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Old 9th January 2006, 08:27 PM   #1
pixpop is offline pixpop  United States
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Default Why is DD bad?

I've seen it said, many times, that direct drive is a bad way to drive a turntable. But I can't seem to find a consistent explanation.

Many folks say the problem is cogging. What does cogging sound like?
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Old 9th January 2006, 08:37 PM   #2
Cobra2 is offline Cobra2  Norway
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Default Re: DD

DirectDrive is just as good or bad as any other drives...
It all depends on how well it is made.
And it is much easier to make a reasonably good belt-drive.
So the "war" was made up in the marketing departements of europeean manufacturers, to fight off the japanese DD invasion...

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Old 9th January 2006, 08:40 PM   #3
cwise is offline cwise  United States
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Default It isn't bad...

it is just harder to make a high quality DD than it is to make a high quality tape/belt drive. With DD you have to deal with the motor noise/rumble being directly translated to the platter, record, and cartridge.
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Old 10th January 2006, 01:18 AM   #4
pixpop is offline pixpop  United States
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--> it is just harder to make a high quality DD than it is to make a high quality tape/belt drive. With DD you have to deal with the motor noise/rumble being directly translated to the platter, record, and cartridge. <--

But what is causing 'motor noise' ? There's only one moving part, the same part that all TTs have.. the platter.
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Old 10th January 2006, 02:12 AM   #5
cwise is offline cwise  United States
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Default You have to consider the motor as a 'moving part'

On a DD turntable, the motor is in contact with the platter via a solid connection (idler wheel). If the motor vibrates, that vibration is transfered from the motor, to the idler wheel, to the plater, to the vinyl to the cartridge. Belt drives are able to decouple the motor from the platter. The belt has a tendency to absorb some of the vibrations.
Try holding a motor in your hand, even with very well made motors, you will feel a small amount of vibrations.
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Old 10th January 2006, 04:07 AM   #6
maxro is offline maxro  Canada
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Default Re: You have to consider the motor as a 'moving part'

Quote:
Originally posted by cwise
On a DD turntable, the motor is in contact with the platter via a solid connection (idler wheel). If the motor vibrates, that vibration is transfered from the motor, to the idler wheel, to the plater, to the vinyl to the cartridge. Belt drives are able to decouple the motor from the platter. The belt has a tendency to absorb some of the vibrations.
Try holding a motor in your hand, even with very well made motors, you will feel a small amount of vibrations.
A direct drive table and an idler wheel table are not the same thing. Direct drive is just that- the motor spindle is the platter spindle.

I don't know why people think that belt drive is better than direct drive. Perhaps it is the same arguement of tubes vs solid state (Gotta love that warm 2nd harmonic distortion).

I beleive that direct drive has gotten its stigma from the influx of cheap plastic tables during the early '80s, many of which were direct drive. It wasn't the direct drive that was bad about them, it was the cheap, resonant plastic plinths and thin platters.

Direct drive is more expensive to make well and that is why it is rarely seen on "high end" esoterica. One needs the economy of scale to offset initial design and manufacture costs. With belt drive you don't even have to make your own motor.

The advantage of direct drive is speed stability. There is no lag or overshoot that can happen with a belt under transient loads. Many belt drive tables are using massive platters to provide a flywheel effect to minimise transient load effects.

As far as one being better than the other; well, I'll just mention that I sold my Rega Planar 3 (belt) when I compared it to a "cheap" Denon DP-30L(?) (direct) that I picked up at a thrift store. I have since replaced that with a Kenwood KD650, also direct drive, but with a much more substantial plinth.

Max
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Old 10th January 2006, 04:41 AM   #7
pixpop is offline pixpop  United States
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--> I beleive that direct drive has gotten its stigma from the influx of cheap plastic tables during the early '80s, many of which were direct drive. It wasn't the direct drive that was bad about them, it was the cheap, resonant plastic plinths and thin platters. <--

Yes, I have a couple of those. Actually, I have one that's direct drive, and one that's belt drive. I've seen quite a few of them for sale at thrift stores. They all look like they came from the same factory, regardless of the brand. The direct drive motor is quite good actually, very precise speed control. The rotor has a shaft that sits in an oil bath, in a brass sleeve. At the top end, the shaft tapers, to fit a matching taper in the underside of the platter. You can just lift the platter off, and the motor still works. The rotor is about 3" diameter, and has the multi pole cylindrical magnet underneath.

As you say, the platter is very thin, about 1/8" thick aluminum, and rings like a bell when struck. The motor is very strong, and will still maintain speed if I add a large amount of weight to the platter.

I'm thinking about putting the motor into a more substantial plinth, and adding a good arm. But I was wondering if it was worth the effort, as I had heard so many folks badmouthing direct drive.
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Old 10th January 2006, 04:53 AM   #8
rjm is offline rjm  Japan
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With DD, the armature of the motor is attached to the spindle. This makes it more difficult to engineer a really good bearing.

That and the sophistication of the control electronics as kept it out of the DIY circuit. Here in Japan though its a cottage industry taking the big Technics commercial decks (the SP-10 being a fave) and re-building your own motor driver and control circuitry.

"Cogging" - I think - refers to the fact that the encoder is made up of discrete steps, and the feedback is thus given control info only, say, a hundred or so times per revolution. So you can imagine that the motor speed "updates" in bursts, which would sound bad...

From what I know of the control circuits used, this appears to me to be marketing hogwash. Any competent design would filter the encoder signal with time constants low enough to sufficiently smooth these steps out.

But I'm a happy Denon DP-2000 owner, so I would be biased, wouldn't I?

-rjm


PS I should add that belt drive designs normally have no feedback from the platter ... relying on inertia only. Its simple and the engineering challenges can all be solved by brute force. Thus the audiophile appeal.
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Old 10th January 2006, 05:03 AM   #9
maxro is offline maxro  Canada
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For more info about high-end direct drive tables, check out:

http://de.geocities.com/bc1a69/index_eng.html

Max
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Old 10th January 2006, 06:35 AM   #10
pixpop is offline pixpop  United States
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--> I should add that belt drive designs normally have no feedback from the platter ... relying on inertia only. Its simple and the engineering challenges can all be solved by brute force. Thus the audiophile appeal. <--

For making a DD motor controller, the challenge would be generating 3 sine waves whose phase could be controlled precisely. I think this would need to be done without PWM, as the PWM circuits generate too much radiation. Better to have them driven by power op amps I would think.

For me, I'm going to start with the motor I have, and only change it if I find something wrong with it.
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