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Ideal potentiometer for post Op-Amp

  1. Mar 28, 2010 #1

    I have built a theremin that runs an output to a Velleman Mono 7W Amp. From the amp it then splits 3 ways. 1 to a speaker. The other two run directly to the horizontal and vertical connections of a deflection yoke in a 12V CRT television. This system works. I have sound from the speaker and a circular visualization from the theremin sound wave.

    However, I would like to control the horizontal yoke with a pot to change the size of the wave form. Since I am splitting a mono signal after the amp, I have to put the pot after the amp as well.

    Here is the amps Datasheet:

    * music power output: 7W / 4ohm
    * RMS output: 3.5W / 4ohm or 2W / 8ohm
    * total harmonic distortion: 0.05% (1W / 1kHz)
    * frequency response: 20Hz to 20kHz (-3dB)
    * signal/noise ratio: 86dB (A weighted)
    * input sensitivity: 40mV / 150Kohm
    * overload and short-circuit protected
    * supply voltage: 15V DC (8 to 18V DC possible) / 0.5A
    * dimensions: 2.2 x 1.4"

    Could anyone help me to pick a potentiometer for this task, and maybe explain how to figure that out?

    Thank you so much.
  2. jcsd
  3. Mar 28, 2010 #2


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    Interesting idea.

    To calculate this, you would need to know the resistance and inductance of the coils in the yoke.

    As a test, you could try the effect of a 22 ohm resistor in series with one of the coils. This would give you some idea of the resistor sizes needed.

    To get a different pattern, maybe you could feed the horizontal coil with a few volts at 60 Hz AC.

    Or, you could restore the frame drive from the TV. This would have previously gone to the vertical deflection coil. Make sure this is isolated from the horizontal coil.

    This would give you something like an oscilloscope display.
  4. Mar 30, 2010 #3
    Thanks for the reply. I did mess around with oscilloscope type wave form displays.

    I had a potentiometer doing exactly what I needed it to do (post op amp signal) on the horizontal coil but it eventually started to smoke.

    I would really like to know what potentiometer would be ideal for a post amp set-up based on the datasheet I provided. If anyone else could lend a hand, that would be great.

  5. Mar 30, 2010 #4


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    Science Advisor

    You would need to know the details of the coils in the yoke, first.
    You could get a pot and have all the control at one end if the coils are very low impedance.
    Either measure it directly, if you can, or put a 22 ohm 5 watt resistor in series with one of the coils and observe the effect. Then come back here.

    If you can't measure the properties of the coils, you can estimate the sort of pot that might be suitable.
    If you already have a speaker connected, the potentiometer would have to be no less than about 20 ohms. The amplifier can only deliver 3.5 watts and there is a speaker connected so it doesn't need to be very big, but probably bigger than the typical Radio Shack stuff..

    You can get large wire wound rheostats with connections to use them as potentiometers.
    Like this one:
    http://www.thesciencefair.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=1856-3&Category_Code=ET [Broken]

    You would connect the ends across the speaker leads and take the output to the deflection coil between one end and the slider.
    But they are expensive.

    These people have some cheaper ones:

    A series resistor may be all that you need if you just want to make the display more circular.
    Last edited by a moderator: May 4, 2017
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