Hmm. Something not mentioned so far with regards to speed stability is soundstage. Anyone who has heard a good tape machine knows what I mean.
When the 'table speeds up and slows down the skating forces on any radial tracking arm will change. This in turn places forces upon the stylus. In effect, the lateral tracking force of the stylus oscillates. IOW we hear the speed variation as an instability in the soundstage.
Tape is immune to this sort of thing. So are straight tracking arms.
When the machine is really good at speed stability, and if you have a concentric LP(!) then the soundstage will match that of tape.
When the total speed is off, sometimes I hear it due to the pitch, other times I hear it due to the timing. Musicians play things in certain keys and tempos for a reason. With some pieces a speed error is of no consequence and with others it means the heart and soul of the piece is not transmitted to the listener... |
That is one thing about the original Empire table that they got right- the platter and the motor mounts are not floating with respect to each other. Wow and flutter on even the first models had excellent specs. The motor on those machines are also quite powerful and have a lot of torque.
A powerful drive is really helpful to speed stability and is why the best vintage machines are still respected today. |
But in general, a good direct-drive motor WiLL have more torque than a good belt-drive motor. As a generalization that is incorrect. A DD drive has to spin at 33 1/3 rpm. In a belt drive, there is a tremendus amplification of torque of the motor afforded by the ratio between the motor diameter and that of the platter diameter. Add to that the possibility of a powerful motor, and you see what I mean. By no means is this an excuse to use a weenie little motor to drive a 20 pound platter though. As an example the Atma-Sphere 208 is up to speed in 1/2 a revolution- not that far off from the SP-10, which gets there in 1/4 of a turn. Most of the delay on the 208 is the belt slipping on the motor, as it is up to speed before you can take your finger off of the 'On' button. However the platter of the SP-10 is about triple that of the 208 in weight. The SP-10 in particular is an exceptional turntable, and thoroughly disproves that old idea that DD was somehow flawed. But it is the exception rather than the rule regarding DD, IOW the SP-10 motor has more torque than most any other motor in use in turntables. I am sure that is a large part of why it works so well. A robust drive is really important in a good turntable! |
The platter also does not keep rotating very far after the motors are switched off indicating perhaps a bearing designed for friction so that the motors have a load against which to pull? Halcro, I can't think of a reason for a bearing to provide drag in a turntable- that seems like a Bad Idea to me. Are you sure the bearing is OK? |
Halcro, I'm not sure I agree with Peter on this one. Our model 208 has the motor quite close to the platter (opposite the tone arm; there are no hum problems even with Grados). However the 208 uses a motor that would easily qualify as a flywheel in its own right. Keep in mind that in a belt drive, cogging is almost a non-issue due to the rotational speed of the motor, and then really only applies to DC motors for the most part.
AC motors are not particularly prone to cogging effects. The best tape drives all have direct-drive capstan motors, usually AC, and they often have rock-solid speed stability that turntables struggle to match.
The Empire that our machine is based on was one of the most speed stable made for quite a long time. Julian Hirsch complained in one of his reviews of it that the speed variation was "buried in the noise" of his instruments, making an exact number difficult to gauge (although Empire did publish specs at the time). Sounds to me like he could have used a Timeline...
I've played with a variety of belts on the machine. Interestingly, even mylar variants sound exactly the same. Its my opinion that if the belt alters the sound then there is a problem in the drive system somewhere.
So I don't buy the idea that drag in the bearing results in speed stability at all although it would not surprise me to find that some designers may have created this story for reasons of their own (obviously the 208 flies right in the face of Peter's comments). OTOH I don't doubt that the belt-drive Thorens had some problems! I think that is why the earlier Thorens are the ones people are looking for. |
Which Empire TT are you referring to? AC or DC drive ,belt type? The Empire is of course AC synchronous drive. But most of my experience with it is actually with our variant of the machine, which we have been doing since the mid-90s: http://www.atma-sphere.com/Products/#Atma-208 |
Pabst is indeed a company name and does not refer to a type of motor. The AC motor in our 208 is indeed the eddy current AC model and was also used by Rek-O-Kut and a variety of tape machines. With its external rotor, its flywheel effect is prodigious! As good as it is, IMO/IME their servo DC motors left much to be desired. Were it not for the motor in the 208, I'm not sure I would even be messing with belt-drive at all. |
However, he did also comment that with a true 3-phase AC synchronous motor controller (so you can throw away the phase-splitting capacitor), the Papst would have been MUCH quieter and therefore might outperform the Thorens motor in all other respects. So, Ralph, have you ever used such a motor controller with your Atmasphere [sic] 208 turntable? Hi Lew, If the motor was noisy then its likely that there was some sort of defect. All I have ever heard from them is a hiss related to the air moving around the rotor. In my application it is isometrically mounted, so noise simply isn't an issue. What I have heard about the controllers is they are used to get greater stability than the AC line offers, but I am dubious about that- the AC line is quite stable! |
Atmasphere - I think you are too optimistic on the quality of the US power supply. The US grid is split into 4 quadrants and the supply agreements in place with the power providers have delivery standards etc, but there are NO standards in the agreements on frequency stability. As you will know frequency will vary with load fluctuations on the grid. I have seen figures of 60hz with an accuracy of 5%. This issue is currently being investigated. Frequency variation does occur but happens over a period of hours rather than seconds, at least, that's what they teach you in school. You may well be right- every time I have checked the speed with a strobe, its spot on, but that probably does not mean a lot as the strobe relies on the line frequency. So all it does say is that the 'table is spot-on relative to the frequency on the AC line. The Speed-Nic thingy says its OK, but IME it does not have a lot of resolution... |
Servo design in a DD 'table certainly includes the expected mass of the platter. If that changes significantly it sill alter the response of the servo; likely to the underdamped side of things. Many cheaper DD tables have a platter that can be removed and the motor can be operated without it- in this case the cogging effects are often quite visible. This an extreme example of course, but illustrates an under-damped condition.
So you probably can decrease the electrical damping by increasing the mass, but you should also not be surprised if overall speed stability is also compromised. Servos often have to operate within some fairly tight parameters. |
I picked up a Timeline as I was interested in how our Scully Lathe was doing while cutting lacquers.
The Scully uses a 1/8 horsepower Bodine AC synchronous motor, driving a transmission that then drives the platter via a drive shaft.
With the cutterhead cutting on the plate, the laser does not move- over the entire side you can't see the change even when you use a micrometer. Not bad for being built in the 1940s :) |
Dover, it has been sitting for some decades, I suspect.
The trust bearings are interesting. They are a set of polished platforms that reside beneath the platter. The drive shaft comes through the center and has nothing to do with them! |
