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...
Arne K
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...
Arne K
--> 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.
But what is causing 'motor noise' ? There's only one moving part, the same part that all TTs have.. the platter.
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.
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.
Re: You have to consider the motor as a 'moving part'
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
cwise said:
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
--> 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.
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.
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.
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.
For more info about high-end direct drive tables, check out:
http://de.geocities.com/bc1a69/index_eng.html
Max
http://de.geocities.com/bc1a69/index_eng.html
Max
--> 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.
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.
Cogging is due to the motor and is a problem in either direct drive or belt drive. Essentially, a motor works by attracting a rotating magnet to a fixed electromagnet. Once it is aligned with the fixed electromagnet, we switch current to an electromagnet in a different position. If we keep on doing this, we can achieve continuous rotation. The problem is in achieving smooth rotation. One way of doing it is to modify the shape of the signal fed to the electromagnets. Another is to mechanically filter out the cogging by means of a compliant belt or idler wheel and heavy platter. Direct drive turntables can add compliance by matching the power of the motor to the mass of the platter. If you take the platter off a Technics SP10 it will cog like mad, but add the platter and it's fine. Eliminating cogging requires precision motor contsruction and precision electronic drive. The electronics are cheap, but the motors aren't.
Hope this helps.
Hope this helps.
I stand corrected on the cogging issue.
Though not for the faint of heart, you can get an idea of the guts of a DD turntable by studying the service manual for the SP10, kindly archived at The Vinyl Engine.
The idea is that all those transistors could be replaced by a few power op-amps and off the shelf logic chips, though doing that is WAY beyond my humble abilities.
It would be cool to design a microprocessor control that gave real time telemetry data and some AI to actually optimise the control algorithm. See what it says at the bottom? "this schematic can be modified at any time with the development of new technology." Hey, its 2006. What are you waiting for, drop a Cell processor in there!
-rjm
Though not for the faint of heart, you can get an idea of the guts of a DD turntable by studying the service manual for the SP10, kindly archived at The Vinyl Engine.
The idea is that all those transistors could be replaced by a few power op-amps and off the shelf logic chips, though doing that is WAY beyond my humble abilities.
It would be cool to design a microprocessor control that gave real time telemetry data and some AI to actually optimise the control algorithm. See what it says at the bottom? "this schematic can be modified at any time with the development of new technology." Hey, its 2006. What are you waiting for, drop a Cell processor in there!
-rjm
Attachments
As you say, the Technics could undoubtedly be simplified with op-amps and more highly integrated digital electronics. As it happens, I have a couple of SP10 and a spare motor. But working up the enthusiasm to redesign the wheel...
I suppose that the nicest way to do it would be to synthesise the analogue waveforms digitally and present them to three DACs, then follow them up with power motor drivers. The tacho signal would need some interfacing too. The amount of design work needed really does make it a mass production piece of equipment.
I suppose that the nicest way to do it would be to synthesise the analogue waveforms digitally and present them to three DACs, then follow them up with power motor drivers. The tacho signal would need some interfacing too. The amount of design work needed really does make it a mass production piece of equipment.
All of that electronics can now be replaced by a single chip. Many manufacturers make them. They are designed for use with the so-called Brushless DC (BLDC) motors. These are really just 3 phase permanent magnet motors, and are becoming very popular in many applications.
Generally, the single chip controls control the current through the windings using PWM and current feedback. They usually use hall effect sensors to sense the motor position, and derive speed from that.
The control circuit in my DD Sansui looks very similar to that shown above for the SP-10. This certainly could be replaced by some power op-amps and a microcontroller such as a PIC to control the speed. But as far as I can tell, there's no need, as the discrete transistor circuit shown above controls the speed fine. I guess the main reasons to replace the circuit would be if it was faulty, and couldn't be repaired, or if it generated cogging noise somehow because it's sinewaves were distorted.
Generally, these motors generate their sinewave drive signals by simply amplifying the signals from the hall sensors. These are not actually sine waves, but rather more complex trapezoidal signals. But they work fine for most applications, and they allow the circuit to be simple. The trick is that the hall sensors must have a precise physical phase relationship relative to the motor windings.
Replacing this control circuit with a higher powered linear circuit would also allow you to apply more power to spin up a more massive platter. I think that's what I'm going to try if I find that the motor as is can't cope.
I'm currently experimenting with a similar motor from a VCR, the motor that spins the heads. I'm trying to see whether it can be made to run at a slow speed, using the motor control chip that came with it. (BTW, it's educational to pull these motors apart. The VCR head motors are just a beautiful piece of technology, managing to communicate multiple signals between the stationary and moving parts with no sliding contacts).
Generally, the single chip controls control the current through the windings using PWM and current feedback. They usually use hall effect sensors to sense the motor position, and derive speed from that.
The control circuit in my DD Sansui looks very similar to that shown above for the SP-10. This certainly could be replaced by some power op-amps and a microcontroller such as a PIC to control the speed. But as far as I can tell, there's no need, as the discrete transistor circuit shown above controls the speed fine. I guess the main reasons to replace the circuit would be if it was faulty, and couldn't be repaired, or if it generated cogging noise somehow because it's sinewaves were distorted.
Generally, these motors generate their sinewave drive signals by simply amplifying the signals from the hall sensors. These are not actually sine waves, but rather more complex trapezoidal signals. But they work fine for most applications, and they allow the circuit to be simple. The trick is that the hall sensors must have a precise physical phase relationship relative to the motor windings.
Replacing this control circuit with a higher powered linear circuit would also allow you to apply more power to spin up a more massive platter. I think that's what I'm going to try if I find that the motor as is can't cope.
I'm currently experimenting with a similar motor from a VCR, the motor that spins the heads. I'm trying to see whether it can be made to run at a slow speed, using the motor control chip that came with it. (BTW, it's educational to pull these motors apart. The VCR head motors are just a beautiful piece of technology, managing to communicate multiple signals between the stationary and moving parts with no sliding contacts).
I doubt if a drum motor that was designed to spin at 1800 rpm (USA) or even 1500 rpm (UK) will take kindly to being forced to spin at 33 1/3. But you're right, the rotary transformers etc are wondrous to behold.
Do you have any suggestions about BLDC chip types for a Technics motor? One of my Technics works but won't stop (a simple fault, but still a fault), the other is missing its power supply and clock generator (it was BBC-modified).
Do you have any suggestions about BLDC chip types for a Technics motor? One of my Technics works but won't stop (a simple fault, but still a fault), the other is missing its power supply and clock generator (it was BBC-modified).
The Fairchild KA3080 is a representative device for use with motors that use hall sensors. But I think putting such a device into a TT that has some other control circuit is non-trivial. I would be trying to repair the existing one first.
many other manufacturers make similar devices. Some are designed to work without sensors using the motor's back EMF for feedback.
They all seem to control speed by balancing torque against friction. This is because the power is always applied at a constant phase offset from the rotor position. I would rather operate the motor more like synchronous motor, with the power more constant, but the phase relationship varying.
BTW, is the SP-10 worth buying as a reference DD implementation? I don't know much about them. Is it a good TT?
many other manufacturers make similar devices. Some are designed to work without sensors using the motor's back EMF for feedback.
They all seem to control speed by balancing torque against friction. This is because the power is always applied at a constant phase offset from the rotor position. I would rather operate the motor more like synchronous motor, with the power more constant, but the phase relationship varying.
BTW, is the SP-10 worth buying as a reference DD implementation? I don't know much about them. Is it a good TT?
The Technics SP10mkII was the standard broadcast turntable. We had them at the BBC where they replaced our Garrard 301. They were designed to have good performance and to be able to start and stop quickly (essential for sound effects and DJs). It's certainly one of the best DD, if not the best.
--> I doubt if a drum motor that was designed to spin at 1800 rpm (USA) or even 1500 rpm (UK) will take kindly to being forced to spin at 33 1/3. <--
Just for grins, I just sat the platter from my DD turntable on the spindle motor from the VCR, and applied the power. It did begin to spin up, drawing a constatn 0.5A from the motor controller. I had to stop it before it got too fast, because it was starting to wobble. But I think that means it would actually work if I servoed the speed at 33.3 rpm instead of 1800.
The interesting thing is, there are no speed sensitive components in the motor. The mechanical location of the hall sensors guarantees the correct phase relationship for maximum torque. The servo simply controls speed by controlling torque.
I might start looking around for a used SP-10 to use as a reference if I get serious with these experiments.
Just for grins, I just sat the platter from my DD turntable on the spindle motor from the VCR, and applied the power. It did begin to spin up, drawing a constatn 0.5A from the motor controller. I had to stop it before it got too fast, because it was starting to wobble. But I think that means it would actually work if I servoed the speed at 33.3 rpm instead of 1800.
The interesting thing is, there are no speed sensitive components in the motor. The mechanical location of the hall sensors guarantees the correct phase relationship for maximum torque. The servo simply controls speed by controlling torque.
I might start looking around for a used SP-10 to use as a reference if I get serious with these experiments.
Of course, it's probably no good to use a VCR drum motor in a DIY direct drive TT, because the VCR motor has ball bearings. Similarly for a hard drive motor, which works essentially the same way.
Although.. looking at the one I have, it does look possible to separate the rotor and stator from the bearings, so it may well be possible to use them with a quiet bearing. The tolerances between rotor and stator are not terribly close.
The downside is that it spins up very slowly. I'll try more current, and see what happens. At the moment, it looks as though it will take about 20 seconds to get up to 33.3 rpm, and that's just with a light platter.
The real DD turntable motor is much larger, and generates much more torque.
Although.. looking at the one I have, it does look possible to separate the rotor and stator from the bearings, so it may well be possible to use them with a quiet bearing. The tolerances between rotor and stator are not terribly close.
The downside is that it spins up very slowly. I'll try more current, and see what happens. At the moment, it looks as though it will take about 20 seconds to get up to 33.3 rpm, and that's just with a light platter.
The real DD turntable motor is much larger, and generates much more torque.
Has anyone tried using one of the DD motor drive assemblies and modifying it to be used in a high end DIY bearing and platter? I have been pondering doing a turntable, but have been hung up on not having a motor design that I know will work to my expectations without needing a whole lot of fiddeling.
It seems to me that the design around a DD motor and control has been done. Why re-invent the wheel. Just pick up a good motor and controller and install it in a heavy plinth with a heavy platter and isolate the tonearm. Seems like that would be a pretty easy way to avoid designing your own optical or hull feedback.
Any feedback on that idea?
DaveM
It seems to me that the design around a DD motor and control has been done. Why re-invent the wheel. Just pick up a good motor and controller and install it in a heavy plinth with a heavy platter and isolate the tonearm. Seems like that would be a pretty easy way to avoid designing your own optical or hull feedback.
Any feedback on that idea?
DaveM
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.