Restoring a Roksan Xerxes (original)... Wow!

[pankon]

Hi,the motor only spins on it's axis when the platters spins up apparently to compensate for the torque needed to start up the very heavy plater. Once up too speed it stabalises & remains in place.If you put the outer platter on upside down,which Roksan have provided the lip cut-out on the underside exactly for this purpose,you can see the motor spin to it's maximum extent & then gradually return to it's original position, assisted by the return spring, as the platter reaches a stable speed.With the platter on upside down you can also check that the belt is running true.Quite a neat solution.

Apologies if seeming to tell you what you already know but may be of benefit for others less well acquainted with the idiosyncrasies of this design.

Stewart,
I understand the design reason of placing the motor on the bottom plinth (and not on the top), so that vibrations are not carried over to the cartridge through the tonearm. However, I still remain rather sceptical about the motor rotating around its axis. This may help the platter to start easier, but I wonder whether it is beneficial with respect to the sound quality. Should the motor give in when there is even a momentary increase of resistance due to higher cartridge friction? The answer should be negative, I guess...
By the way, the OL DC motor can easily spin up the platter in a matter of a few (4-5) seconds.

[pankon]

Although Origin Live recommend placing their DC motor on the top plinth with a single screw, and although the OL DC motor is quiet, still there are inevitably vibrations that may be carried from the motor to the cartridge through the tonearm.
So I took the decision to experiment a bit and placed the motor on the bottom plinth, as the original Xerxes (AC) motor used to be. With the only difference that I did not allow the motor to rotate around its axis.
Initially, my impression was that the sound had become somewhat 'smaller' in comparison to when the motor was placed on the top plinth. Then, a few tracks and a few minutes later, I realized that the sound was somewhat cleaner and more precise and that gave the above-mentioned impression. It was perhaps the lack of motor vibrations and hence added energy around each note that made things smaller; but also more precise.
So, I will continue my experiment by having the motor on the bottom plinth, but I think that this is how it is eventually going to stay.
I do not know whether any other fellow forum member has had a similar experience with the two alternative motor placements.

[YNWaN]

My experience is that reducing the vibration transmitted from the motor and/or main bearing significantly increases the signal to noise ratio and so enhances dynamic range - for the same playback level (as previously used) this can initially give the impression of a 'smaller' sound.

I think mounting the motor on the same mechanical plane as the arm and bearing can work but it requires a lot more care with the motor mounting than just a simple rubber shim/gasket.

[pankon]

My experience is that reducing the vibration transmitted from the motor and/or main bearing significantly increases the signal to noise ratio and so enhances dynamic range - for the same playback level (as previously used) this can initially give the impression of a 'smaller' sound.

I think mounting the motor on the same mechanical plane as the arm and bearing can work but it requires a lot more care with the motor mounting than just a simple rubber shim/gasket.

Mark,
your feedback confirms my initial impression. Slightly 'smaller' sound but also slightly more detail.
I will keep the motor positioned on the bottom plinth, until my 'experiment' is concluded. It may take a few more weeks. But it seems that the placement on the bottom plinth is better (for now).

[YNWaN]

Hi,the motor only spins on it's axis when the platters spins up apparently to compensate for the torque needed to start up the very heavy plater. Once up too speed it stabalises & remains in place.

That's not quite right I'm afraid. The rotating motor isn't to do with startup torque - it doesn't help this aspect at all and the AC motor used would have no difficulty overcoming the static inertia of the platter anyway. The actual idea is to compensate for variable stylus drag and momentary variations in motor speed. As you say, once up to speed the motor is held stable, in equilibrium, though the spring holding it is under tension. For the idea to work the strength of the spring must be matched to that of the suspension and drive belt compliance - that is to say that it should be significantly less resistive than either (which it is). The concept is thus; the platter and motor are supposed to stay in a fixed relationship in terms of rpm and linked by the drive belt. However, is the rotation of one alters (due to variation in stylus drag or motor flutter) it will react independently initially, stretching the belt until the belt is able to compensate and regain the equilibrium. The idea is that instead of the forces acting to stretch the belt they will cause the motor to rotate by the same amount so maintaining even belt tension and so even speed. It's rather an interesting, but controversial, idea. Speed stability is one of the foundations upon which the success of the Xerxes was based though and so it certainly doesn't seem to do any harm (which on first thought one would rather expect it to).

[pankon]

That's not quite right I'm afraid. The rotating motor isn't to do with startup torque - it doesn't help this aspect at all and the AC motor used would have no difficulty overcoming the static inertia of the platter anyway. The actual idea is to compensate for variable stylus drag and momentary variations in motor speed. As you say, once up to speed the motor is held stable, in equilibrium, though the spring holding it is under tension. For the idea to work the strength of the spring must be matched to that of the suspension and drive belt compliance - that is to say that it should be significantly less resistive than either (which it is). The concept is thus; the platter and motor are supposed to stay in a fixed relationship in terms of rpm and linked by the drive belt. However, is the rotation of one alters (due to variation in stylus drag or motor flutter) it will react independently initially, stretching the belt until the belt is able to compensate and regain the equilibrium. The idea is that instead of the forces acting to stretch the belt they will cause the motor to rotate by the same amount so maintaining even belt tension and so even speed. It's rather an interesting, but controversial, idea. Speed stability is one of the foundations upon which the success of the Xerxes was based though and so it certainly doesn't seem to do any harm (which on first thought one would rather expect it to).

Mark,
I see where you are coming from, but I think there is an issue (potential weakness?) with the whole concept. Let's assume the classical case of momentary increased cartridge friction (stylus drag, as you call it). In that case the platter will tend to slow down. Then, if we assume that the belt does not stretch, the motor will tend to slow down as well. If however the motor tended to keep its rotational speed unchanged and overcome the platter deceleration, the fact that the motor is attached to a spring, it will not allow the motor to exert the required extra torque, but just give in instead and rotate around its axis, because -as you say- the spring is more compliant that the inertia of the platter.
So, unless my thinking is wrong, I believe the solution would be to have an extra-strong motor, keep it fixed in terms of self-rotation and also have a belt from a material that cannot stretch.
I am not contradicting your line of thinking just for the sake of it, I just think that the self-rotation principle is questionable.

[YNWaN]

I must tell you that the concept I expounded was not my own but that of Touraj Moghaddam designer of the Xerxes, I just phrased it for the purpose of this thread. Personally I have taken a different path. There is however an issue I am personally aware with with the mechanism that you propose; motor torque is proportional to load - during steady running friction is very low and load is also approaching zero as is torque. When the load does vary there is significant compliance surrounding the magnetic field within the motor, this compliance must be compressed before motor torque can increase and this injects delay into the system.

When you write 'extra strong motor' you presumably mean one capable of developing high torque. However, that torque only exists when under load, when loaded by the static inertia of a heavy platter at startup for example. What few seem to realise is that as load approaches zero so does torque - the maximum potential torque of the motor makes no difference in such a situation and once up to speed a turntable platter does offer very minimal load if the platter is balanced and the main bearing is low friction, the mass of the platter is irrelevant.

[pankon]

I must tell you that the concept I expounded was not my own but that of Touraj Moghaddam designer of the Xerxes, I just phrased it for the purpose of this thread. Personally I have taken a different path. There is however an issue I am personally aware with with the mechanism that you propose; motor torque is proportional to load - during steady running friction is very low and load is also approaching zero as is torque. When the load does vary there is significant compliance surrounding the magnetic field within the motor, this compliance must be compressed before motor torque can increase and this injects delay into the system.

When you write 'extra strong motor' you presumably mean one capable of developing high torque. However, that torque only exists when under load, when loaded by the static inertia of a heavy platter at startup for example. What few seem to realise is that as load approaches zero so does torque - the maximum potential torque of the motor makes no difference in such a situation and once up to speed a turntable platter does offer very minimal load if the platter is balanced and the main bearing is low friction, the mass of the platter is irrelevant.

I see where you are coming from with respect to requiring minimal torque when the platter is rotating at nominal speed. The issue is what happens when a deceleration force appears ('stylus drag') and how this can be overcome without being audibly noticeable.
By 'extra strong motor' I mean that the motor can provide the required torque so quickly, that any momentary increased cartridge friction will be instantly overcome without allowing for any deceleration and hence audible consequences. I assume that the belt will not be stretched (deformed).
The mass of the platter is relevant, because if properly distributed at the edge, it will create a flywheel effect, i.e. it will have a high angular momentum, and thus be less prone to deceleration (or acceleration).

[YNWaN]

The absolute torque of the motor is not the same as the speed it can alter its speed. In addition, it can't 'steamroller' through load variations as it is actually running at near zero torque (this is irrespective of the maximum torque the motor can generate when under load).

With regard to the mass of the platter, yes it will form a flywheel but not a very effective one when you consider the very low rotational velocity. I put to you that there is a dilemma in the logic of the drive system, a kind of chicken and egg issue or, more correctly, a master and slave problem. Is the platter inertia supposed to be governing the motor or is the motor supposed to be governing the speed of the platter? Or, as often seems to be the case, are both the platter and motor attempting to work against each other?

[pankon]

Well, it is now evident that the placement of the Origin Live DC motor on the bottom plinth (or chassis), instead of the top plinth as proposed by Origin Live, provides the following advantages:

• better fine detail
• lower noise floor
• slightly more realistic overall sound, especially with voices and separate instruments

The only tradeoff is the somewhat 'smaller' sound, but if I had to choose I would go for the motor on the bottom plinth.

I wonder whether there are other users of the OL DC motor or other motors who have made the same experiment of motor placement.