i_should_coco

Turntables DD vs BD vs Idler

137 posts in this topic

Let's discuss.

Haing recently nailed my colours to the DD mast, I am very happy, the pitch stability is second to none. But what are the disadvantages and why would anyone use belt drive if there aren't any?

What about idlers? What benefits does this system confer?

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i_should_coco wrote:

Let's discuss.

Haing recently nailed my colours to the DD mast, I am very happy, the pitch stability is second to none. But what are the disadvantages and why would anyone use belt drive if there aren't any?

What about idlers? What benefits does this system confer?

A very interesting question! Here's my view on this, and by the way, I'm also a DD user for two of my TTs and idler drive for my other two.

Belt drives have the advantage that motor rumble and motor Wow and Flutter (cogging) are isolated by the rubber belt from the turntable/arm system. It is also easy to suspend the turntable/arm system on springs, rubber bands etc to provide isolation from building-borne vibrations, including audio feedback and microphony.

This allows an inexpensive motor to be used, and light-weight plinth and other mechanicals, all reducing cost for a certain level of performance.

DD requires that the motor has very low levels of noise and speed fluctuations, especially if the DD turntable has very low mass, as is typical in Broadcast applications requiring instant-start. For non-broadcast use where start-up time is less critical, W&F can be reduced as in belt-drives by having a heavy platter, but the motor still has to be superbly engineered.

The servo that controls TT speed needs to be very fast acting with very low overshoot as otherwise dynamic wow can be a problem, but again, this can be largely overcome by a heavy platter.

DD turntables could be isolated on springs, just like a Belt drive, but their greater weight makes this more difficult, and sufficient isolation can usually be achieved with rubber shock-mounts, feet etc. Nevertheless, DD turntables are seldom as well isolated as a belt-drive suspended sub-chassis.

Idler drives also require a very good motor, as the isolation provided by the rubber idler is small,and they also require very accurate machining of the drive surfaces. Otherwise, I would say there's little difference in performance between a DD or ID, speed stability and isolation is about the same, although speed accuracy is much better on a DD due to electronic speed control.Having said that,not that anyone's done it, but if an ID were given the samesort of servo control that is common on DD, then I would say that their speed stability and accuracy should be substantially the same.

S.

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SergeAuckland wrote:

i_should_coco wrote:
Let's discuss.

Haing recently nailed my colours to the DD mast, I am very happy, the pitch stability is second to none. But what are the disadvantages and why would anyone use belt drive if there aren't any?

What about idlers? What benefits does this system confer?

A very interesting question! Here's my view on this, and by the way, I'm also a DD user for two of my TTs and idler drive for my other two.

Belt drives have the advantage that motor rumble and motor Wow and Flutter (cogging) are isolated by the rubber belt from the turntable/arm system. It is also easy to suspend the turntable/arm system on springs, rubber bands etc to provide isolation from building-borne vibrations, including audio feedback and microphony.

This allows an inexpensive motor to be used, and light-weight plinth and other mechanicals, all reducing cost for a certain level of performance.

DD requires that the motor has very low levels of noise and speed fluctuations, especially if the DD turntable has very low mass, as is typical in Broadcast applications requiring instant-start. For non-broadcast use where start-up time is less critical, W&F can be reduced as in belt-drives by having a heavy platter, but the motor still has to be superbly engineered.

The servo that controls TT speed needs to be very fast acting with very low overshoot as otherwise dynamic wow can be a problem, but again, this can be largely overcome by a heavy platter.

DD turntables could be isolated on springs, just like a Belt drive, but their greater weight makes this more difficult, and sufficient isolation can usually be achieved with rubber shock-mounts, feet etc. Nevertheless, DD turntables are seldom as well isolated as a belt-drive suspended sub-chassis.

Idler drives also require a very good motor, as the isolation provided by the rubber idler is small,and they also require very accurate machining of the drive surfaces. Otherwise, I would say there's little difference in performance between a DD or ID, speed stability and isolation is about the same, although speed accuracy is much better on a DD due to electronic speed control.Having said that,not that anyone's done it, but if an ID were given the samesort of servo control that is common on DD, then I would say that their speed stability and accuracy should be substantially the same.

S.

Assuming that a TT's job is to spin the record at a constant speed, the tight coupling that this method produces would seem to be the best from an engineering point of view, given the caveat that the motor is extremely quiet. I guess the bad reputaion of cheap Japanese DDs came from attempting to compromise the design too much. Perhaps DD is only a good method if done properly, whereas belt or idler designs are more forgiving?

From what you are saying, if it's not possible to use the best quality motor (e.g. for commerical considerations of cost, etc.) then the idler method followed by the belt drive would seem to be the order of things.

One other thing I have against belt drives, is that the belt is an absolutely critical part of the system - any variation from ideal would compromise the design. As rubber is not a stable compound and changes over time, this would mean that it's hard to maintain consistency with time, no? What I also don't understand is the expense no power supplies, when the perfect speed control is transferred through a floppy rubber belt - this would seem to defeat the object...

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i_should_coco wrote:

SergeAuckland wrote:
i_should_coco wrote:
Let's discuss.

Haing recently nailed my colours to the DD mast, I am very happy, the pitch stability is second to none. But what are the disadvantages and why would anyone use belt drive if there aren't any?

What about idlers? What benefits does this system confer?

A very interesting question! Here's my view on this, and by the way, I'm also a DD user for two of my TTs and idler drive for my other two.

Belt drives have the advantage that motor rumble and motor Wow and Flutter (cogging) are isolated by the rubber belt from the turntable/arm system. It is also easy to suspend the turntable/arm system on springs, rubber bands etc to provide isolation from building-borne vibrations, including audio feedback and microphony.

This allows an inexpensive motor to be used, and light-weight plinth and other mechanicals, all reducing cost for a certain level of performance.

DD requires that the motor has very low levels of noise and speed fluctuations, especially if the DD turntable has very low mass, as is typical in Broadcast applications requiring instant-start. For non-broadcast use where start-up time is less critical, W&F can be reduced as in belt-drives by having a heavy platter, but the motor still has to be superbly engineered.

The servo that controls TT speed needs to be very fast acting with very low overshoot as otherwise dynamic wow can be a problem, but again, this can be largely overcome by a heavy platter.

DD turntables could be isolated on springs, just like a Belt drive, but their greater weight makes this more difficult, and sufficient isolation can usually be achieved with rubber shock-mounts, feet etc. Nevertheless, DD turntables are seldom as well isolated as a belt-drive suspended sub-chassis.

Idler drives also require a very good motor, as the isolation provided by the rubber idler is small,and they also require very accurate machining of the drive surfaces. Otherwise, I would say there's little difference in performance between a DD or ID, speed stability and isolation is about the same, although speed accuracy is much better on a DD due to electronic speed control.Having said that,not that anyone's done it, but if an ID were given the samesort of servo control that is common on DD, then I would say that their speed stability and accuracy should be substantially the same.

S.

Assuming that a TT's job is to spin the record at a constant speed, the tight coupling that this method produces would seem to be the best from an engineering point of view, given the caveat that the motor is extremely quiet. I guess the bad reputaion of cheap Japanese DDs came from attempting to compromise the design too much. Perhaps DD is only a good method if done properly, whereas belt or idler designs are more forgiving?

From what you are saying, if it's not possible to use the best quality motor (e.g. for commerical considerations of cost, etc.) then the idler method followed by the belt drive would seem to be the order of things.

One other thing I have against belt drives, is that the belt is an absolutely critical part of the system - any variation from ideal would compromise the design. As rubber is not a stable compound and changes over time, this would mean that it's hard to maintain consistency with time, no? What I also don't understand is the expense no power supplies, when the perfect speed control is transferred through a floppy rubber belt - this would seem to defeat the object...

You're right about all the above. DD needs very good engineering and the results tend to be physically heavy, so adding to shipping and handling costs, but if done well, seems to me to be the best engineering solution.

Idler drive, to me, isDDbefore it was possible to make very quiet slow-turning motors, and accurate servos.

With the exception of isolation from motor noise, I can't see any engineering benefit from belt drive, and I absolutely agree that expensive external power supplies and crystal speed locking seem rather pointless when it's all isolated by a rubber belt. If you use a stiff belt, like silk or linen threads, then you lose the isolation that rubber affords, and increase slippage, so again, what's the point?

Simple belt drives like the original Connoisseur BD1, then Regas, have the advantage of simplicity and low cost, but I too can't see any point in some of the current belt-drive designs.

I suspect that fancy power supplies and speed control are another way ofextracting more money from the gullible.

S.

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mosfet wrote:

Just out of interest.. Is anyone building turntables with the newer brushless motors?

Brushless motors are notably quiet, with high efficiency and plenty of torque.

You mean DC motors I take it? I know PT are a big advocate of DC motors and in the Anniversary the motor was on the subchassis, so was certainly quiet enough. It just had piss poor torque. I think the problem there was that the soft sub-chassis limited the weight of the motor, hence the weedy little thing.

I think the old Technics DDs and probably others are brushless, not 100% sure, though.

I can see when using a cheap AC motor why you'd want it well de-coupled from the platter.

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i_should_coco wrote

You mean DC motors I take it?

Well I’m not sure, I don’t know much about turntables.

The motors I was thinking of are popular with model aircraft builders; they are powered from a DC source and use a digital switching controller. The ones I’ve seen would have enough torque to overcome start-up inertia of a turntable I think. Almost silent in running.

eflite_brushless_motor_17b.jpg

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I've always found that DD (as a general rule)has a very solid soundstage, piano doesn't suffer with pitch instability as the stylushas little, if any,'braking' effect on the platter due to the massive amounts of torque applied via the motor. This would of course apply to the likes of a monster such as the various SP10 incarnations etc, etc. This doesn't mean that a DD is the bestway to overcome friction

There are otherways around this breaking effect, or friction induced pitch instability, and that's to build a massive platter so the inertia, fly wheel effect,overcomes the friction of the stylus acting in the groove. This should result in a morestable speed/pitch.

I've found that belt drives on low mass platters suffer with pitch instability, this is obviously more noticeable on certain types of musicwhich display sustained frequencies/notes or notes in a state ofdecay such as from piano.

After moving from one belt drive deck (Gert Pedersen Orbe)to another with a platter which is a number of kilogramsheavier (AS One), bass is more resolved and piano works have a much more focused and stable presentation. Its a lot more stabe sounding than I remember the likes of my SL1210 sounding.

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mosfet wrote:

i_should_coco wrote

You mean DC motors I take it?

Well I’m not sure, I don’t know much about turntables.

The motors I was thinking of are popular with model aircraft builders; they are powered from a DC source and use a digital switching controller. The ones I’ve seen would have enough torque to overcome start-up inertia of a turntable I think. Almost silent in running.

eflite_brushless_motor_17b.jpg

That's about the size of the motor that PT/Funk Firm uses and it's proabbly quite high RPM so would need a belt or some kind of reduction mechanism. Compare with the motor from the Teres Certus (DD) TT! (The shaft is over 1" in diameter fr comparison).

DD_Motor.jpg

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With the exception of isolation from motor noise, I can't see any engineering benefit from belt drive, and I absolutely agree that expensive external power supplies and crystal speed locking seem rather pointless when it's all isolated by a rubber belt. If you use a stiff belt, like silk or linen threads, then you lose the isolation that rubber affords, and increase slippage, so again, what's the point?

Simple belt drives like the original Connoisseur BD1, then Regas, have the advantage of simplicity and low cost, but I too can't see any point in some of the current belt-drive designs.

I suspect that fancy power supplies and speed control are another way ofextracting more money from the gullible.

S.

Sege what proof do you have of speed instability in belt driven decks with decent motor controllers, it sounds like you are making assumptions about all decks based on those at the bottom end of the spectrum.

one of the joys of a belt drive deck is that i can isolate the main bearing vertically with magnets for very little added complexity, not something that can be done so easily with an idleror at all with a DD AFAIK.

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sq225917 wrote:

With the exception of isolation from motor noise, I can't see any engineering benefit from belt drive, and I absolutely agree that expensive external power supplies and crystal speed locking seem rather pointless when it's all isolated by a rubber belt. If you use a stiff belt, like silk or linen threads, then you lose the isolation that rubber affords, and increase slippage, so again, what's the point?

Simple belt drives like the original Connoisseur BD1, then Regas, have the advantage of simplicity and low cost, but I too can't see any point in some of the current belt-drive designs.

I suspect that fancy power supplies and speed control are another way ofextracting more money from the gullible.

S.

Serge what proof do you have of speed instability in belt driven decks with decent motor controllers, it sounds like you are making assumptions about all decks based on those at the bottom end of the spectrum.

one of the joys of a belt drive deck is that i can isolate the main bearing vertically with magnets for very little added complexity, not something that can be done so easily with an idleror at all with a DD AFAIK.

I'd have to agree with you sq.

Serge, how would a platter weighing several kilo's and suffer from slipping as you've put it? Any passages that areintensively transcribed will occur momentarily. The sheer inertia of a heavy platter being belt driven (or in my case, driven with a thin cottonthread) willovercome the issue of friction induced pitch instability. The record is more likely to slip on the platter before the belt slips. I also use a very heavy machined recordweight. If I touch the platter when its spinning on my deck, its throw off unless I press hard.

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kingsxfan wrote:

sq225917 wrote:

With the exception of isolation from motor noise, I can't see any engineering benefit from belt drive, and I absolutely agree that expensive external power supplies and crystal speed locking seem rather pointless when it's all isolated by a rubber belt. If you use a stiff belt, like silk or linen threads, then you lose the isolation that rubber affords, and increase slippage, so again, what's the point?

Simple belt drives like the original Connoisseur BD1, then Regas, have the advantage of simplicity and low cost, but I too can't see any point in some of the current belt-drive designs.

I suspect that fancy power supplies and speed control are another way ofextracting more money from the gullible.

S.

Serge what proof do you have of speed instability in belt driven decks with decent motor controllers, it sounds like you are making assumptions about all decks based on those at the bottom end of the spectrum.

one of the joys of a belt drive deck is that i can isolate the main bearing vertically with magnets for very little added complexity, not something that can be done so easily with an idleror at all with a DD AFAIK.

I'd have to agree with you sq.

Serge, how would a platter weighing several kilo's and suffer from slipping as you've put it? Any passages that areintensively transcribed will occur momentarily. The sheer inertia of a heavy platter being belt driven (or in my case, driven with a thin cottonthread) willovercome the issue of friction induced pitch instability. The record is more likely to slip on the platter before the belt slips. I also use a very heavy machined recordweight. If I touch the platter when its spinning on my deck, its throw off unless I press hard.

That's quite right, a record on a platter weighting many kilos is unlikely to suffer from dynamic wow caused by modulation. And I'm not saying that belt drives are incapable of good performance, equivalent to DD and ID, and they do have the advantage that motor cogging noise is isolated.

What I am saying is that the very isolation afforded by the rubber belt makes rather a nonsense of sophisticated motor control circuits as they control the motor which is isolated from the platter by the belt. Consequently, they can only have an indirect effect on the platter rotation. It may be possible, and for all I know has been done, to put a tachogenerator on the platter, and use that signal to control the motor, thus including the belt in a feedback loop. However, the speed of response of the servo due to the inertia of the platter would make it a very difficult job to avoid overshoots, and therefore speed hunting, and consequently has probably been tried and discarded.

In your particular case, having a cotton thread would couple the platter closer to the motor, except for the fact that your platter is very heavy, and consequently the cotton will slip on either the platter or motor pulley if the motor tries to control the platter too quickly. As an illustration of this, what is the start-up time for your turntable? Several seconds I would expect, perhaps 15?

A DD turntable, especially one with a lightweight platter, can have a fast-acting servo and avoid dynamic wow by electronics rather than high mass. It won't necessarily be better than a belt-drive, but I find it a more elegant engineering solution. In the case of my own two DD turntables, start-up time to full stabilisation of speed is about 200mS, consequently I always put the stylus down onto the stationary record then press the start button.

My own preferences for DD and ID turntables has nothing to do with performance, BD is equally capable of good results, I just find DD a more elegant engineering solution, and my two IDs rathernostalgic.I have in the past owned a BD1, then LP12, Gyrodec, Pink Triangle, Thorensand finally Ariston before getting my first DD. That was the one I kept.

S.

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