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Drive System and Motor Enclosure, Part 2

Online forums also had a lot of suggestions for motor power supplies. Mark Kelly from Australia wrote a great article in an on-line publication from BasAudio.Net called DIYMAG. (Go to http://basaudio.net/blog/diymag/ and look for “early 2006.”) In the back of the publication was 10-page article about AC and DC turntable motors and how to drive them. Mark’s article impressed me, and he certainly understood more about platter drive than I, so I used his design for a DC motor driver with error correction.

There used to be a kit marketed by diyAudio.com for the controller, but it was no longer listed. Instead I laid out a PC board using Kelly’s circuit (see Figure 23). I still have a couple PC boards left from my first run.

Figure 24 shows the controller board mostly loaded with components. The op amps (U3, U4) and voltage reference (U2) have not been plugged in, and the resistors that set platter speed as also missing. The pots (R7, R27) that adjust platter speed were actually mounted on a piece of perforated board and suspended below the top of the motor enclosure so platter speed could be adjusted while the enclosure was assembled (see Figure 22).

John Parker of Choir Audio indicated that I needed a belt material that addressed two major points:

1. Properly transfer energy from the drive pulley to the platter without slippage or expansion (stretch).
2. Low-Noise Operation: Surface area causes friction, and friction causes noise and unwanted energy (vibration).

John preferred round belts because they addressed both points. He also suggested that I try 3/16”- and 1/8”-diameter belts, which he was able to provide once I used a piece of string to determine the length of belt I needed, which turned out to be 45”.

Now for a pulley: There is nothing like friends. One of mine made a bronze ¾ ” diameter pulley with a ½”-diameter groove for the pulley (see Figure 25). The groove had a 1/8” radius (so my round belts would fit nicely and maximize contact) that was located 3/8” from the bottom of the pulley, and the edges were flared at 45° so the belt wouldn’t get pinched. The pulley had a 3mm hole drilled ¼“ into the bottom, and was glued to the Maxon motor shaft to drive the platter using a round-cross-section rubber belt.


Copyright 2012 Guy W. Riffle. All rights reserved.

_________________
- Guy

Drive System and Motor Enclosure, Part 3

I mounted the motor between two pieces of ½“ Delrin with a 1” hole in the middle that pinched the motor and was held tight by two screws threaded into the Delrin (see Figure 26). This mount is attached parallel to the underside of the motor-enclosure cover with four screws (see Figure 22). Note that the two sides of the motor mount did not touch, and pinch the motor tightly between them.

To make the motor mount, I drilled all holes first, and then cut the piece in half, as described in Figure 27. Because the motor was a little bigger than 1” in diameter (actually is 26 mm), I had to enlarge the 1” hole with file so it would fit the motor. I then tapped the holes in the upper half of the mount (the piece farthest from the screw heads) for 10-32 hardware, and assembled the two halves using socket-head screws. The motor was adjusted up and down until the top of the motor was flush with the top the enclosure cover, and then the two locking nuts were tightened.

I placed the motor enclosure to the left of the plinth. The further away from the platter the drive pulley was located, the more precise the alignment had to be to keep the belt from moving up and down and coming off the platter. I placed the motor enclosure so there was about 2” between it and the plinth, so I needed a 45” belt.

Like the plinth, the motor enclosure had three spikes that thread into T-nuts so the enclosure’s height (and the belt’s height) could be set and the enclosure could be leveled so the belt didn’t move up or down when the platter turned. One of the three spikes can be seen in Figure 22. The spikes were locked in the leveled position with wing nuts that turned against the T-nuts.

Copyright 2012 Guy W. Riffle. All rights reserved.

_________________
- Guy

System Setup

There was an initial setup to align the tonearm to the platter. The cartridge was attached to the head shell of the tonearm wand. The tonearm should not be moved on the air beam without air applied, so the pump was plugged into the wall, and the black tonearm air sleeve was slid onto the silver air beam. A retainer screws onto the end of the air beam to keep the tonearm from sliding off. This feature also allows the user to buy another tonearm and quickly exchange cartridges. A tracking force of 1 – 2 grams was placed on the cartridge, and one of the templates provided with the tonearm was put over the spindle on the platter (see Figure 28).

The template was rotated until it was about parallel to the air beam, and the tonearm was moved above the template to see if the air beam was truly parallel. With the air-beam assembly snugged to the plinth but not tight, the air-beam assembly was rotated until the stylus followed one of the parallel lines on the template. This indicated that the assembly was parallel to the desired stylus path.

Because the stylus was not tracking on top of the line running from the spindle to the “Rest” point, the wand retaining screw was loosened (with a hex key provided by the manufacturer) and the wand was extended until the stylus tracked the line. The cartridge vertical angle was set by lowering the cartridge on a mirror on the platter, and making sure that the stylus showed no angle to the left or right. If an adjustment was needed, I loosened the same wand setscrew and rotated the wand. Afterwards, the setscrew was snugged down.

With the plinth hanging over the edge of a table so that the tonearm mounting screw was accessible from below, a bubble level was set on the platter, and the three spikes were adjusted to level the plinth. The tonearm level was then prepared for adjustment by first adjusting the counterweight until the tonearm had neutral balance (no weight on the cartridge). Because the tonearm level was more sensitive than the plinth balance, the tonearm drifted toward either end of the air tube. I used a 4-mm hex wrench to adjust the three leveling screws at the back end of the tonearm assembly until the tonearm sleeve hovered in the middle of the air tube. The tonearm mounting bolt was snugged and the tonearm level checked again so the sleeve continued to hover in the middle. Small adjustments to tonearm level can be made without changing the mounting-bolt tightness. The tonearm-sleeve assembly was removed from the air tube so I could move the plinth.

The turntable assembly was next placed on a stable shelf with enough room to the left for the motor enclosure. The air pump and surge tube were placed on the right side of the room. Remember that the air pump can be moved to another room if it is too loud for the listening room. A bubble level was set on the platter, and the three spikes were adjusted to again level the plinth. The motor enclosure was then adjusted to be level at the same height as the plinth. The belt was wrapped around the drive pulley and platter, and the motor enclosure was carefully drawn away from the plinth to apply reasonable tension to the belt. The motor enclosure was turned on, and a strobe disk was used to adjust the platter speed to 33-1/3 RPM with a jeweler’s screwdriver through the left access hole in the enclosure cover.

The air pump was again plugged into the wall, the tonearm was slid onto the bearing tube, and the retainer was screwed onto the end of the air tube. A stylus scale was used to adjust the initial tracking force to 2 grams. Then I had people with experienced ears sit down and listen to vinyl played on the turntable, and give suggestions on VTA adjustment. The optimal adjustment was determined as time went by and more listening proved the setting.

Conclusion

That’s it! This project looks easy when I read it because it doesn’t include all my false starts and travels onto tangents (pun?). Because I don’t have easy access to a machine shop, I bought a bearing-and-platter kit and an air-bearing tangent-tracking tonearm from other sources. I built a wooden plinth, a motor enclosure, and an air-supply system to support the two subassemblies. Once combined, I ended up with a vinyl playback system that other people say has respectable performance that exceeds the money I invested in the project.

Copyright 2012 Guy W. Riffle. All rights reserved.

_________________
- Guy

Guy:

This is very well designed and implemented project. Kudos for the effort. It truly must have been a labor of love. If you have not already published it, you should consider submitting it to "AudioXpress" magazine. They would love it. I can provide you with contact info if you need it.

Tom

Impressive work!

This is great

Craig

Tom,

I tried to submit this to AudioXpress, but they considered it more of a product review than a construction project because I bought the platter and tonearm.

At that point club members started trying to decide if amplifiers are construction projects because we buy resistors, capacitors, transistors, and tubes. Well, the magazine rules, and has changed after being consumed by Elektor.

_________________
- Guy

Thanks, Everyone!
See, even a person of limited experience can build something interesting!

_________________
- Guy

Guy,

Great article about your turntable. I'm interested in following your plans and building one. Save me one of those controller boards.

Jim