Hiho, The point is that if you don't know enough about the circuit to fiddle with the servo at the level of its electronics (which includes me), you can at least affect damping by changing the rotating mass of the platter, which is a parameter built into the servo system. Easiest way to do that is to alter the mass of the platter mat. Adding mass does not "overwhelm" the servo; it adds damping to the system as a whole, if you take the "no platter" condition as "undamped". I think in engineering terms this is the correct way to think of it, but I am more than willing to be corrected by an engineer.
But I agree with the sense of what you wrote, mostly. The L07D and perhaps the aforementioned TT101 may be examples of servos that don't attempt to control speed as tightly as does, for example, the Technics system. The engineers that worked for Denon, Kenwood, Victor, Technics, Pioneer, etc, each had a go at deciding upon the optimum way to achieve the same goal, and they came up with different answers. |
Doesn't this discussion on DD servos highlight the question, does a very high mass platter, with very high inertia, driven by a high torque motor with a belt, thread or fluid drive with built in slippage, such that the platter mass will drive through any load fluctuations sound better than a DD with its constant speed correction.
Correct me if I'm wrong, but isn't the need for servos because DC motors inherently do not respond in of themselves to changes in loads.
For those that are keen to experiment - here are a couple of decks worth trying :
Rossner and Sohn - latest special custom-made turntable is called The Mott (The Mother of Turntable). The rotating platter itself weighs 232 kg. Choice of air or oil bearing.
Sati - 50kg platter, interesting comments on motors on their website. |
Dear Dover, You wrote, "Correct me if I'm wrong, but isn't the need for servos because DC motors inherently do not respond in of themselves to changes in loads." OK. You are wrong. First you are wrong because your statement carries the assumption that all DD motors are DC motors. I am not sure you meant to say that or perhaps you meant to type "DD" and out came "DC". In fact, a major fraction of DD motors are 3-phase AC synchronous types. DC motors are also used in some pretty fancy and expensive belt-drive turntables. Second, you have failed to define what you mean by "load"; I assume you refer to variations in forces due to stylus drag, etc, that occur during the course of playing an LP. Third, you are wrong because your statement carries also an assumption that all DD turntables use servo feedback. Some of the new ones don't, e.g., The Beat. In any case, I think that the adoption of servo mechanisms has to do with the fact that a DD turntable motor has to turn constantly at the relatively slow speed of 33 rpm, and using servo feedback is one way to reach that goal. As we have been saying, there are many many different ways to apply servo feedback. Belt-drive motors run much faster and for some the belt itself is used to smooth over small changes in speed. Also, it's cheaper not to use any feedback. But major point is that in principle DD motors qua motors (thank you, Nandric) are no more and no less inherently speed constant than are belt-drive motors. I do not think it is possible to argue from generalities that one way is better than another. |
I believe the Brinkmann Bardo and Oasis use the gentler servo speed control. Here's what they wrote in their white-paper; it's a little long but it's a good read: PROBLEM WITH DIRECT-DRIVE
Studios (radio stations in particular) demand quick start-up times turntables typically have to reach their correct speed within half a revolution. For LPs this means 0-33 1/3 rpm within 0.9 seconds. Such acceleration figures can only be achieved through use of high-torque motors and correspondingly tight coupling between the drive and platter. It isnt a surprise then that for decades idler wheel drive designs were the defacto standard in studio applications. But idler wheel turntables also had seriously high maintenance costs in order to be up and running 24/7 and to avoid rumble and other sound degrading issues caused by worn out idler wheels to affect the sound negatively. Thus out of necessity, in the late 1960s manufacturers of studio turntables began to look for low(er) cost maintenance alternatives. They came up with direct drive, whereby the platter was placed directly on the motors shaft, ie the stator was mounted around the bushing and the shaft was used as the rotor and voila, the goal was achieved; at least in theory.
But start-up times of less than 1 second necessitated high torque motors, which designers achieved by using motors with 32 and more poles. The penalty they paid were heavy cogging effects accompanied by high wow & flutter numbers. The cure was found in quartz locked motors and phase locked regulators; which corrected for any deviations from their preset with an iron fist. On paper at least, these corrected direct drive turntables boasted hitherto unimaginable low wow & flutter numbers down to 0.001%. But the rigorous iron fist regulation prevented the platter from spinning smoothly; instead, the regulation caused the platter to oscillate continuously between speeding up and slowing down. These start/stop motions translated into an unpleasantly rough and hard sound; odd as wow & flutter numbers in the 0.001% range are deemed inaudible.
Once the direct drive technology had gained a foothold in pro audio applications, the benefits of mass production (ie. trickle down effect) made sure that very soon even $100 turntables were equipped with direct drive and advertised as having less than 0.01% wow & flutter. This is precisely where direct drive got its bad rap sheet. Under closer scrutiny however, this assumption were based on some misunderstandings. For one, in home audio application use, turntables are not really required to reach 33 1/3 rpm in less than a second, thus 32 pole motors and phase locked regulators are not really necessary either.
THE BRINKMANN SOLUTION
Having decided to utilize a direct-drive mechanism for our new turntable, we began searching for the appropriate motor. Sadly (or luckily, as it should later prove) and despite much effort we were unable to find a motor that was up to our stringent quality requirements. We therefore decided to design and build the motor in-housewhich had the nice side effect that we did not have to cut any corners and instead were in the fortunate position of defining all relevant parameters ourselves. The motors stator consists of four specially designed field coils, which are mounted concentrically with high precision around the platter bearing. Based on listening and tuning sessions, we decided to forgo the typical 90-degree mounting angle in favour of a non-standard 22.5-degree raster, which, due to the magnetic fields overlapping, further reduced cogging. The motors rotor also acts as the sub-platter and carries a magnetic ring with 8 poles on its underside.
The drive mechanism, based around Hall sensors and an encoder disk, is designed in such a way that there is just enough power to bring the 10 kg heavy platter up to 33rpm in about 12 seconds. Conversely, only a minimal amount of energy is actually necessary to keep the rotational speed at a constant. While the drive mechanism is indeed direct, power
is actually transferred without any contact. This soft coupling via a low power magnetic field translates into a silent drive, which reduces cogging further. One of the main attributes behind the sound quality of the Oasis has to do with our proprietary motor control. It works proportionally, i.e. it transfers just enough energy to the motor for it to remain at constant speed. Conversely, due to our ultra low-friction bearing, only a small amount of energy is actually necessary to keep the motor at constant speed. Previously available regulators typically work disproportional and rather abruptly: they speed up and slow down the motor very rapidly when necessary.
During the development phase of the Oasis turntable, we spent many long hours auditioning several different regulator designs; it became quite evident that utilizing our concept of proportional regulation always resulted in better sound: typical harder motor control concepts produced a sound significantly lower in quality, with less color and drive. I suspect their decision "to forgo the typical 90-degree mounting angle in favor of a non-standard 22.5-degree raster" is influenced by Dual's design of the EDS-1000 motor, which also has the same coil arrangement... _______ |
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. |
