| pixpop |
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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| Cobra2 |
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 |
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| cwise |
| 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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| pixpop |
--> 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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| 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. |
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| maxro |
| 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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| pixpop |
--> 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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| rjm |
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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| pixpop |
--> 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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| EC8010 |
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. |
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| rjm |
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 |
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| EC8010 |
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. |
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| pixpop |
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). |
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| EC8010 |
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). |
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| pixpop |
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? |
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| EC8010 |
| 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. |
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| pixpop |
--> 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. |
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| pixpop |
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. |
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| DaveM |
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 |
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| pixpop |
That's exactly what I'm working on. I started this thread just to make sure I was on the right track, and that there wasn't something inherently bad about direct drive that would make the whole thing pointless. Apparently, there isn't.
The only thing I can't control is the quality of the original bearing. It may be possible to retrofit the motor onto a quality bearing if that becomes an issue. |
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| DaveM |
That was my idea, I have access to a machine shop with grinding equipment as well. I will probably buy a high polished shaft from McMaster Carr and fit that into an oil-lite (brass impregnated with lubrucant) bushing that I will grind to fit the shaft with minimal clearance. Then press in a polished carbide steel ball riding on a thin piece of teflon in an oil bath. I know it would be rediculous overkill, but that would be what I am after.
DaveM |
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| EC8010 |
| Teflon can't take that point loading. But sapphire or diamond can. |
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| dice45 |
.. i must know it since my homebrewn TT's platter bearing has a ruby ball of 3mm dia and a teflon trust plate. And one trust plate lasts for more than 5 years. Platter weight is about 4kg .
The trick is that the ball is a calibration ball with extremely low form deviations and ultra-low surface roughness. Any steel ball would eat itself thru the teflon within weeks.
With the ruby ball, there will be a stable indentation in the teflon surface.
And you have heard that thing, too, EC8010, as we met at the ETF in Langenargen 2004. My TT was the one with the fancy modified Rabco tonearm on it. You remember? |
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| davidsrsb |
I read somewhere that the VCR head was the highest precision component in mass production.
I suspect that the hardest part of building a DD is keeping that magnetic field out of the cartridge. |
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| maxro |
| quote: | Originally posted by davidsrsb
I suspect that the hardest part of building a DD is keeping that magnetic field out of the cartridge. |
An aluminum platter makes a great diamagnetic shield.
Although, I have read that Grado carts don't work well with some tables for lack of sheilding. My dad had one on his direct drive KD-500 and had no issues. Ironically, if I recall, the problem is with Regas (belt drive) and Grado cartridges. Hmm...
Max |
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| EC8010 |
| quote: | Originally posted by dice45
The trick is that the ball is a calibration ball with extremely low form deviations and ultra-low surface roughness. Any steel ball would eat itself thru the teflon within weeks. |
Well, well. I live and learn. That's very interesting. Presumably, it also has to be very accurately centred.
Sadly, I remember the Rabco but not the turntable it was on. |
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| Shaun |
| I have a spare SP10 Mkii with what appears to be a worn bearing (the platter rocks). It would be interesting to see what you guys come up with as a bearing to mate to this motor, as I have been told that it's not possible to replace the bearing on this unit. |
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| KimBOlesen |
Hi
Why not use the alu plate as a DD Lenz motor ? Only a bearing and a plate. No magnets, only two C-core electro magnets and some control electronics.
Best regards Kim |
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| EC8010 |
| quote: | Originally posted by Shaun
I have a spare SP10 Mkii with what appears to be a worn bearing (the platter rocks). It would be interesting to see what you guys come up with as a bearing to mate to this motor, as I have been told that it's not possible to replace the bearing on this unit. |
Perhaps the bearing is dry? If you take it apart a bit, you will find that you can clean the bearing and oil it.
I'd say it would be difficult to replace the bearing, but not impossible. |
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| pixpop |
--> I suspect that the hardest part of building a DD is keeping that magnetic field out of the cartridge. <--
Why... what do you think will happen? What would it sound like?
--> Why not use the alu plate as a DD Lenz motor ? Only a bearing and a plate. No magnets, only two C-core electro magnets and some control electronics <--
What is a Lenz motor? Is it the kind of motor that drives a power meter, with the spinning alumminum disk?
--> I have a spare SP10 Mkii with what appears to be a worn bearing (the platter rocks) <--
Yes, at least take it apart and find out why it rocks. If the bearing really is bad, then it will be interesting to see if the motor is constructed in such a way that you can put a different bearing into it, assuming some mechanical skills.
Neil |
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| davidsrsb |
The usual belt drive arrangement keeps the motor a long way from the cartridge and arm tube.
A typical DD motor either under the platter or at the rim is going to have 360 degree coil windings, so the cartridge will pass straight over them at some point, a couple of cm. The coils create a rotating field, which from a fixed point looks like an ac field. The cartridge body is plastic or non ferrous metal so the magnetic field will act directly on the coils. An aluminium platter is poor at blocking magnetic fields.
One way out is to put permanent magnets around the platter edge in a NSNSNS.. sequence and arrange coils to attract and repel at the correct time at only a small part of the edge, well away from the arm. I believe one Japanese manufacturer used to do this. The problem will be that a fancy coil waveform would be needed to avoid cogging, but a microcontroller and DAC could do this. |
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| pixpop |
It's true that aluminum would make a poor shield, but in the DD motors I've seen, as well as in VCR motors, the magnet is encased in a very effective ferromagnetic shield. You can put a peice of metal very close to it, and not be able to sense the presence of the magnet.
How strong would the disturbance to the cartridge be? I guess it would appear as some very low frequency pulses at some multiple of the platter rotational frequency. E.g. if there were 6 poles, at 33.3 rpm you might expect to see pulses at 3.33 Hz. But I don't know how much energy they would carry.
It's different for a belt drive motor perhaps, because the frequency is much higher. |
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| DaveM |
I guess that means you would want to make the platter nice and thick to get the cartrige away from the motor field;) Just another rationalization for a big platter.
If you made a layer of the platter of steel and then grounded through the bearing you could in effect make a very effective shield. That in combination with mu-metal around the motor would completely eleminate any noise or field effect created by the motor.
This didn't seem to be a problem with the Technics or Denon tables. They were fairly well reguarded in their day, so I have to assume that there was not a ton of noise from the motor to begin with.
DaveM |
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| rjm |
| quote: | | The usual belt drive arrangement keeps the motor a long way from the cartridge and arm tube. etc etc |
With all due respect magnetic field coupling is much less of a problem in DD tables. This is because the frequency of rotation of the motor is only 0.5 Hz. The change in field at the cartridge is very slow, its inaudible. A belt drive has the motor running at 60Hz, for example, thus the change in field is 60hz, or multiples thereof causing that much loved HUMMMMMM particularely in non-shielded MM or MI cartridges.
You have to consider the difference in motor speed both for vibrational and magnetic coupling. The motor in a DD table runs about 100 times slower, which is a big offset to the inherent disadvantage to mounting it directly on the spindle.
-R
PS There was some mention of BLDC motors, but aren't most DD tables powered by AC servo motors? I know mine is. I confess I dont know much about the difference, though. |
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| KimBOlesen |
Hi
Yes it is runs like a power meter.. An old Beogram8000 used this sort of motor. I have a schematic if it has any interest..
Best regards Kim Olesen |
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| davidsrsb |
Even a low frequency field could cause saturation and inductance modulation.
I saw one DD motor that had fixed coils and hundreds of poles on the rim. While the rim may be doing 33.3 rpm, the poles coil drives would be switching at 100s x 0.55Hz and audible |
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| pixpop |
| quote: | Even a low frequency field could cause saturation and inductance modulation.
|
Yes, of course you need to control both the frequency and strength of the field. It would be insanity to put a strong unshielded magnet close to a cartridge. But it if was close enough to affect the signal, I think you would be having other more serious effects, such as tracking problems due to the force field of the magnet.
| quote: | | I saw one DD motor that had fixed coils and hundreds of poles on the rim. While the rim may be doing 33.3 rpm, the poles coil drives would be switching at 100s x 0.55Hz and audibl |
It doesn't matter how many coils there were. They would have wired them as three phases, so the commutation frequency doesn't multiply as you suggest. However, if there were a large number of magnets at the rim, these would generate an alternating mag field as they swing past the cartridge. They would need to be shielded. Such a design is asking for trouble because of the difficulty of matching a large number of magnets. |
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| pixpop |
| quote: | Yes it is runs like a power meter.. An old Beogram8000 used this sort of motor. I have a schematic if it has any interest..
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yes, I'd like to see the schematic please. If you have a manual, can you check for any patent numbers in there as well?
Neil |
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| EC8010 |
| I opened up my Technics SP10mkII motor to see what was inside. It's an outer rotor motor with a 15 pole stator (remember it's driven by three phases) inside a multipole (can't be 15) annular ferrite magnet rotor with a pressed steel screen around it. In the middle is another rotating annular ferrite magnet and three small fixed coils, presumably to provide the tacho signal. |
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| pixpop |
| quote: | | inside a multipole (can't be 15) annular ferrite magnet |
Most likely has 3 North poles and 3 south poles. What's the radius of the outer ring of coils, and how big are they?
Do you see hall effect sensors? I'm guessing not, as these motors are probably driven with relatively pure sine waves.
I wonder about the two sets of coils and magnets. Seems like a lot of trouble just to get a tacho signal. |
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| EC8010 |
The outer coils aren't all that big. The laminations are perhaps 3/4" high and the coils are about 1/2" wide. The whole thing is about 4 1/4"" diameter.
There are more than three North and three South poles on the magnet. I very crudely tested by wiping the steel blade of a screwdriver around the inside of the magnet and found twenty places where it would stay.
No, I didn't see Hall sensors. The reason for three coils for the tacho is to try to increase the tacho frequency. Trying the same screwdriver trick, I found that the inside of the magnet was serrated. I think this may be a variant of the Studer/Revox trick where they use the leakage field from their (AC) capstan motor. Their 4" outer rotor motor has lots of little fingernail slots cut into it so as it passes near a single coil the flux changes rises and falls with the slots to give a nice high tacho frequency. Cunning. |
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| pixpop |
Yeah, they've certainly been doing their homework.
I want to get one. Maybe someone has one they don't use and would part with for a reasonable price. Otherwise, I guess I'll have to watch for a good one on eBay. From what I've seen, folks tend to hang on to them though. ;-( |
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| EC8010 |
| quote: | Originally posted by pixpop
I want to get one. Maybe someone has one they don't use and would part with for a reasonable price. Otherwise, I guess I'll have to watch for a good one on eBay. From what I've seen, folks tend to hang on to them though. ;-( |
If you only need a motor, without platter, and without electronics, make me an offer for my Technics SP10mkII motor. |
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| pixpop |
| quote: | | If you only need a motor, without platter, and without electronics, make me an offer for my Technics SP10mkII motor. |
Well, I actually need a complete unit, but I'll offer you $25 for your motor anyway (and I'll pay shipping, of course). I'm assuming you have reason to believe it's not dead. |
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| tade |
| so, how do i go about using a vcr motor as a belt drive motor? |
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| Cobra2 |
$ 30 + shipping.
Arne K |
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| pixpop |
| quote: | | so, how do i go about using a vcr motor as a belt drive motor? |
The first thing you need to do is get the speed under control.
I'm assuming that you have the motor driver electronics from the VCR, as well as the motor. These work using a DC control voltage. The higher the voltage, the more torque, and the faster it spins.
You'll need to build a PLL. You feed a reference frequency into the PLL, along with tach pulses from the motor controller. The PLL will output a DC voltage that will increase or decrease the motor speed until it matches the reference frequency. A PLL exists inside the VCR, but I doubt you'd be able to find and separate it out.
You cannot just put a steady voltage on the control pin, as you will have no speed regulation. If the load changes, the speed will change.
You'll need to source a belt of some kind, and do some math to figure out what speed the motor should run at in order to generate 33.3 rpm.
If all you have is the motor, then you'll have to source a BLDC motor control chip, and get that working with your motor.
Some VCRs have two motors you could use: the head drum motor, and the capstan motor (The one that controls the speed of the tape through the machine). The drum motor has low torque and high speed. The capstan motor has high torque and low speed, which might be more suitable for a belt drive. It usually has a nice flywheel as well so the speed control should be easy. |
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| EC8010 |
pixpop and Cobra2: I have a naked motor stripped from a SP10mkII that was missing some external BBC electronics and I have an SP10mkII chassis that would need to be restored to the original Technics circuit to compensate for the lack of external BBC electronics (power supply and master clock). I didn't bother offering the chassis because I assumed that the freght would be prohibitive, but it might not be too bad. If this is of any interest I can email you tomorrow evening.
As I made the offer to pixpop and don't want to turn this into an online auction, he has first refusal of the naked motor at the price he offered. Converting currency on this amount will be expensive, so I suspect a barter deal is probably best. |
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| pixpop |
EC8010,
It probably makes more sense for me to find something closer to home, so I think I'll back out at this point. Go ahead and sell to cobra2 if he still wants it. I'm watching eBay for a complete unit that doesn't have to ship too far.
Thanks,
Neil |
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| EC8010 |
| Fair enough. USA is quite a way from the UK. |
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| KBK |
I haven't read all the five pages yet, but from what I know about the situation... DD turntables are bad if they have NEGATIVE FEEDBACK. The 'free floating' kenwood KD series DD turntables were fine. They had no feedback. Those were teh big corian based decks, the ones where you could put a seprate arm on. the KD 500, 550, and 600, I believe.
I've had 4 of them from that series. Not my favorite but the consensus among the audiophile crew was and is that the non-feedback design was and is superior. Similar characteristics to a belt drive unit, in terms of a ALCK of 'micro cogging' of the motor...leading to superior temporal characteristics presented to the listener.
Beware the KD series though, inspect the bearing closely before purchase. They have a tendacy to collapse. |
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| KBK |
Reading, I find reference to self made platters etc. If machining, be aware that Linn has found that the platter can have severe resonance alleviated to a large extent...by taking a full month to machine.
The way Linn does it, is they only machine a small amount of the platter at one given time. then put it in a rack, pick up the next platter, etc.. do a bit more, move on to the next platter..etc..etc. One month to machine each platter. They don't allow the metals to heat.
This kills the hard 'bell' resonance that would normally be in the platter. Pick up a linn platter and tap it. You'll see that it resonates far less than you would expect. A different frequency, character and duration to that bell ring.
The main bearing, sleeve, thrust plate, and platter are all machined and matched together..and are only available as a set, if one part is damaged. |
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| maxro |
| quote: | Originally posted by KBK
Reading, I find reference to self made platters etc. If machining, be aware that Linn has found that the platter can have severe resonance alleviated to a large extent...by taking a full month to machine.
The way Linn does it, is they only machine a small amount of the platter at one given time. then put it in a rack, pick up the next platter, etc.. do a bit more, move on to the next platter..etc..etc. One month to machine each platter. They don't allow the metals to heat.
This kills the hard 'bell' resonance that would normally be in the platter. Pick up a linn platter and tap it. You'll see that it resonates far less than you would expect. A different frequency, character and duration to that bell ring.
The main bearing, sleeve, thrust plate, and platter are all machined and matched together..and are only available as a set, if one part is damaged. |
Man, that's complete madness. There is such a thing as coolant one can use to keep the platter and the tool cutting it from heating up during machining. Also, repeated re-clamping of a job is a great way to introduce errors in concentricity and flatness.
Oh, and I think that it's only the KD 500/550 that had this floating feature you speak of. The KD 600/650 had a phased locked loop.
And, yes, my KD 650's platter (without its mat on) rings like a bell.
Max |
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| KBK |
| The proof inthe pudding is that you can barely get an LP12 platter to ring, at all. What exactly that translates to, is well, a thing to be figured out. :) |
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| pixpop |
| quote: | | Similar characteristics to a belt drive unit, in terms of a ALCK of 'micro cogging' of the motor...leading to superior temporal characteristics presented to the listener. |
What does that 'micro cogging' sound like? Can you describe it?
What is the bad effect that negative feedback has on a turntable? |
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| pixpop |
| quote: | | DD turntables are bad if they have NEGATIVE FEEDBACK |
What about belt drive turntables. Are they bad if they have negative feedback? |
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| KBK |
It has to do with how the human hearing machanism works. We hear like a diode and in time, with level peaks being the critical point. So, we principally only hear the positive edge transients, in time.
If you think of how a horn speaker works, you find it has a very high amount of distortion to it's negative part of the waveform, but the positive edge is treated well, if you have designed a good horn. When you analyse the components of exactly why such a high amount of distortion of the negative wave corelates to the human perception of a horn's sound (positive!), you see that the timing, level, and placement of a transient becomes critical to satisfy human perception. The exact envelope of the frequency range of that warble or jitter (it has relation to digital jitter and it's perception) plays a part in it's perception as well.
Belt drive could sound as bad or worse than DD, but not that often, which is suprising to some. It all depends on how the platter speed is controlled/maintained by each differing technique. Ie, the nature of the distortion. |
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| maxro |
So KBK, I take it you are a fan of single-ended amplifiers, if you don't like negative feedback turntables. ;)
Max |
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| pixpop |
| quote: | It all depends on how the platter speed is controlled/maintained by each differing technique. Ie, the nature of the distortion.
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What I'm trying to understand is, how would I be able to tell if my turntable was suffering from this problem. What would I listen for? |
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| Jezz-the-Fezz |
If you think about it, cogging (from the perspective of the symptom) is a variation in pitch commensurate with the variation in torque created by the motor whose action is not linear. If you have a 3 - pole motor running at say 60 rpm, you'll get 3 "cogs" per second.
Sonically, you're likely to get intrusion at the "cogging frequency", but perhapse just as importantly, the entire bandwith is affected with every "cog". Although with micro-cogging you'd perhapse not immediately identify the issue at hand, it would contribute towards a generally less coherent signal.
That's my best guess. |
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| KBK |
Rythmic integrety, transient punch and bass slam are primarily affected or are the components that seem to show motor/'platter drive' issues the best. At least in my personal experience of examining the issue. Also, a overall blurring of the signal, yes.
On a LP 12, you can mess with the pot on the Vahalla board and you can put your ear on the chassis, listening for the changes in the motor's drive....and hear the differences in the sound of the table. You're not supposed to play with the pot on the valhalla board....but hey..I have a screwdriver..it was there..so....:) |
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| Jezz-the-Fezz |
| :rolleyes: well, I mean; come on! it's just asking to be tweaked!:D |
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| Cobra2 |
-please e-mail me!
Arne K |
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| KimBOlesen |
| Diagram... reduced to 36%.. Beogram 8000 Tangential Drive |
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| hifidaddy |
Direct drives are not bad, really. But there are so many less good sounding DD turntables even from respectable companies, though. Take the Micro DD1500, which sounds really awful like it should be thrown out of the window immediately after the needle hits the groove, but it could be enhanced with a standalone motor unit into a much better string drive turntable. Then take the Technics SP10II, which sounds OKish, when and only when used in its proprietary turntable base. I have had 40+ turntables at home so far, but the EMT938 based Thorens TD524 was the first and only one which I heard in my listening room, which could compete with a simple Garrard idler wheel turntable. And yes, it uses feedback. Remember that on European Triode festivals there is regularily an EMT 938 with FR-64S tonearm playing. Unfortunately, the EMT938 is handicapped by its own tonearm, so it takes another 10 inch tonearm to get the best out of it.
best regards,
Hartmut |
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| pixpop |
Did some experiments with a VCR drum motor and BLDC driver chip this morning. I built a PLL, and fed it a reference from my audio oscillator. I connected the PLL filter output to the BLDC chip control voltage.
This experiment definitely shows that these BLDC driver chips are not appropriate. Because they use trapezoidal drive, they generate harmonics that inject acoustic energy into the platter. It's very obvious cogging that can be heard just by listening a couple of inches from the platter.
Another problem is that the BLDC chips generate a tach signal (usually called FG, for some reason) from an inductive or hall sensor mounted close to the rotor magnet. However, this signal fails at very low speeds, and is quite erratic at speeds you would need for a DD motor. Once it gets above about 100 RPM it stabilizes, so it may be suitable for a belt drive. Also, the drum (at least the one I have) has a series of fine grooves machined around its circumference, which may be useable for a thread drive.
The cogging problem would be avoided by driving the stator with 3 phase sine waves, but I need to build a 3 channel linear amplifier to test that. Also, an optical tach signal would enable superior speed control, as you would get many more transitions per revolution.
However, in addition the abovementioned problems, the VCR drum motor contains ball bearings, and these of course inject acoustic energy into the platter. |
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| EC8010 |
FG = Frequency Governer.
Shame your experiment didn't work, but it's useful information nonetheless. |
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| Cobra2 |
with a small pulley and belt?
This way you can keep the revs up...
Arne K |
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| pixpop |
| quote: | | Why not try ... with a small pulley and belt? |
yes, I'm sure that would work fine, It's just that I'm not building a belt drive at the moment. I'm building a DD. |
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| KBK |
Just today got a 30hp motor and controller that are stable down to 0.1hz. Ouch.
Kinda pricey, though. It's for a big mixer...
Oh yeah, there are some brutally high precision motors and controllers from that world, on ebay.
For example, we have a $15k (original price) lab mixer and PS, which is self adjusting under load, to maintain exact speed. You can run it at lower speeds..look at the RPM and current meters, grab the shaft and try and brake it. Then watch the current consumption rise and the RPM stay the same. It goes out to 20k rpm, and is definitely precision.
I'm sure there are used and simialr units on ebay.
Heck, I remember about 15 or more years ago, I complied info on the kinds of stuff that are used in the big Rockport and the like. (I had never heard of anyone using such beasts in turntables, at that time) I was looking at over $2k for the motor alone. Sure was a cool one, though! |
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| dice45 |
Hello EC8010,
| quote: | Originally posted by EC8010
... Presumably, it also has to be very accurately centred.
Sadly, I remember the Rabco but not the turntable it was on. |
Yes, indeed. But as i had my shaft (as well as the bushing) precision-ground to dimension, it had to have conical centering bores at the shaft's end faces. As the whole cylindircity and concentricity of the shaft is **referenced** to those centering bores, the centering bores are indeed very accurately centred.
And i use one of the centering bores as the reference face to center the ball.
No wonder you don't remember the TT. It's home-brewn, looking intentionally unspectacular, IOW it is living-room-compatible :) but it has inner values indeed. |
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| EC8010 |
| quote: | Originally posted by dice45
No wonder you don't remember the TT. It's home-brewn, looking intentionally unspectacular, IOW it is living-room-compatible :) . |
Hello Bernhard,
I fear you have failed in the ethos of DIY Hi-Fi. It has to be big, ugly, and inconvenient to use. If it can consume vast amounts of electricity and get dangerously hot as well, then that's all to the good. Otherwise, we might as well all use B&O. |
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| Nordic |
| Basicaly it must not be posible for your significant other to operate the unit without your supervision.... |
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| Pjotr |
| quote: | Originally posted by EC8010
Hello Bernhard,
I fear you have failed in the ethos of DIY Hi-Fi. It has to be big, ugly, and inconvenient to use. If it can consume vast amounts of electricity and get dangerously hot as well, then that's all to the good. Otherwise, we might as well all use B&O. |
Hi,
Still my favourite: http://reviews.iwon.com/cat/audiore...33_1597crx.aspx
:D |
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| dice45 |
EC8010,
| quote: | Originally posted by EC8010
... I fear you have failed in the ethos of DIY Hi-Fi. It has to be big, ugly, and inconvenient to use. If it can consume vast amounts of electricity and get dangerously hot as well, then that's all to the good. Otherwise, we might as well all use B&O. |
my coming TT-design is big, no question. It is a floor-standing unit (as no shelf would be able to carry it :) ). But consuming vast amounts of N'R'G, no, it doesn't . It needs two tiny wall outlets, one for AC and one for pressurized air :D . And it has to be beautiful because i have to be able to stand it over the years.
My current TT is unobtrusively beautiful ... i had put a lot of effort into its design, appearance- as well as engineering-wise. And i was able (and still am) to like its looks.
And if i come to think of it, i didn't spent my design time for appearance at all. I simply strived to make things look "right", "plausible", "balanced". i do so when i started designing a component. So do i today.
Once there was a Swiss construction engineer; his specialty was designing and building bridges from steel-reinforced concrete. His name was Robert Maillart. When a journalist askend him one time to which percentage he spent his design time for architectural appearance, he answered: "Not a single minute, becaúse what is well designed is also inherently beautiful" .
Methinks, he's got a point with that. |
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| dice45 |
| quote: | Originally posted by Nordic
Basicaly it must not be posible for your significant other to operate the unit without your supervision.... |
quite the contrary. My technical specification for my tonearm requires "basic function must be layman-performable and evident without the layman having to consult the manual" ... and one of the consequences of that is that the linear tracker's active positioning must be fast enough (it is!) to follow the SO's hand trying to put the headshell from the lead-out groove to the parking position, as she would do with a pivoted arm.
After all, my (hypothetical) SO would have to stand my 190x120cm open baffle speakers and the amps driving it. Making the TT inoperable for her would be too much ... all the more as i never met a female preferring the CDP's sonics to that of vinyl :) |
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| EC8010 |
| quote: | Originally posted by dice45
"Not a single minute, becaúse what is well designed is also inherently beautiful" .
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Three cheers for him. I agree absolutely. |
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| baggins |
Hi guys I know this isnt a class piece of gear but I think the bearing
I made for my home built TT may be of use and its dead simple-
When you make the spindle support bush it will probably have the drill
angle left at the bottom of the tube, 118 degree vee for a normal twist drill. Drop a suitable ball bearing into the bush about 1/4 or 5/16 depending on the spindle size .Drill the end of the spindle so that a
1/8 or 3/16 ball bearing fits in to just over half its depth and peen it in with a dot punch. eh voila hardened steel point contact. fill with oil to taste. |
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| nghiep |
| Has anyone consider the magnetic effects of the magnet on the platter spindle? The magnets act as a damping material absorbing sonic energy from the stylus and record interface. The belt drive bearing allows the sonic energy to resonate back to the turntable. This contribute the different sounds of boxy turntable and single solid piece plinth. |
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| pixpop |
-->The magnets act as a damping material absorbing sonic energy from the stylus and record interface. <--
How does a magnet absorb sonic energy? |
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| nghiep |
| There is a magnet on the spindle and many fixed magnets surrounding the spindle to create the turning torque. As the spindle magnet attempts to resonate/dissipate the vibration from the stylus, the surrounding magnets also apply magnetic field to turn the spindle. This magnetic field prevents the spindle and its magnet to resonate naturally with the sonic energy and conform to the magnetic field. It's like to have one magnet attached to a fork of the musical instrument tuning fork and another magnet attached to a fixed location. The magnetic field interaction between these two magnets affects the tuning fork ability to reasonate naturally to mechanical vibration. Some tonearm used magnetic field to dissipate stylus energy and stabilize the arm. It's just an elementary schoolboy observation. |
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| Construct |
| quote: | Originally posted by pixpop
-->The magnets act as a damping material absorbing sonic energy from the stylus and record interface. <--
How does a magnet absorb sonic energy? |
Although I'm not entirely sure, I believe he's talking about eddy current damping.
See this page for a much better explanation than I could possibly come up with:
http://farside.ph.utexas.edu/teachi...res/node73.html |
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| pixpop |
I understand eddy current damping, e.g. as used in the dynavector tonearm, and also those with the magnetic suspension.
I don't see how it relates to acoustical energy in the spindle of the TT. I think, because the magnetic fields are balanced around the spindle, the net changing field seen by the metal of the spindle is very small.
Nghiep, do you know how much damping is introduced by this mechanism? |
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| Cobra2 |
are we getting theoretical!
I just heard an interesting interview with Simon Yorke ( http://www.recordplayer.com )
He had much of the same preferences as I suspected...
Everything in the design should not be dampened.
But to find the right resonances that work together.
Do as he do, just build, try, replace, modify, build again...
Arne K |
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| pixpop |
-->Do as he do, just build, try, replace, modify, build again... <--
Good advice, I think.
I'm on my way. Ordered my reference tonearm today, and on the lookout for a nice chunk of slate for a shelf in my room. I'll post pictures as soon as I have something worth looking at.
Neil |
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| nghiep |
| Technics SP-25 direct drive unit driving VPI TNT Jr. |
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| baggins |
| Is the tone arm wooden & uni-pivot? can I see the bearing? |
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| lgreen |
As with anything, Direct Drive can be great if implemented correctly.
Take the Denon DP-59L, one of the best DD ever. I just so happen to have one up for sale on EBay. . Speaking of resonances, its 2 tonearms (one is serpentine) had different resonant frequencies as I recall.
With this and my Nakamichi RX-505 you can have some sweet analog sound.
(sorry about the shameless plugs, figured you all would like to see some classic stuff....plus I can't let go.....ouch.......) |
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| maxro |
| quote: | Originally posted by lgreen
As with anything, Direct Drive can be great if implemented correctly. |
So, if tin-can telephones were just implemented correctly...
Max |
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| baggins |
With this and my Nakamichi RX-505 you can have some sweet analog sound.
Till the tape wraps itself round the capstan! |
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| Cobra2 |
| quote: | Originally posted by baggins
With this and my Nakamichi RX-505 you can have some sweet analog sound.
Till the tape wraps itself round the capstan! |
Hardly with a dual capstan machine...even rarer on a Naka...
-unless you keep your lunch inside the tape-comparment.
Arne K |
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| MRupp |
| quote: | | So, if tin-can telephones were just implemented correctly... |
Apparently yes .....here is a brand new DD turntable that come out recently. Since this manufacturer has some reputation to lose, I presume he knows what he is doing: http://www.brinkmann-usa.com/oasisturntable.php |
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| baggins |
Had it happen on aiwa dual capstans .I,m not an expert on cassette
technology but I thought they were reasonable transports & yes they can sound great when everythings tickety boo .But I lost a nice
copy (oops) of some opera arias and Ive been wary of them since
incidentally it was TDK ordinary tape ,I dont Know if that makes any difference |
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| lgreen |
| quote: | Originally posted by baggins
***
it was TDK ordinary tape ,I dont Know if that makes any difference |
I think it does. With dual capstans and a thicker tape you will not have that problem. I've played and recorded thousands of tapes in the last 20 years and only one time, on my Technics, did a tape wrap itself up, and it was one of my cheapie tapes, which for some reason had a lot of good tunes on it. Never had any problems with TDK SA, SAX or Maxell XLII, XLIIS. |
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| maxro |
| quote: | Originally posted by lgreen
As with anything, Direct Drive can be great if implemented correctly. |
| quote: | Originally posted by maxro
So, if tin-can telephones were just implemented correctly...
Max |
| quote: | Originally posted by MRupp
Apparently yes .....here is a brand new DD turntable that come out recently. Since this manufacturer has some reputation to lose, I presume he knows what he is doing: http://www.brinkmann-usa.com/oasisturntable.php |
Actually, I'm a fan of direct drive over belt drive turntables. It is the "as with ANYTHING" part that I take issue with.
Max |
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| baggins |
I have in front of me a TDK D type and a Maxell UR cassette and no where on either package is the thickness of the tape stated are you
advocating only the use of chrome etc are all standard tapes suspect? |
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| Cobra2 |
-all tape would have same thickness. This will however vary more between brands, and length, than type. That is why lenghts over C-90 (C-120+) often not are recommended.
But it is a minor detail.
Regular maintenance of the Deck is more important.
Arne K
can we get back to DD TT ? |
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