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Denon dp 80 vs yamaha gt 2000

Discussion in 'Turntables' started by filipealm, Oct 27, 2017.

  1. bimasta

    bimasta Super Member

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    When I had the JVC QL-YF5, I was surprised when I saw the motor. I was struck by its resemblance to the motor in my computer's external hard drive. Did TT makers adopt this new technology? It was cheap, reliable. Does anyone know?
     

     

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  2. totem

    totem AK Subscriber Subscriber

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    Quite so one having a magnetic strip, the other being bi-drictional servo and coreless.
     
  3. EngineerNate

    EngineerNate AK Subscriber Subscriber

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    Was that one a coreless design or not?

    The motor in the SL-1200mk2 and it's relatives bears a striking resemblance to the motors in computer CDROM drives.

    Brushless "outrunner" type motors where the magnets are in a can that spins with the shaft as opposed to on a central shaft inside of a can filled with windings are hugely popular in industry. It's an easy way to build a brushless motor with good torque characteristics and wrapping windings on a fixed internal stator is easier than wrapping them inside a can from a manufacturing standpoint.

    Coreless motors are also widely used in hard drives etc. because they are simple to manufacture and eliminate the additional parts needed for the core. With mass production, the windings can be "printed" onto the circuit board along with some sort of quick drying adhesive/binder, making them very easy and cheap to produce in large quantities.

    @totem - I believe the Denons use an AC Induction type motor as well as opposed to the coreless DC brushless in the Victors. I kind of like the induction type motors because the mechanism of speed control can be made quite a bit simpler. DC servo motors must accurately track the rotating magnetic field of the permanent magnets on the rotor to maintain synchronous operation and control the rotating frequency of the field windings as well. An AC induction motor can be made to run at a fixed frequency and simply vary the input voltage to vary the amount of slip between the field winding frequency of rotation and the rotor's speed.

    I'm sure we could have a drawn out debate over which has the ability to reach the nth degree of speed accuracy, but my gut tells me it's easier to make smooth corrections to the speed by varying the slip than it is to change the rotational frequency of the drive. I know the earliest Sony DD TTs with their AC servo induction motor are well liked for having quite a smooth presentation, with some stating that they have some of the positive sonic characteristics you'd normally find in a good belt drive rather than your typical direct drive.
     
    Last edited: Feb 12, 2018
    Pio1980 likes this.
  4. Pio1980

    Pio1980 AK Member Subscriber

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    The Denon has a large outrotor AC drive platter motor with an electronic voltage throttle speed control powered directly from a winding on the power transformer, regulated with a quartz clock standard reference on a rim generated tachometer signal feedback. Iirc, speed correction is bidirectional.
     
  5. Pio1980

    Pio1980 AK Member Subscriber

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    The older Denon AC drive and Sony AC drive are very similar, being non-quartz clock regulated electronic throttle controlled single direction speed correction.
    Much simpler than bidirectional quartz regulation.
    The throttle is wonderfully simple, a power transistor across a rectifier bridge in series with the motor windings and AC power source. The fb and speed control circuit drive the power transistor to control AC voltage to the motor and the platter rotation velocity. The rim tach signal provides true fb signal to the throttle drive circuits.
     
  6. Pio1980

    Pio1980 AK Member Subscriber

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    The bidirectional quartz systems are considerably more complex.
     

     

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  7. EngineerNate

    EngineerNate AK Subscriber Subscriber

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    Do all of the tables in question do Bi-directional? I know the Victor/JVC TT-101 does.
     
  8. Pio1980

    Pio1980 AK Member Subscriber

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    That I don't know. I'm guessing those with Quartz phase lock loop probably are.
     
  9. EngineerNate

    EngineerNate AK Subscriber Subscriber

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    I know on the Victor that the tt-71 is not bi-directional. I believe the 81 is bi-directional with a single quartz feedback loop and the 101 is bi-directional with dual servo loops.

    Interestingly, the QL-F6 has an identical motor (theory of operation wise) to the tt-101, but can often be found going cheaper than the QL-7 or QL-5. Unfortunately being a full automatic, you're stuck with it's built in tonearm unless you do major surgery.

    Edit-Swype and AudioKarma do not like one another. Sorry for multiple corrective edits haha.
     
    Pio1980 likes this.
  10. JP

    JP 7480 74111110101115

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    I haven't met a quartz locked drive that wasn't bi-directional.

    Conversely I'd love to know where the dual servo loop info came from on the TT-101. It use an F/V loop to get the motor within lock range of the PLL, as every other quartz-locked drive has some variation of this. I see no other evidence of dual loops.

    DC motor speed is controlled by varying current, not varying frequency.
     
    Pio1980 likes this.
  11. EngineerNate

    EngineerNate AK Subscriber Subscriber

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    Unless the motor is brushed (zero direct drive tables that I know of are) there has to be control of both frequency and current. Permanent magnet DC brushless servo motor is really a misnomer. The motor effectively sees AC, in general in a three phase setup. The frequency sets rotational speed, current sets torque.

    Info regarding the dual servo loops was from the vintage knob.

    http://www.thevintageknob.org/jvc-TT-81.html
     

     

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  12. JP

    JP 7480 74111110101115

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    The drive frequency of an SP-10 MKII, as an example, is 5.55Hz for 33.33RPM as dictated by motor geometry. Speed variations "at speed" are actually variable load conditions, which means a torque adjustment is what's required for "speed" regulation. You can verify this behavior with a scope on a motor drive phase. The frequency does not vary.

    TVK sometimes gets more wrong than right.
     
    fiddlefye likes this.
  13. EngineerNate

    EngineerNate AK Subscriber Subscriber

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    A permanent magnet brushless DC motor must have the ability to adjust the electronic commutation frequency to maintain lockstep control of the permanent magnet rotor. If lockstep control is not maintained the motor ceases to function. Torque is a function of the current in the field windings and the strength of the permanent magnets. Rotational frequency is a function of speed or vice versa. 5.55Hz may be the target frequency at 33.33rpm, but unless the Technics is an induction type motor and not permanent magnet, the control system must have the ability to vary this to keep the platter under control at all times. Otherwise dragging your finger on the platter with enough force to slow it would result in a complete breakdown of the control loop.

    I used to build brushless motors for another hobby and I studied them in school. If there's some control mechanism I haven't learned (quite possible) I'd love to be enlightened, but my current understanding was backed up by reading the service manual of the pioneer Pl-400 and it's bretheren. It has a detailed description of how the commutation is set via feedback from the hall effect sensors in the motor driver board and then adjusted to speed based on the PLL loop once synchronous operation is established. If the motor were not synchronous there would be no need for the hall effect sensors.
     
  14. JP

    JP 7480 74111110101115

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    Within the lock range of the PLL on the vast majority (I haven't worked on them all) of Technics drives, TT-101, etc. the frequency simply does not change. I've measured them all.
     
  15. Pio1980

    Pio1980 AK Member Subscriber

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    The Halls trigger coil commutation for direction and energization timing, the frequency generator coil provides the speed info, which may be converted to a voltage by an integrator in the non-quartz ref'ed versions. A quartz pll system would likely need the tach frequency signal to function.
     
  16. EngineerNate

    EngineerNate AK Subscriber Subscriber

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    I could see the rotational frequency of the drive remaining stable within the quartz lock regime. That is a very narrow band of speeds.

    I could have stayed the above post more clearly.

    At steady state, the coils are energizing such that as the magnets pass each coil, the coils are energized such that they push/pull each magnetic pole on the rotor at the desired rate. Torque is fixed at some preset amount by the steady state current set in the drove circuit. If the rotation of the permanent magnet rotor and the coils gets out of sync, the coils will repel/attract the wrong poles on the rotor and all hell breaks loose.

    If the rotor/spindle slows and the commutation frequency remains fixed, the magnetic poles on the rotor will be influenced by the wrong coil fields and rotation will break down. So, the controller will both reduce the rotational frequency and increase the current (put a DBT on a big DD and drag your finger on the platter. It'll light up) to give the rotor a bigger push each time the magnets pass the coils-speeding it up. As it speeds up, the coils are commutated faster to keep up until the desired rotational speed is achieved.

    The reverse happens if the rotor spins too fast-depending on the drive function, the current can simply be cut and the rotor will slow, or the controller can time the coil energization to actively retard the rotor, then return to the steady state once a stable speed is achieved.

    Thinking about it, most DD turntables have something between 6-24 poles on the field windings. That gives some range of degrees of rotation (greater than a few degrees) where the motor coil pulse would still most strongly affect the correct magnetic pole, as for some sweep of degrees the correct pole would a. Still be closest to the coil as compare to all the other poles and b. on the correct side of the coil to react properly to the magnetic field. So within some small range of transient speed variation, the rotational frequency of the windings need not change:as long as the speed is corrected within that window with an applied torque, synchronous control has been maintained without adjusting the drive frequency. Since turntables operate within an extremely small range of rotational frequencies, outside of startup and fringe conditions like scratching, a fixed frequency on the drive as described by @JP makes sense. As soon as an event causing large variation in speed is encountered, the drive frequency must compensate to maintain control of the platter.

    Cheers
    Nathan
     

     

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  17. JP

    JP 7480 74111110101115

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    Most don’t. If you apply sufficient load to drop the motor out of lock they usual go to a stall condition.
     
  18. EngineerNate

    EngineerNate AK Subscriber Subscriber

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    They still need some ability to get it up to speed from zero.

    I know my pioneer and JVCs if I drag a finger I can get it down to zero rpm momentarily and it keeps applying torque the whole time.
     
  19. JP

    JP 7480 74111110101115

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    Do you have a scope?
     
  20. EngineerNate

    EngineerNate AK Subscriber Subscriber

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    On the list. Need an actual workspace so that I can start collecting more specialized gear.
     

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