I think putting the record direct on the brass platter is not such a good idea.
TW acoustics does that anyway, they use copper for the platter though and that is a little softer. I talked to MiiB and he suggested a decoupled plastic mat on top.
I found a record mat that we had cast many years ago that can do the job.
I thought that decoupling with gel pads could be fine. These pads are used by drummers to tune the drum set. One maker is Slapklatz in denmark.
I will post a simple drawing how it can be done.
TW acoustics does that anyway, they use copper for the platter though and that is a little softer. I talked to MiiB and he suggested a decoupled plastic mat on top.
I found a record mat that we had cast many years ago that can do the job.
I thought that decoupling with gel pads could be fine. These pads are used by drummers to tune the drum set. One maker is Slapklatz in denmark.
I will post a simple drawing how it can be done.
Attachments
There's also the matter of phase.
Hold a magnet near an iron pole of a rotor (or equivalently, hold a piece of iron near a magnetic rotor). The rotor will turn so a magnetic pole aligns (perfectly, assuming no friction) with the iron pole piece. It may swing back and forth a bit, but we can assume some (otherwise negligible) friction eventually stops it The magnet wil then pull with a force that goes directly through the axis.
Now put a small turning force on the platter (not so large as to disengage the magnet and the iron pole piece). It won't stay in the same place - it will turn slightly so that there's a new equilibrium between the applied turning force and the pole/magnet force. The magnet and pole piece change angles so the line between them no longer goes through the axis. It's like putting up a long taut string and then hanging a weight on it - the string where the weight is hung WILL drop down.
Spin the magnet or pole piece around the rotor (the equivalent of a rotating magnetic field generated by coils), and the story is the same, just in a rotating frame of reference. With no load, the pole piece is perfectly aligned with the magnet. With a slight load, the rotor lags behind, even though it remains "at the same speed" as the rotating magnetic field. When the load is applied this change in phase is effectively a temporary dip in rotational speed. When the load is removed, the rotational speed increases momentarily as the rotor poles catch up to and realign with the rotating field.
I did some really crazy experiments tonight. sq had send me a PM with a photo that showed a small diameter carbon fiber disk that floats 2/3rd of the record ca. 1 - 2mm over the platter so it does not touch it. That was the result of research a friend of him did that tried all kinds of platter mats. I thought that a CD has aprox. the same diameter and thickness so a floated a record on a CD. I used a clap to hold the vinyl down so that i had enough drag. The sound i got was interesting to say the least. Somehow the disturbance from scratches and dirt was less and it sounded a little bit darker and more sweet.
It also had an effect on the soundstage that is a bit hard to describe. Somehow it had a bit less height information and focus but it also sounded well fleshed out, say a bit rounded.
I am still at bit speechless how to describe it and i do not know yet if i like it better or worse. VTA had of cause changed too and i know that when i lower the arm rest i get also a somewhat fuller sound. Tomorrow i will try to raise the arm so that it is parallel to the record again. Maybe i can restore the focus and enyoy the quiet at the same time.
The effect is not drastic, i think the sandwich platter ( steel, vinyl, graphite, decoupled spindle ) of my SG1 is well optimized already. Ultimately, with the CD in place, i found the sound a bit artificial so i replaced the CD with a felt disk of the same diameter and aprox. the same thickness. The sound i got now was very natural. The slight artifice was gone. Tomorrow i will raise the arm and do some photos.
I never thought that a CD could be used that way. This world is full of miracles and wonders.
It also had an effect on the soundstage that is a bit hard to describe. Somehow it had a bit less height information and focus but it also sounded well fleshed out, say a bit rounded.
I am still at bit speechless how to describe it and i do not know yet if i like it better or worse. VTA had of cause changed too and i know that when i lower the arm rest i get also a somewhat fuller sound. Tomorrow i will try to raise the arm so that it is parallel to the record again. Maybe i can restore the focus and enyoy the quiet at the same time.
The effect is not drastic, i think the sandwich platter ( steel, vinyl, graphite, decoupled spindle ) of my SG1 is well optimized already. Ultimately, with the CD in place, i found the sound a bit artificial so i replaced the CD with a felt disk of the same diameter and aprox. the same thickness. The sound i got now was very natural. The slight artifice was gone. Tomorrow i will raise the arm and do some photos.
I never thought that a CD could be used that way. This world is full of miracles and wonders.
O.k. I see what you are telling me. With any system selected there will be system specific anomalies. To solve these we need to update/amend the system. DC motors also have there system anomalies. In the case of the AC motor this lag-response-anomaly could be solved by using a 6 or even 12 pole motor, and then (I presume) the motor will become really expensive (I think). If I had time then I would try to construct something, that I call, a 'odd'-phased-motor. Probably running on 3 phases and having 8 or 10 pole's.
Concluding, maybe using an AC motor is not the best choice. On the other hand, I always love (loved) systems running on AC motors. DC motors are harder to control, but can have greater benefits, one of them is that they are cheaper and easy to get. Until now I tried to defend AC motors, but I can see the DC motors can be a good choice. Give me a little more time and, maybe, I will change my vote in favor of an DC motor.
FDW, if you run that AC motor into a high drag load it forces the field in the motor to compress and this has the effect of hugely reducing torque/cogging ripple.
Get an AC turntable motor and hold it in your hand, turn it on. You can feel the vibration right? Now rest your finger on the pulley as it spins. What do you notice? The vibration drops off to nothing when you apply moderate load. A spinning platter isn't typically enough load, you need real drag.
There's a good reason why Garrards and Thorens decks used to use eddy brakes- it linearises the motor by adding drag.
Get an AC turntable motor and hold it in your hand, turn it on. You can feel the vibration right? Now rest your finger on the pulley as it spins. What do you notice? The vibration drops off to nothing when you apply moderate load. A spinning platter isn't typically enough load, you need real drag.
There's a good reason why Garrards and Thorens decks used to use eddy brakes- it linearises the motor by adding drag.
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This is exactly what my 'odd-phased-motor' would do, it would create it's own load/drag. But this is not in the scope of this project (designing such a motor).
For now, I can see how a DC motor would be a better fit for this project. And for that I will refrain from promoting AC motors (in this project 🙂). Concluding that an DC motor will have my vote. I would still like to see a high-mass high-torque motor.
Delaying the audio signal would not work. It would be necessary to linearly timeshift the analog signal exactly equal and opposite to the platter drift.
The important thing is the "tightness" of the system. As was described, there will be a force based deflection from the ideal rotational position and the platter position, and that force needs to be high to limit speed modulation.
jn
Hard to beat this slow turning ,large capstan ,Technics motor:
Technics RS-1500 Capstan-Motor, das Original | eBay
When i use your link i can not see the Technics motor.
Yes, both the Bauer DPS and the Spiral Groove turntables use a bearing that has friction.
That improves the speed stability of the AC motors.
Yes, both the Bauer DPS and the Spiral Groove turntables use a bearing that has friction.
That improves the speed stability of the AC motors.
Bizarre, try this:
http://www.ebay.de/itm/Technics-RS-1500-Capstan-Motor-das-Original-/130879650192
Artikelnummer:130879650192
http://www.ebay.de/itm/Technics-RS-1500-Capstan-Motor-das-Original-/130879650192
Artikelnummer:130879650192
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Here's a set of better pictures of this mother of all capstan motors:
http://www.ebay.ca/itm/Technics-RS-1500US-Reel-Reel-Motor-Original-Part-/151052975328
http://www.ebay.ca/itm/Technics-RS-1500US-Reel-Reel-Motor-Original-Part-/151052975328
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I see, but can't understand why.
In any case the Canadian link is better.
This has become an interesting conversation here about the rotational ripple in the platter and where that is created. I would think that the heavy mass of the platter that Joachim has proposed would have a large effect on rotational speed and would smooth out most of the variations? On the application of bearing drag that sounds like it could create its own problems with bearing noise from any drag that it would induce. Wouldn't you be better off using a bearing with a Viton seal that would cause the drag?
Yes, the heavy platter smoothes out.
I asked the guys at Transrotor how heavy a platter should be.
They said that they did not find any more improvements when the platter weight was 12kG. So with our 24gG platter i think we are on the safe side.
That did not hinder Rosner&Sohn to show a 500kG !!! plater at the Analog Forum in Krefeld.
Verdier also did a new big table with an extremely high mass platter.
I asked the guys at Transrotor how heavy a platter should be.
They said that they did not find any more improvements when the platter weight was 12kG. So with our 24gG platter i think we are on the safe side.
That did not hinder Rosner&Sohn to show a 500kG !!! plater at the Analog Forum in Krefeld.
Verdier also did a new big table with an extremely high mass platter.
Joachim,
That last turntable is going to the absurd level of mass at this point. I have seen some of the large mass turntables for many years now and just don't see the point of that exercise. To take that mass from a standing start up to rotational speed must take an extreme amount of torque to just get it moving. I guess you can just leave the turntable rotating at all times and not bring it to a stop, but that doesn't seem to be very practical. I can imagine the slippage of any belt drive turntable with that much mass driven by the small spindles that are shown in the pictures. I would expect some flat spots to form on the inside of the belt or O-ring drive from the start-up of the turntable and slippage in those first couple of seconds. Rather silly at some point.
That last turntable is going to the absurd level of mass at this point. I have seen some of the large mass turntables for many years now and just don't see the point of that exercise. To take that mass from a standing start up to rotational speed must take an extreme amount of torque to just get it moving. I guess you can just leave the turntable rotating at all times and not bring it to a stop, but that doesn't seem to be very practical. I can imagine the slippage of any belt drive turntable with that much mass driven by the small spindles that are shown in the pictures. I would expect some flat spots to form on the inside of the belt or O-ring drive from the start-up of the turntable and slippage in those first couple of seconds. Rather silly at some point.
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