Tremolo control - input signals seems to be riding the output

Discussion in 'General Electronics Chat' started by atferrari, Feb 5, 2015.

  1. atferrari

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    LTSPICE - .asc file, model and sub attached.

    In this basic tremolo control, I cannot avoid what seems to be the input signal riding the output.

    As is, even if the JFet is cut (??) I still get the equivalent of the input signal, at the output.

    Sure it is more than 100% modulation; done on purpose to put in evidence that the JFet is cut.

    Around this for many hours, I supose I am wrongly biasing the opamp but cannot see where or how.

    Comments will be appreciated.
     
    Last edited: Feb 5, 2015
  2. Alec_t

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    It's not a bias problem, Agustin. When J1 is switched off, U2 becomes a ~unity-gain amplifier; hence its output ~equals the signal at its non-inverting input.
     
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  3. atferrari

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    Yes Alec, a follower!

    Way too obvious now (as usual)...:(

    Gracias for replying.
     
  4. Alec_t

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    Agustin, you can use your method of controlling the amp gain with a JFET if you follow U2 with another opamp to subtract the source signal from U2 output.
    Tremolo.gif
     
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  5. atferrari

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    Wow, nice solution Alec.

    Gracias for taking the pain of getting a solution. Good reasoning thus a clever way to solve my "problem" (in fact kind of an uncomplete design).

    Myself, I did different, not messing anymore with gain but processing the signal at the input. I am learning a LOT of things but it takes time.

    The attached .plt includes the settings for displaying data and the other is the current .asc in force.

    Please, have a look.
     
  6. Alec_t

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    Looks good. If you wanted a much greater tremolo depth (which I doubt) you would need to increase the value of R14.
     
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  7. atferrari

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    Yes, right. It took me some time to realize what is the actual difference of using a Jeft as part of a divider and in series with the signal. Always learning.

    Now playing to build a proper envelope in a simple way.
     
  8. Alec_t

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    Here's another way of making a voltage divider to control tone amplitude, using two opto-isolators connected in inverse-parallel. Attack/decay and tremolo waveforms modulate the opto-diode current and hence the effective resistance provided by the opto-transistors.
    BellSoundGenerator.gif
     
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  9. atferrari

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    Quite compact Alec, and creative.

    Meantime, few minutes before entering here I completed (in LTSpice) the basic circuit to generate the sound of a chime. I was stuck with reducing the amplitude of an envelope and shifting its level down. Now, I am convinced that in a single opamp stage you cannot attenuate without inverting until I recalled the trick of the divider at the input. Just two resistors. :p
     
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