plinth, Part 1

Many questions arose when I tried to start building the turntable: how big should the plinth be? What materials should I use to make the plinth? Should the plinth incorporate a suspension system? Should the platter-drive motor be mounted on the plinth or on a separate box? What the heck is a plinth?

Starting with the last question, John Parker said, “A plinth is the enclosure or stand used to support your platter and bearing assembly. This will be the foundation of your design. Please remember that the plinth design will need to be executed in such a way as to offer complete stability. Stability can be achieved through a build that offers both weight and precise construction. If the plinth is not stable and level, the entire project will suffer.”

After struggling to choose a direction, I realized I had to start somewhere that might not have the optimal solutio, but would give me a starting point for future comparisons. I decided to make the plinth out of layers of birch-veneered plywood. A suspension system was too complicated to consider, so I skipped it and hoped that I could isolate the turntable in other ways. The platter was designed for belt drive around its perimeter, and John Parker said, “If you mount the motor and drive pulley very close to the platter, then you will need to consider the reduced wrap angle of the belting material and the reduced surface area that can be used to retain tension on the belting material. This could cause very irregular speed control because of slippage.” For this reason, the motor was mounted in its own box separate from the plinth.

How did I answer the question of how big to make the plinth? Well, it should probably be bigger than the platter and tonearm, but not so much that I couln’t fit it on a shelf. An interesting source online at http://www.vinylengine.com/tonearm_database.php described mounting requirements for many commercial arms, and I started to find a plinth size that would support as many arms as possible. Then I realized that I probably wouldn’t change the tonearm anyway, so I just made the plinth big enough for the Lin tonearm. In the end, I made the plinth 16” wide and 14½” deep, as shown in Figure 6. These dimensions increased by 1½“ when the plywood plinth was wrapped with ¾“ oak.

I chose birch-veneer plywood because it was available, had more plies and less voids than regular plywood, and provided a finished surface when assembled. Voids could cause resonant cavities in the plinth, and should be avoided (so to speak). By using a 2’ x 4’ sheet of plywood, I could get all my pieces with at least one manufactured edge to allow square trimming later, as long as I didn’t mind joining two pieces to make the bottom layer. See Figure 4 for a cutting plan.

Figure 5 shows the five pieces of plywood cut to size. The two narrow pieces were slotted for gluing biscuits and glued together, giving me four equal-size pieces. The fourth biscuit was missing because the section of seam it would occupy was cut out to provide access to the bearing’s torque nut.

I drilled the two top pieces to receive the bearing, which was designed for a 1½“ plate. I put blue painters’ tape on the wood and marked my layout lines on the tape so I wouldn’t have to sand off the layout lines later, and the tape reduced splintering when sawing and drilling. Before drilling the bearing hole, you should lay out the tonearm mounting holes that key off the center of the platter. The tonearm came with laminated templates that are harder to use once you replace the bearing center point with a big hole! I had to go back and cover the bearing hole with painters’ tape, locate the center again, and then locate the tonearm mounting holes.



I laid out the two top pieces carefully using the factory edges for reference and marked the bearing hole 5¾” from the back and 6¼“ from the left as laid out in Figure 6. I drilled the two holes separately on a drill press because it’s easier to drill thinner pieces then thicker – the bits get very hot when you drill thick stuff. I used a 1¼” hole saw because I didn’t have a Forstner bit that big, and, in my experience, a spade bit usually makes a horribly rough hole – good for rough carpentry but not for cabinetwork.

Copyright 2012 Guy W. Riffle. All rights reserved.

_________________
- Guy

Plinth, Part 2

Only the top two plywood layers supported the bearing (remember, the bearing was designed for a 1½” plate), so the bottom two layers needed an access hole cut to allow my hand in to tighten the torque nut. On the bottom two plates, I laid out a 4” square centered on the bearing location (remember to measure twice, cut once!). I drilled four 1” holes centered ½“ in from the square’s sides at each corner, and cut out the square with a saber saw. On the bottom plate, I routed the edge of the square using a 3/8“ round-over bit with guide bearing to make the edge smooth (see Figure 7).

All four pieces were glued together using normal yellow wood glue and a wood-glue applicator roller, as shown in Figure 8. The roller applied a thin, even layer of glue between each piece. I used a factory-cut edge to line up the panels – the other edges were later trimmed to be even. Once all four pieces were stacked together, I put the bearing in the holes in the top two pieces to line up the hole. I then clamped the four pieces together, removed the bearing, and made sure to clean off any glue from the bearing. You can never use too many clamps!

I had to provide a means to level the platter, and many people in the club swore by isolation spikes or cones to control vibration. If I could mount a trio of spikes (easier to level than four) on the bottom of the plinth, I could use them to level the platter and hopefully isolate the plinth from some vibration. I found a set of speaker spikes that were threaded to screw into a speaker at Parts Express (see Figure 9). I could replace the threaded studs with longer ones, drill holes through the plinth from top to bottom, and use a screwdriver to adjust the spikes from the top. You can see the mockup results in Figure 10 in the next posting. Look at the matte finish and bevel on the platter! I still used painters’ tape to locate measurements, and I tried to center the spikes immediately around the platter – two in the back and one in the front. This kept the spike adjustments out of the way of the tonearm, but I found that the plinth was very tipsy with this arrangement – the mass of the plinth was about the same as the mass of the platter and bearing. After seeking suggestions from club members, they pointed out that two spikes on the side facing the motor pod would stabilize the plinth against the pull of the belt, and the likely position of the third spike was opposite the first two. That’s the arrangement I used (see Figure 6), and it was much more stable than my first attempt. If you look in coming plinth figures, you can see where I plugged the first spike-mounting holes.

The threaded rods I bought for the spikes were long enough to stick through the top of the plinth. Instead of trying to find a way to dress the rod holes and carry a screwdriver, I decided to top the rods with knurled handles and check nuts. I bought four sets (in case I lost one) from Carr Lane (through Novatool Inc. in Baltimore) for a total cost of $33.02. The parts were CLM-6-KK Knurled Knobs and CLM-6-TSJN Check Nuts, which were an inch around. Once the platter is leveled, I will hold the knobs and twist the check nuts down against the plinth top to hold the adjustment. I thought about super-gluing the knobs in place, but permanence scares me so I put locks nuts in the tops of the knobs and tightened them against the rods inside: Pretty and functional!

You can see in Figure 8 that the plinth edges toward the camera weren’t perfectly aligned. Once the glue was thoroughly dry, I ran the plinth block through a bench-top joiner to even out the edges. Figure 10 shows that the four sides were then smooth and ready for a dressing up with oak. Well, all accept for the back edge. When I ran the plinth through the joiner, I should have placed a sacrificial piece of lumber on the plinth top to prevent splintering: I didn’t, and it did!

Copyright 2012 Guy W. Riffle. All rights reserved.

_________________
- Guy

Plinth, Part 3 (Oops)

Figure 11 shows many things: tonearm mounting holes, bearing hole, old and new spike (point) holes, and a splintered back edge. I used dowel rod to plug the old spike holes – I drilled the holes larger to the size of the dowel rod, glued dowel in, and smoothed it down. I then used my router to cut a ¾“ notch about ¼“ deep across the back of the plinth so I could glue a strip of hardwood (in this case, red oak) into the notch, and smooth it down using a plane and a sander.

Figure 12 shows the plinth ready for its oak inlay. The two old spike holes are plugged and smooth. Because I was worried about glue mottling the top’s finish, I sanded the top and applied a few coats of polyurethane. After that, any glue on the top would easily wipe off with a damp rag.


Copyright 2012 Guy W. Riffle. All rights reserved.

_________________
- Guy

Plinth, Part 4

I used red oak to cover the sides of the plinth and make it more presentable. I’ve made mitered corners before, but never easily. This time I used simple butt joints for the corners. I wanted the plinth sides to be a little above the plinth surface, so I got a piece of 1/8” hardboard and cut a piece 12” square so it was smaller than the plinth top. I bought 1” x 4” red oak, cut two pieces 14¾ “ long for my sides, and two pieces 17¾“ long for the front and back (plinth width plus thickness of two sides). I put waxed paper on top of my table saw, laid the 1/8” hardboard in the center of the table, and placed the plinth upside down on the hardboard. Again using my glue roller, I applied glue to both sides of the plinth, laid the red oak against the plinth sides so they lay on the table top, and made sure they overhung the front and back of the plinth evenly. This left about 1/8” overhang on both sides. I then clamped them in place and waited for the glue to dry. Figure 13 shows the plinth after adding the oak sides. Once the glue cured, I trimmed the front and back edges of the sides flush to the front and back of the plinth body using a flush-trimming router bit with guide bearing. Look at all those plies in the plinth!

I again put waxed paper on top of my table saw and placed the plinth upside down on the hardboard. This time, the oak sides held the plinth surface above the saw table. I applied glue with my glue roller to the front and back of the plinth, laid the 17¾“-long red oak against the front and back of the plinth so they lay on the tabletop, and made sure they overhung the plinth sides evenly by about 1/8” on both sides. I then clamped them in place and waited for the glue to dry. Figure 14 shows the plinth after the front and back trim pieces had cured in place. Note how the side edges of the front and back extend past the sides. The front and back trim pieces were trimmed flush to the sides using the same flush-trimming router bit with guide bearing. In Figure 15, you can see the flush corners as well as the trail of the guide bearing. You can also see the oak inlay on the plinth back that fixed the splintering issue.


It seems that square edges are always getting knocked off of wood, so I beat it to the punch: I used a 1/8“ round-over bit with guide bearing in my router to round the vertical and horizontal edged of the plinth trim. After lots of sanding with a dual-action sander and progressively-smoother sandpaper, I applied a coat of Minwax pre-stain conditioner followed by two coats of Minwax golden-oak stain (#210B), always following their instructions. Once the stain was dry, I applied three coats of semi-gloss water-based polyurethane.

Copyright 2012 Guy W. Riffle. All rights reserved.

_________________
- Guy

Plinth, Part 5
With the finish dry, I mocked up the turntable by adding the bearing, platter, and tonearm to the plinth (see Figure 16).

I wanted to maximize my VTA adjustment around its center point, and the mockup showed I needed to mount the tonearm ½“ higher than the plinth top. It may have been better to raise it even higher because the arm should never be below the platter top – maybe I’ll adjust that later. For now, I drilled a hole in a piece of ½” thick Delrin for the tonearm mounting screw, placed a large steel washer on the Delrin so the leveling screws wouldn’t dig into the Delrin, and mounted the tonearm through the Delrin and onto the plinth. I needed to countersink the tonearm-mounting hole from the bottom because the supplied tonearm mounting stud was not long enough to reach the bottom (it would have to be about 4” long to do this!).

You can see in Figure 3 that the tonearm was delivered with a pair of RCA jacks attached to the cable. I couldn’t see a way to get the connector box through a 5/16” hole, so I cut off the connectors. Next, I stripped the plinth (again) and drilled a pair of ¾ “ holes from the back for a new pair of Cardas RCA jacks. Once the holes passed about 1 ½ “ into the plinth, I drilled two hole from the bottom of the plinth to meet the holes from the back at 90°. I threaded the cable from the tonearm down through the plinth, and then up and back through the new holes in the plinth back. I cut a rectangle of 1/8” aluminum, mounted the RCA jacks on it, soldered the cabled to the jacks, and screwed the plate with jacks to the back of the plinth (see Figure 17). If I’d thought about this, I’d have drilled the holes before I finished the plinth. Once I added the spikes and knobs one last time, the turntable was ready. All I needed was an air supply for the tonearm and a motor to turn the platter.


Copyright 2012 Guy W. Riffle. All rights reserved.

_________________
- Guy

Air Supply

The Ada-Lin MG-1 tonearm needed a constant supply of low-pressure air to support its air bearing. Mr. Lin offered the Alita AL-15A linear air pump on his web site, and I’m not experienced with air-bearing arms but it appeared that no additional valving or controls were required. I thought I could get an air pump for less cost that listed on Mr. Lin’s site. I considered something from the HiBlow family, but stuck with the tonearm-manufacturer’s recommendation. I bought an Alita AL-15A online (http://www.PondMerchant.com) for $115.

Online forums warned about air-pump pulses and moisture in airlines. To counter this, I built a surge tube with an air drier. The surge tube was made of PVC (see Figure 18). I bought a 4” closed PVC toilet flange, a PVC pipe cap, and a 5’ length of PVC (the shortest I could buy). I drilled two 9/16” holes, one in the top and one in the bottom, and tapped both for 3/8”-18 NPT. I loosely packed the length of PVC with Dacron pillow stuffing from a crafts store, and glued the three pieces together with PVC glue. I roughed up the outside, sprayed it with PVC primer, and finished it with a few coats of white paint. It was a little unstable, so I attached it to a wooden cutting board with lag screws. A pair of ¼“ tube fittings were screwed into the threaded holes so ¼“ tubing could be attached.

At the suggestion of a friend in the club, I bought a laboratory-quality air drier from http://www.drierite.com and added it to the side of the surge tube (see Figure 19). I got the gas-drying unit (with hose barbs, #26800), a mounting clip (#26809), and a 1-lb. jar of 8-mesh desiccant (#23001). I drilled two holes in the side of the surge tube and tapped them for 10-32. I then dabbed black RTV on the holes and screwed the mounting clip to the surge tube with 10-32 binding-head screws. When the desiccant absorbs water vapor, it turns pink (look again at the bottom of the drier in Figure 18); it’s ready to change when it’s all pink. The cool thing is that I could rejuvenate the desiccant by heating it in the oven; then I could use it again! I went overkill on the air treatment, as can be seen in Figure 18: I added a water separator on the right side of the surge tube in addition to the air drier. This part added no benefit to the project and was not needed.

The tubing started at the air pump and connected to the bottom of the drier. A second piece of tubing connected from the top of the drier to the bottom of the surge tank. The last piece of tubing connected from the top of the surge tank to the tonearm. I may find that the air pump is too loud to keep in the room while listening. In that case, I’ll move it to the next room and use a long piece of air hose and a remote switch to control the air supply.
My surge tube was pretty tall – I thought I’d use all the tube I bought. A 3’ tube may be sufficient, or you could make two 3’ tubes and connect them with a piece of hose. The result would fit better into normal room décor, maybe behind the stereo stand. Other on-line suggestions for a surge tank included a one-gallon or five-gallon plastic gasoline jug filled loosely with stuffing as a surge tank and fitted with hose fittings.

_________________
- Guy

Drive System and Motor Enclosure, Part 1

I chose to use a separate motor enclosure to isolate the motor and associated vibrations from the platter and tonearm. It included the motor and pulley, the servo-controller board, AC transformer, and assorted controls (see Figure 20). I used 4” wide red oak to match the plinth: ½“ thick for the sides and ¼ ” thick for the top. The overall dimensions were 10¼” x 5½“. The front and back were glued to the sides, just as the plinth was assembled, as seen in Figure 21 and Figure 22. The top was attached with six screws (three on each side) so it could be removed. I didn’t want the six mounting screws too close to the edges, so I glued two strips of ½“ red oak to the top edges of the sides so I could place the screws about ½“ from the edge.



A SPST toggle switch was mounted on the top panel to select platter speed (33-1/3 or 45 RPM). A power-indicating LED was included, along with two variable resistors to adjust the platter speeds through two small holes. An AC power switch was located on the back of the enclosure (see Figure 21).

How fast should the motor turn to run the platter at the required speeds? The answer is:

(pi x platter diameter)/(pi x pulley diameter) x platter speed

If I had a drive pulley with a ½ “ diameter, then the motor would have to run at the two following speeds:

(pi x 12")/(pi x 0.5") x 33.33 RPM = 800 RPM
(pi x 12")/(pi x 0.5") x 45 RPM = 1080 RPM

Online forums showed a lot of discussion about Maxon DC motors (probably for replacing failed motors). I chose the Maxon 110191 DC brushed motor, and I ordered one from Maxon for $135.

Understand that a DC motor provides maximum torque at zero speed, that the selected Maxon motor was rated for 48 Volts, and that its speed was proportional to the applied voltage at 127 RPM/Volt. Therefore, the controller needed to provide about 6.3 and 8.5 Volts for my needs. These voltages were well below the rated maximum, so the resulting torque was close to the maximum torque the motor could provide. The motor had no problem starting the platter from a stop and getting up to full platter speed in a few seconds.

Copyright 2012 Guy W. Riffle. All rights reserved.

_________________
- Guy