Even so, I'm in the Audio business as a professional, it's also still my hobby.
So I'm here as a hobbyist, playing with turntables, building tube stuff and recently working on a backloaded horn. The last one goes back to 1982 (Lowther horn with Coral driver as far as I remember).
So please treat me as a hobby person. Thanks! Of course, I'm happy to help out on speaker things if I can
Hello, and welcome. I think there are a few professionals here; most people with a serious hobby interest manage to extend it into their job. I've spent a lot of time improving my metalworking skills with the intention of making a turntable from scratch. Really from scratch, so that means taking the capstan motor from a VHS machine, and throwing everything away except the rotor and the pole pieces with coils wound on them. Resurrected motor will be driven by a three-phase sine wave from analogue oscillator. Bearing/shaft will be hardened shaft from floppy drive in lignum vitae. Belt drive. Platter with similar bearing and probably a little magnetic assistance to relieve pressure on thrust pad.
I’ve had some turntable ideas in the back of my mind for years, mostly involving brushless motors but the electronics are a bit of a mystery to me. Where’s the best place to learn? I’m thinking more of an idler design so speeds will be a bit higher than most belt drives.
What will happen comes closer. What has happened goes further away.
Well, for whats its worth, the eddy current direct drive turntable I brought to the last Owston has stalled as the company doing the metalwork has hit the financial barriers.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
shane wrote: Mon Apr 29, 2024 10:31 pm
Where’s the best place to learn? I’m thinking more of an idler design so speeds will be a bit higher than most belt drives.
Tricky. There are lots of books on motors, but they are all concerned with extracting maximum power for a given amount of electrical energy, not with smoothness of rotation. I started from first principles and equations of motion. Being firmly analogue, I developed my own oscillator, but look up the Altmann turntable for some easy-to-implement ideas for motor drive.
Idler drive doesn't need a faster motor, its just that they have traditionally used four-pole induction motors (1475 rpm) whereas belt drive turntables traditionally used 250 rpm motors intended for clocks or air conditioning controls (yes, that was the design use for the Philips/Airpax/Premotec/McLennan motor used by Linn and most other manufacturers). There was a fallacy put about that the more poles, the smoother the rotation (less cogging). However, if you note that the energy stored is mw^2r, the heavy rotor of a fast-spinning induction motor is a far better flywheel than the light ferrite blob of a slow synchronous motor. But the synchronous motors were cheap, whereas the fast induction motors needed expensively honed shafts and sleeve bearings (Garrard referred to it as super-finishing in Wireless World adverts).
The flywheel effect of the motor was the main reason why I was thinking about idler drive. I think that is to a large extent what gives the Garrard and Lencos of this world their dynamics. I’ll have a look at the Altmann though. Not come across that one before.
What will happen comes closer. What has happened goes further away.
shane wrote: Tue Apr 30, 2024 11:59 am
The flywheel effect of the motor was the main reason why I was thinking about idler drive. I think that is to a large extent what gives the Garrard and Lencos of this world their dynamics. I’ll have a look at the Altmann though. Not come across that one before.
You may be right, but I am not sure why the moment of inertia of the motor is any more important that that of the platter. Or for that matter the entire rotating system.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
I guess it’s the sum of the parts. Idler platters are more or less similar to belt drive platters, but the flywheel effect of a big fast rotor is enormous compared to that of a small synchronous motor, which is also decoupled by a belt.
What will happen comes closer. What has happened goes further away.
Nick wrote: Tue Apr 30, 2024 2:05 pm
That may be true for some value of "belt drive platters"
Hence why I said more or less.
Nick wrote: Tue Apr 30, 2024 2:05 pm
Yes, but the effect of the "big fast rotor" is also stepped down by the reduction ratio of the motor to platter.
Surely it’s stepped up, in the same way that a car in 1st gear has far more engine braking effect than in top?
Nick wrote: Tue Apr 30, 2024 2:05 pm
I guess, what I am trying to politely suggest is that you have made some claims but I can't see any proof of those claims other than "it feels right".
Absolutely. I’m not trying to make any claims. I’m just exploring whether what feels right actually is.
Nick wrote: Tue Apr 30, 2024 2:05 pm
If you are going to make your own turntable, you can make the platter as massive as you want.
True, but I (and more importantly my wife) have an aversion to those turntables that have platters a foot thick with a record looking a bit silly perched on top. Also, I have to hand a spare TT2 platter which, for those not familiar, is a more normal 30mm thick.
Last edited by shane on Tue Apr 30, 2024 4:35 pm, edited 1 time in total.
What will happen comes closer. What has happened goes further away.
Surely it’s stepped up, in the same way that a car in 1st gear has far more engine braking effect than in top.
Yes, but in that case the platter is driving the motor, if the motor is driving the platter then it would be a step down.
But my point is that even if its stepped up, a 401 platter and motor will have less inertial than a big Clearaudio belt drive dual platter turntable, but still the 401 is doing something different to the Clearaudio in terms of sound.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
What I was pointing out in the motors is that although the fast induction motor suffers more torque ripple because it only has four poles, it has a very effective local flywheel because it spins so fast. Conversely, although there's less torque ripple in a correctly driven synchronous motor, it's just as well because it has a negligible flywheel because it's way lighter and spins at a sixth of the speed (typically). OK, so we compliantly drive the platter via a belt, but I suspect there's not as much decoupling there as we'd like, and I too would like to get away with a slim platter rather than something that looks as though it fell off a train or an industrial print roller. Besides, really heavy platters impose a lot of force on their thrust bearing, making it difficult to keep the rumble down. Quietening motor noise at source seems best to me. I suspect it's that omega squared that makes a Garrard sound good. Nevertheless, homebrew turntable will use a three-phase synchronous motor driven so as to produce minimum vibration (oh, and it will be outer rotor, so a much more effective local flywheel).
On a slightly different topic of smooth rotation, some model engineers have improved the cut finish produced by their mills by adding a substantial flywheel. Unlike a lathe, a mill takes a discontinuous cut. It's a bit like using a wood plane; you aim for a smooth motion, rather than jerky that rips and tugs at the surface.
I hope Karl-Heinz is finding all this rambling on his introductory post of interest?
A VHS capstan motor. They're all very similar, with a 68mm diameter outer rotor and 3.5mm shaft. Calculation says that at standard VHS speed it should rotate at 180 rpm, which is a perfectly usable speed. The construction is way superior to the Airpax. VHS was pretty dreadful, but you need tight control of linear tape speed and phase to recover pictures. As standard, they have a bunch of electronics on their PCB that includes a servo that senses motor speed and phase. If I knew what the (typically 8-way) connector needed, I'd use that electronics. With a working VHS machine, one could probe that connector whilst adjusting replay speed to see what does what. In lieu of that, I've taken a motor apart to reveal twenty four poles wound with individual coils that require three-phase drive. If you have an oscillator that produces sine and cosine, appropriate addition of the two will produce 120 degrees and -120 degrees, giving you the other two phases in addition to the sine. You can buy dirt cheap TDA2030 amplifiers on Fleabay, so I bought three of those to drive the motor (cheaper than buying a TDA2030 and components).
When I built my own turntable (using parts from a Logic DM101) I used a video motor off Ebay mounted in a nylon bush in a 2.5 diesel Iveco connecting rod with 3 rubber buffers. 2 big end end and 1 small end end.
The power supply for the motor was from our Nick which he generously donated to the project.
The turntable is in the bottom of my wardrobe and has not seen the light of day for many years.
A proper thread for this topic might be a good idea ?
The only thing necessary for the triumph of evil is for good men to do nothing.
