DC Motor controller for Turntable

Discussion in 'The Projects Forum' started by TransFi, Feb 7, 2010.

  1. TransFi

    Thread Starter New Member

    Feb 6, 2010
    Hello everyone

    I'm involved in small production of a hi-fi tonearm & would like to develop a turntable to accompany it. Problem is, while I'm ok with mechanical things, I know little about electronics........!

    I am posting here to see if anyone is willing to help with the electronic side of things in this venture. I am happy discuss remuneration for ones time & expense for a successful outcome. Please PM me.

    The design brief appears simple.....but in reality it is a challenge. What is required is to rotate a spinning platter at speeds of 33.3 & 45rpm EXACTLY. This has to be done in a linear fashion. Because I will be using the motor to be directly coupled to the side of the platter using a pulley with an O-ring, any rubbish going into the motor will go into the platter, which in turn will be amplified by the stylus to the detriment of the sound. This will rule out all PWM type of control.

    I have this deck up & running using a triple cascade of voltage regulators. I need about 6V for 33 & 7.5 for 45. The speed stability does not remains constant affected by:

    1. The oil in the bearing warming up
    2. Electronics warming up
    3. Stylus drag on the groove records
    4. Temperature

    3 is an interesting problem because the drag on the grooves is greatest on the outermost part of the record, where the perceived speed is faster.

    I need some kind of feedback control, but that is stable without 'hunting' as this would cause wow & flutter. I dont want the digital type of quartz locked control that direct drive decks use as this causes a hardness/harshness to the overall sound. Bear in mind, we are talking 'high-end' audio.....not MP3 player stuff. Simplicity is the key.

    If anyone is interested, or indeed has already worked on such a project, please get back to me.



    BTW....I am UK based, Hertfordshire/Northampton area.
    Last edited: Feb 7, 2010
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    If the PWM frequency is high enough, you won't notice any problem. The motor is an inductive load, which will smooth the pulses. The turntable has mass, which will further dampen them. If you still have concerns, just pick a PWM frequency in excess of what humans can hear or your audio amps can pass.

    Have you considered some kind of shaft encoder? This would work in similar principle to the little mirrors we used to have around the turntable.
  3. Duane P Wetick

    Active Member

    Apr 23, 2009
    Consider a motor that is held tightly by torsional springs. As the load changes the springs give slightly, but always fall back to their rest position. The type of control that this simulates is called a phase locked loop (PLL). Search on this forum for more information on this subject.

    Another way may be to monitor the drive motor current, and compare to a setpoint value. By the use of an integrator, the rate of compensation that you want may be adjusted to suit any audiophile.

    Cheers, DPW [ Everything has limitations...and I hate limitations.]
    Last edited: Feb 8, 2010
  4. TransFi

    Thread Starter New Member

    Feb 6, 2010
    Thanks guys!

    PLL sounds like what I need.........something gentle that will control the speed. I will read , about it but I still wouldnt have the knowledge to construct.......so if anyone is out there that wants to get involved in this project.............