Posts: 7738
Location: Powell, Wyoming
Joined: July 23, 2007
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WHOO HOO! I finished the tonearm tonight and as soon as the needle hit the groove I knew I had a winner. Excellent sound reproduction. Sharply focused with superb detail. PRaT, it's all there.
I compared some WAV transfers (a couple of Cult 45 rpm 12" singles and a Classical solo piano LP) that I'd done with my modified Technics SL-1200 w/Grado to the same transfers done with the DIY turntable/tonearm with the Grado. Doing this, I can easily make A:B comparisons. Both sounded great and the differences are subtle. I think the DIY tonearm/turntable produces a slightly sharper sound with a bit more dynamic range. That it sounds as good as my Technics has me thrilled. I've got all of fifty bucks into this DIY machine and it's definitely producing 'big league' performance.
Not the best sounding recording, but it sure makes for a cool picture.
MFSL DSOTM spinning and sounding great. Effective length of tonearm is 303mm or just shy of 12".
Here's the business end of things. The pivot is stainless steel with jewel quality finish. Nothing loose or sloppy here. The fit into the tonearm is very tight. The pivot point and stylus tip are at the same distance from the tonearm tube centerline (and bottom of the headshell).
The matching sapphire jewel bearing is epoxied into the head of an 8-32 socket-head bolt. I drilled a pilot hole into that monster lag screw and tapped it with 8-32 threads. The jam nut ensures a rigid connection between bolt and lag screw.
Just to give you an idea of the quality of the bearing.... I had the tonearm assembled and balanced out in my shop, with the lag screw held upright in a vise. No tonearm wires, so it could move freely. I blew lightly on the headshell/cartridge. The tonearm spun slowly and kept spinning for five minutes before it stopped.
Designing and building the tonearm required a great deal of thought. I do not have any formal training in Physics, but I quickly realized that many factors such as quality of materials, distribution of mass, center of gravity, pendulum effect, moment of inertia, etc.... had to be carefully considered.
The lag screw makes a firm connection to the plinth. Note that the socket head screw (which hold the vee jewel bearing) can be adjusted up or down, allowing changes in VTA. Note the two 'outrigger' bolts on either side of, and in line with the pivot. The brass thumbscrews allow for easy adjustment of azimuth. As you might imagine, the unipivot design lends itself to rocking or 'wiggle' action, side to side which is undesirable. The low-slung weights help provide stability and balance.
The counterweight was taken off the Marantz tonearm. Note that the bolts which secure it allow for fine adjustment of the weight up and down, thus altering center of gravity. Center of gravity in relation to the pivot is highly critical. Too high (COG above pivot) is unworkable of course, as the tonearm would fall over to the side. So the trick is to have the COG slightly below the pivot. Just enough to provide stability, but not so much that tracking of warped records is compromised.
Yes, that is a wire brad between the pivot and the counterweight. I don't yet have antiskating, but the wire brad will provide an anchor point for a string-and-weight antiskate device.
Pull the two acorn nuts and this small panel is easily removable, a nice feature since the Cardas tonearm wires are soldered directly to the RCA jacks.
This was my first DIY tonearm project. I'm very happy with the outcome. However, there are two design flaws that became evident well into the process. They both can be seen here.
First, note the offset of the headshell and position of stylus. That's fine if this were a conventional gimballed bearing tonearm. However, as a unipivot, I realized that the stylus tip should ideally be inline with the center of the tonearm, pivot, and COG. In theory, with the stylus offset from the COG (as I built it) a warp would create a twisting motion, altering azimuth. However in actual practice, it tracks warped records quite well. It seems the mass of the tonearm is sufficient to resist the twisting motion.
Secondly, the tonearm lift also has that same COG problem. Lift it and the tonearm wants to twist. Not good! I should've anchored the tonearm lift in the center of the tonearm. This is something I will fix. I don't plan to install a cueing mechanism, as I prefer to cue by hand. BTW, that tonearm lift is nothing more than a wire brad with its head flattened out.
I hope you've enjoyed seeing my DIY project. I just wish you could hear it.
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