Rega P5 running too fast

[watercourse]

Hi mr.datsum, what do you mean by the motor seating a bit off?

As you were talking about adjusting the position of the pulley to make the belt tauter , I wondered whether the motor was in the exact right spot. Now I realise that as I have no experience of the P5, I may talking out of my arse. With the Planar 3 the pulley has to be inserted and centred in the hole in the plinth through which it protrudes. When mounting the motor it is important for us Planar 3 owners to get the mounting position right first time as the motor is mounted with high-tack adhesive pads. I imagined that the P5 also has a hole through which the pulley is inserted BUT I have no idea how the motor is mounted. I assume that there is an optimum motor position to get the correct belt tautness, though and wondered whether your motor is in that spot.

Mr. Datsun, your approach also holds for the P5, but instead of mounting via tape, you can adjust the motor mounting distance with screws that attach to the top of the plinth. The left-hand screw is adjustable, the right is fixed in distance.

[watercourse]

To restate Frank's formula:

Gearing ratio = motor speed / platter speed (33.33rpm)

Pulley D = Subplatter D/Gearing ratio

UK speed at 50Hz = 250rpm
US speed at 60Hz = 300rpm

Thanks Frank and Mr. Datsun for covering for my laziness

There seems to be something off with the formulas, though. Try plugging in my measurements into the formula:

Stock pulley diameter ~ 9.65mm
GT subplatter diameter ~ 101.18mm
US motor speed = 300rpm
US gearing ratio = 9

Using the above formulas, the actual pulley diameter of 9.65mm should require a (calculated) subplatter diameter of 86.85, which is significantly smaller than the actual subplatter diameter.

Or conversely, if plugging in the subplatter diameter and gearing ratio, the pulley diameter should be 11.24mm. But I know that the Michael Lim double pulley diameter is about 0.3mm larger than the stock, and the table runs much faster with a larger pulley diameter. Unless we are talking about an 11.24mm effective diameter.

And in my sh*ts and giggles experiment, increasing the subplatter diameter by 2mm brought the table to within 0.3% of 33.33 rpm.

Thoughts?

[Frank_Sm]

In the case of Rega, The subplatter diameter defines what diameter the pulley needs to be since Rega uses a mass produced phenolic resin subplatter. It's not the other way around.
I'm quite sure that Rega manufactured many subplatters in order to determine the repeatability of the process before arriving at the final dimensioning of the pulley.

I've made a lot of pulleys for turntable projects and I normally draw the profile using CAM software to get the "approximate" dimensions. I then machine the pulley and install it and test it before arriving at the final dimension. This has always worked for me, but as they say, YMMV.

You can't rely on the measurement taken from the pulley using calipers. I know of a way you can do it but it still requires modeling it and also knowing the exact angle of the cutting tool.

Anyway, The formula is correct. It's the implementation that's questionable.

[mr.datsun]

There seems to be something off with the formulas, though. Try plugging in my measurements into the formula:

Stock pulley diameter ~ 9.65mm
GT subplatter diameter ~ 101.18mm
US motor speed = 300rpm
US gearing ratio = 9

Using the above formulas, the actual pulley diameter of 9.65mm should require a (calculated) subplatter diameter of 86.85, which is significantly smaller than the actual subplatter diameter.

Or conversely, if plugging in the subplatter diameter and gearing ratio, the pulley diameter should be 11.24mm. But I know that the Michael Lim double pulley diameter is about 0.3mm larger than the stock, and the table runs much faster with a larger pulley diameter. Unless we are talking about an 11.24mm effective diameter.

And in my sh*ts and giggles experiment, increasing the subplatter diameter by 2mm brought the table to within 0.3% of 33.33 rpm.

Thoughts?

watercourse. After Paul's point and with reference to Frank's latest post about how he comes to a correct pulley size, I now know that although the basic formulae are correct, there is one thing that cannot be done easily – and that is to determine the effective diameter of the pulley by measurement.

The pitch diameter of a pulley is not the outside diameter. Or the inside diameter. In fact, the pitch diameter is very difficult to measure directly.
from The Gizmologist's Lair

Having read around the subject a little (wiki and elsewhere) it seems that the point of highest tension in a belt is where the effective diameter would be measured, at least in theory. It appears that you cannot work backwards from an actual pulley/belt system to the real measurements. It's been just as hard for me to find out what the effective diameter of the sub-platter is once a round section belt is looped around it -it may be larger than it's nominal measurement and we can guess that that might be by the diameter of the belt (in motion, under tension).

So, long story short, it appears that without a more complete understanding of the subject, we can only use these measurements and formulae to aid thought experiments and get a general understanding of the problems of designing and using belt drives and hence how difficult is must be to get a perfect speed with any belt driven system. And that goes for a Linn or a Rega!

With regards your figures, I think this:

Your target pulley size is 11.24mm. (As you are using a 101mm subplatter.) And that is, ignoring stated problems about arriving at a true effective diamter of the subplatter, pretty much a fact.

Your stock pulley is ~ 9.65mm. I assume that this is the interior estimate diameter measured at the apex of the 'V'.

Now, looking at my drawing I could make a sensible guess that the effective diameter is in the order of pulley diameter measurement at interior of V groove + 2 x radius of the belt (or 1 x diameter)

In your case that would be 9.65 + 1.78 = 11.43mm which in a thought experiment is not too far off what you need. Of course, it ignores the true point of the belt's highest tension, the belt's true thickness under tension and also the true effective diameter of the sub-platter + belt.

Of course, it's not much use in the practical world. In practice the whole equation of platter speed is made more complicated by differences in manufacturer's pulleys and sub-platters and combinations of them. It's remarkable that any Rega or any belt driven turntable runs at 100% correct speed. I'm just happy that mine runs at a pretty good constant speed.

[mr.datsun]

In the case of Rega, The subplatter diameter defines what diameter the pulley needs to be since Rega uses a mass produced phenolic resin subplatter. It's not the other way around.
I'm quite sure that Rega manufactured many subplatters in order to determine the repeatability of the process before arriving at the final dimensioning of the pulley.

I've made a lot of pulleys for turntable projects and I normally draw the profile using CAM software to get the "approximate" dimensions. I then machine the pulley and install it and test it before arriving at the final dimension. This has always worked for me, but as they say, YMMV.

You can't rely on the measurement taken from the pulley using calipers. I know of a way you can do it but it still requires modeling it and also knowing the exact angle of the cutting tool.

Anyway, The formula is correct. It's the implementation that's questionable.

Frank, all this discussion and what you say about your method for arriving at a correct pulley size, has made me realise that the design of the pulley/subplatter system is more complex than I would have guessed. It makes me wonder whether there is a market for pulley and sub-platter pairs that are matched during manufacture, rather than Rega users arriving at a good match by trial and error...

[watercourse]

Thanks to the both of you. I do understand it a bit better, and I guess it makes sense why the Basis folks charge $100 for a belt, if they are actually testing, measuring, and QA/QC-ing them before sending.

[Frank_Sm]

In the case of Rega, The subplatter diameter defines what diameter the pulley needs to be since Rega uses a mass produced phenolic resin subplatter. It's not the other way around.
I'm quite sure that Rega manufactured many subplatters in order to determine the repeatability of the process before arriving at the final dimensioning of the pulley.

I've made a lot of pulleys for turntable projects and I normally draw the profile using CAM software to get the "approximate" dimensions. I then machine the pulley and install it and test it before arriving at the final dimension. This has always worked for me, but as they say, YMMV.

You can't rely on the measurement taken from the pulley using calipers. I know of a way you can do it but it still requires modeling it and also knowing the exact angle of the cutting tool.

Anyway, The formula is correct. It's the implementation that's questionable.

Frank, all this discussion and what you say about your method for arriving at a correct pulley size, has made me realise that the design of the pulley/subplatter system is more complex than I would have guessed. It makes me wonder whether there is a market for pulley and sub-platter pairs that are matched during manufacture, rather than Rega users arriving at a good match by trial and error...

Indeed that would be a good idea but you would also have to supply the belt with both pieces to take all variables into account. This is assuming the motor rotates at the correct speed too.

Yes, it is much more difficult to nail 33.3 or 45 rpm exactly due to these variables. Dimensions always have tolerances because in the real world of manufacturing, nothing is perfect. The pulley (and subplatter) must be given a tolerance zone so the machine shop can manufacture the part. Imagine what a part would cost if you told the shop to machine it to exactly to the specified dimension.
Even parts going on the space shuttle have limits and tolerances. These tolerances are obviously closely monitored and adhered to,I.E. measured in a climate controlled environment, etc.

I'm glad that you were able to arrive at your conclusion (regarding the process) by going through it and researching it.

[mawari]

>snip<
It's remarkable that any Rega or any belt driven turntable runs at 100% correct speed

Indeed. All this makes me appreciate now why some turntables drive the outside of the platter using thin nylon thread, with a correspondingly larger radius motor pulley. The greater dimensions in use would reduce the effect of pulley and platter machining tolerances, as well as belt "seating" and compression effects.

Somewhat harder to hide the pulley under the platter that way though

I'm just happy that mine runs at a pretty good constant speed

Enjoy the music. Don't freak out.

Absolutely. It can be an interesting exercise trying to work these things out, if for no other reason than to evaluate the potential for improvement ( or not ), but such diversions are not why we bought a hi fi !

Paul

[bacobits]

Wouldn't a better design motor controller with variable speed adjustments incorporated make all this go away?

[watercourse]

Calgon, take me away!!!

[Blueguitar]

OK - so what I get out of this thread so far is the following:

The TTPSU and White Belt definitely help with speed stability. The Groovetracer subplatter may help.

Speed will change over time as the belt ages - as it stretches out the speed will increase. The differences in speed, however, will not be heard, though they can be measured.

Is that right?

Thanks!

[RP3user]

OK - so what I get out of this thread so far is the following:

The TTPSU and White Belt definitely help with speed stability. The Groovetracer subplatter may help.

Speed will change over time as the belt ages - as it stretches out the speed will increase. The differences in speed, however, will not be heard, though they can be measured.

Is that right?

Thanks!

Yes I think that sums it up quite well

G

[brassnwood]

I agree with bacobits - Wouldn't a better design motor controller with variable speed adjustments incorporated make all this go away?
I've enjoyed reading this blog with all the points you guys have made but I'm glad I bought an Origin Live DC motor with variable speed control ttpsu
GC

[mr.datsun]

I agree with bacobits - Wouldn't a better design motor controller with variable speed adjustments incorporated make all this go away?
I've enjoyed reading this blog with all the points you guys have made but I'm glad I bought an Origin Live DC motor with variable speed control ttpsu
GC

brassnwood, the origin live DC models looks good but also see that their pricing climbs rapidly with increased quality. Which one are you using and how do you find it? It's clearly an important option for those Rega owners who have chosen to upgrade their drive.

With regards the overall subject I would have thought that a a better design motor controller with variable quartz locked speed would just cost more than a fixed speed model. I cannot help thinking that introducing variable speed into quartz controlled PSU would introduce either more cost or compromise. It's just that. Weigh it against the cost of the deck.

I mean to get accurate speed on a Linn is going to cost a Linn owner with an AC motor turntable £1,215.00. So in the context of that, I do not think we can say that Rega's approach is wrong or down to poor design. As with anything, it's just design, costing, pricing comprises all the way to our living rooms.

But sooner or later I guess that you are going to be paying half as much or as much for the PSU as the turntable. One thing I see on the Origin Live site is the statement that in their opinion the design of the motor drive is the single most important factor influencing any turntable's performance. http://www.turntable-power-supply.com/info.htm#results.