Complete attenuation

Discussion in 'General Electronics Chat' started by Shagas, Jun 14, 2013.

  1. Shagas

    Thread Starter Active Member

    May 13, 2013
    802
    74
    Hello guys

    - How do I get a '-infinity to 1' gain regulation out of a signal?-

    I'm building a little synth (noisemaker) project.
    I've got 2 oscillators , one of which is a sine-wave and the other a square wave.

    Both oscillators will be fed into a simple Op-amp adder circuit.

    I want to be able to regulate the feed of both oscillators with 1 stereo pot so I can get the waves in their pure form and their combinations aswell by adjusting that pot . The question is:
    When I want a pure sine wave for example , how do I get the square wave osc to shut up? I guess there would have to be some sort of complete attenuation , a gain of - Infinity or something no?
    I've tried doing it with an op amp but it goes unstable at a gain of -3.
    I tried using a potential divider to simply bring the voltage down and then feeding it into the op amp adder , and still no luck
    I've also tried a similar method with a T-network that I read about , still crap.
    I can get the signal down to a few mV with a potential divider and measure it with my oscilloscope , but when I connect it to an op amp amplifier then it just goes unstable . I guess that the resistances come in parallel or something and it upsets the op amp.

    My signal is about 300mV and I can't bring it to lower than about 90mV.

    -->>> So how do I get a -infinity to 1 gain regulation out of a signal? <<-----

    Thanks in advance for any help
     
  2. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    Try AC coupling to the opamp via a small capacitor. Say, 10uF.
     
    Shagas likes this.
  3. crutschow

    Expert

    Mar 14, 2008
    13,000
    3,229
    Post the op amp circuit that doesn't work. It should.
     
  4. Shagas

    Thread Starter Active Member

    May 13, 2013
    802
    74
    opm-amp-att.png

    Here is the problematic part of the circuit.
     
  5. Shagas

    Thread Starter Active Member

    May 13, 2013
    802
    74
    Hmm yes why Didn't I think of that . I'll try it out today , thanks
     
  6. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    It's also a good idea to AC couple audio signals through pots. Because, If DC is allowed across pots, they may produce noise when rotated.
     
    Shagas likes this.
  7. Shagas

    Thread Starter Active Member

    May 13, 2013
    802
    74
    Thanks for the insight
     
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