You spin me right round

[jjgoertz]

I'm curious about your choice of motors, why not an AC synchronous motor? I've used several from Hurst in turntables, these are quiet and inexpensive. The last has sufficient torque to start and drive a 15 pound platter.

It was just easier for me to get a small brushless controller than to find a variable frequency drive. The alternative to a VFD for an AC motor is what AE was talking about with tuning speed by machining the drive pulley to a different diameter, and I don't have a metal lathe at home.

[Æ]

It was just easier for me to get a small brushless controller than to find a variable frequency drive. The alternative to a VFD for an AC motor is what AE was talking about with tuning speed by machining the drive pulley to a different diameter, and I don't have a metal lathe at home.

In my situation, I want to KIS (Keep it Simple). Here in the U.S. the AC line frequency is very stable, so no real problems there. A lot of the synchronous motors that I've come across are kind of wimpy and you can buy them cheaply at surplus. However, I pulled a very nice and serviceable motor from a used BSR turntable, 24 poles and all and it turned very smoothly without cogging.

I used to own one of the earlier REGA turntables and I can tell you just how incredibly cheap it was, including the plastic motor pulley which you could easily see had a slight wobble when it spun. I liked their simple solid concept, but way back then they hadn't quite perfected it yet. So anyway, I retained the glass platter, the plastic sub platter, the brass bearing and the plastic dust cover. The rest of it was junk, real sloppy execution. I have since turned the sub platter on a lathe, almost to perfection and definitely threw out the lousy cheap motor and pulley. So anyway all I really have to do is lathe a precision motor pulley. Actually, it should probably properly be called a sheave, but I digress.

[TN Allen]

Thanks for the explanations.

Regarding the Rega, a friend asked me to look over his, and I agree some of the ideas are nice, but the materials choices and construction seemed lacking, given the cost of the turntable. I don't know how old his Rega is, perhaps the quality has improved since. I was impressed with the simplicity of the "O" ring/rubber band motor suspension, however it seemed a bit dubious as well, though it did seem to work.

I might help with a pulley or sheave, I've turned several, checked turntable speed counting revolutions and comparing these to the stop watch time on my phone.

[Æ]

Thanks for the explanations.

Regarding the Rega, a friend asked me to look over his, and I agree some of the ideas are nice, but the materials choices and construction seemed lacking, given the cost of the turntable. I don't know how old his Rega is, perhaps the quality has improved since. I was impressed with the simplicity of the "O" ring/rubber band motor suspension, however it seemed a bit dubiIt is easier to check platter speed with a ous as well, though it did seem to work.

I might help with a pulley or sheave, I've turned several, checked turntable speed counting revolutions and comparing these to the stop watch time on my phone.

It is easier to check platter speed with a stroboscope pattern and a small neon light. Even better is to play a test record that has selected frequency tracks, typically 1000Hz and then measure that signal. I have three of four, maybe more test records with all sorts of different test tracks. Using a stopwatch is clumsy, requires your instantaneous reaction.

I plan on using a flat drive belt with my remade REGA.

[TN Allen]

The strobe pattern and test record are good for verifying and adjusting platter speed if one has a way to do so, while the platter spins. However, when assembling a platter drive using an AC synchronous motor with a fixed spindle speed, it is useful to know what the actual speed is with a given diameter pulley. A simple calculation provides a target pulley diameter, but a few factors can affect the actual speed, requiring a slight change in the pulley diameter.

A tick mark on the edge of the turning platter and the phone next to the platter where I can watch the tick mark pass the edge of the phone makes it easy to count revolutions, and stop the clock fairly accurately. I haven't calculated the potential error that may be caused, but suspect it is acceptable over 20-30 revolutions and a few trials. Given the speed, I can then do the math to correct the pulley diameter for 33.33RPM.

Were I to want a more accurate method, I would assemble an Arduino circuit using a photo sensor and LED to clock the speed. I suppose I could also use the Arduino to generate a PWM current to drive the motor at an adjusted speed.

[Æ]

For accuracy, playing a 1KHz tone off of a test record would be the easiest and best way, provided you have a way of measuring frequency. Plenty of the better, more expensive True RMS multimeters have frequency measurement capability. If your test tone (on the record) as played by an actual tonearm and pickup measure more than 1KHz then you would instantly know that your turntable platter was spinning a bit too fast. You can easily obtain one percent measurement accuracy with the test record. Plus it would be a real world test scenario, because it would include both the weight of the record and the drag of the stylus.

I'd start with a slightly oversize diameter pulley and go at it from there. It would be easy to extrapolate your actual platter speed with the motor pulley diameter. Heck, you could even do it empirically, just keep removing more pulley diameter until you finally get down to 33.333 RPM. Of course an electronic variable speed option would be the best.

I take it you don't own any test records? Or have never used one? I did a quick look on eBay for test records, I couldn't believe how expensive some of them actually were, but there were still plenty to be had from $10-$20. I have the CBS and the Stereo Review test records, also a few others. I picked up a gently used one at a Thrift Store for just a couple of bucks.

Sorry, but using a stopwatch is kind of problematic because it is only as accurate as your perception, your reflexes and you reaction time. Can you actually press the Star and Stop within a thousandths of a second? Additionally most Test Records contain a plethora of test tracks, which better enable you to set tracking and all that other stuff that goes along with optimizing a tonearm and cartridge combination.

I remember seeing an adjustable diameter motor pulley, it was partially split into quarter sections, with a tapered screw driven in from the top. Drive the screw in deeper and the pulley would spread out slightly, increasing the pulley diameter and of course the platter speed. As long as the AC line frequency holds, my synchronous motor will be spinning at 300 RPMs, unloaded.

[TN Allen]

I'll look forward to reading about your Rega rework and platter clocking scheme and setup.

[Æ]

I'll look forward to reading about your Rega rework and platter clocking scheme and setup.

Unfortunately it is a very stalled project.

After several unsuccessful tries, my replacement for the original plinth ended up being two pieces of 3/4" MDF glued together and then filled with lead shot. Before joining the two halves together, I used a router to hollow out mating pockets in each half of the MDF. After joining together the two MDF halves, I poured in a lead shot-epoxy mixture. Unforeseen, the raw MDF absorbed quite a bit of the epoxy, which caused the two halves to pucker inwards noticeably which then required me to top fill the MDF in the areas that had shrunk.

I've actually tried (emphasis on tried) making a plinth several times, one of my tries was a very nice piece of dark marble which looked great but was very problematic for me to work with and I ended up cracking it. Then I tried a couple of very nice pieces of solid hardwood, but they always ended up cupping or warping. So finally I settled on good old inexpensive MDF, but ran into multiple problems learning how to fill it with lead and epoxy. If you don't want the MDF to absorb too much of the epoxy, you need to pre-treat or seal the raw exposed interior MDF surfaces. The interior of MDF is like a sponge and really absorbs anything liquid. I cannot tell you how much lead and epoxy I've wasted learning just how to get it right.

The next step will be for me to apply some sort of attractive finish to the plinth, either a laminate or paint. I like a nice painted finish, but I'm not very good at it.

[mattk]

RIP Pete Burns, without whom this thread would likely have a different title

[Face]

Unfortunately it is a very stalled project.

Why not try Baltic Birch instead?

[Æ]

Why not try Baltic Birch instead?

Possibilities are endless, if you have plenty of time, tools, money, know how. . . currently I'm short on a couple of those. I cannot tell you how much money I've wasted in failed attempts, always encountering some sort of difficulty. Anyway, MDF is quite cheap, so no major loss if I screw up there. However, I do now have an almost finished lead filled MDF plinth, just needs paint or laminate. Hey, if you think it's easy to do, you should give it a try! LOL

Next step then would be mounting a suitable motor, which I have. Then also reworking and rewiring a tonearm that I intend to use.

[jjgoertz]

I drilled and reamed the bearing shaft and picked up some stainless shaft from McMaster for the spindle.





And opened up the hole through the platter. This fixed all the issues with the the records not being concentric.



I've ordered a new drive belt, but unfortunately it's back-ordered. I'm hoping it's stiffer than the rubber band because I can hear it slow down in some passages, and I'm thinking it's belt stretch that allows that.

[TN Allen]

You do nice work, very interesting choices.

I'm wondering if you considered using ball bearings? Given the weight of your platter I wonder if the ball bearings might be advantageous. I've asked this question of others building turntables, and the opinion usually is that they are noisy. I don't know if that opinion is based upon experience. I have used sealed ball bearings and find they are not noisy. My test is listening with a screw driver tip against the turntable base/plinth, and the handle against the bone directly in front of my ear, it's a way of listening for and locating engine and machine noise, but seems to work well with turntables as well. I've done the same with sleeve bearings, the ball bearings are no more noisy than these. I usually compare the sound from a new turntable to an AR XA. I've found the motors in both new and the AR XA make more noise than either the sleeve bearing in the AR XA or the ball bearings in new turntables. Of the few turntables I've listened to, the sound is all but inaudible about 1 foot from the turntable, and as expected, a heavier base/plinth seems to mask sound better.

Bearings for computer fans are sometimes manufactured to run more quietly than standard bearings, perhaps these might be worth looking at for turntables.

I also wonder if there are other suggestions for testing turntables for noise, perhaps a microphone set on the plinth and a way of measuring the mic output for comparison.

[Woofie]

I've ordered a new drive belt, but unfortunately it's back-ordered. I'm hoping it's stiffer than the rubber band because I can hear it slow down in some passages, and I'm thinking it's belt stretch that allows that.

That's awesome, great job.

[Æ]

You do nice work, very interesting choices.

I'm wondering if you considered using ball bearings?

There is usually a single thrust bearing (ball) down in the bottom of the bearing sleeve supporting the spindle. Really smooth if oiled up correctly.