Limiter/compressor circuit for student project

Discussion in 'The Projects Forum' started by ZachAttack, Nov 7, 2013.

  1. ZachAttack

    Thread Starter New Member

    Nov 7, 2013
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    Hi there,

    I am a new user to the forum and have a question about a project I am working on. I am pulling a signal from a small motor and feeding it to a standard home audio amplifier. I would like to limit/compress the signal amplitude to stay within the input specs of the home audio amplifier. I cannot provide any power to the circuit so it must be passive. Would a passive soft limiter be my best option? If so, I haven't found much information on passive limiter circuits out there so what type of configuration and diodes would be best?

    If there is anything I have left out or need to clarify, please let me know so I can take action.

    Thanks for your advice,

    ZachAttack
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Are you "listening" to your motor? :rolleyes: If so, why???
     
  3. #12

    Expert

    Nov 30, 2010
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    Three ways to do it.
     
  4. ZachAttack

    Thread Starter New Member

    Nov 7, 2013
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    MikeML: The motor is powered by a user with a racing simulation wheel. I am pulling the signal and using it as a sort of tactile feedback for the racing seat that the user is sitting in.

    Thank you for posting those circuits #12, how does the resistor value affect the clipping of the signal? I have seen a few circuits that include LED's, what is the point of these?

    Thanks,

    ZachAttack
     
  5. wayneh

    Expert

    Sep 9, 2010
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    In addition to the signal conditioning #12 has suggested, I'd consider adding a capacitor in series with the signal, for instance 0.47µF ceramic. This would absolutely eliminate any DC current passing thru to the audio circuitry. You can try it with and without, and I bet it'll sound the same.
     
  6. ZachAttack

    Thread Starter New Member

    Nov 7, 2013
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    Thanks for your input wayneh that is a good idea. I'll definitely try that out when I build a test circuit.
     
  7. #12

    Expert

    Nov 30, 2010
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    Maybe you are mistaking a zener symbol for an LED? I only used silicon rectifiers and zener diodes.

    The resistors might represent the impedance of the source or you might need to add resistance. Depends on the circuit. You wouldn't want to let the smoke out of a low impedance signal source!

    The first circuit limits the input to +/- .6 volts.
    The second circuit limits the signal to .6V more positive than the power supply voltage and .6V more negative than "common". The third circuit limits the signal to the voltage of the negatively biased zener diode + .6 volts for the forward biased zener diode.

    There are other ways to do this, but this is a start. Waiting for you to respond and thus give us a feel for how to communicate with you. If those three circuits do not address the problem, I expect you to give more information so we can adjust the circuits appropriately.
     
  8. ZachAttack

    Thread Starter New Member

    Nov 7, 2013
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    Hi there #12. My apologies for not responding sooner. Exams...

    Yes the circuits you posted do not contain LED's, I was just inquiring about another limiter circuit I found which contained LED's. It's figure 4 of the link below.

    http://www.261.gr/limiters.html

    Although I do need to make a passive soft limiter, I was just interested in how the LED's play into the circuit in the link above.

    #12 the circuits you posted will suffice, I just don't understand the reasoning behind why a resistor will make this a soft limiter instead of a hard limiter.

    This problem of not knowing exactly how the resistor affects the smoothing shouldn't deter me from just building the circuit and testing with different values, but I am still interested in the math involved.
     
  9. #12

    Expert

    Nov 30, 2010
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    Those circuits I posted are not soft limiters. They are more like absolute protection devices for the next stage. You can soften them by putting a resistance in series with each limiting device. There are circuits called compressors that have progressive attenuation according to amplitude. I vaguely remember a Siliconix compressor/expander chip that probably isn't in production any more. NE570/SA571 NE/SA572 NE575

    They sucked pretty badly for musicians because the loss of percussives really wimps out the sound. Probably a factor in their demise. Still, they have their uses, like protecting the power amps from the output of the mixer board.

    LEDs have a different breakover voltage than rectifiers. Still, you have to keep them protected from reverse voltages over about 5 or 6 volts or they get damaged. The proof is in the pudding. Build the circuit and listen to it. These are NOT super high-tech experiments. It just takes a lot of time to try the different ways and educate your ear.

    This isn't really an encyclopedia site where somebody is going to write you a book. If you want a good answer you have to ask, "Soft limiter for what?"
     
  10. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Motorola also made a single chip compressor/expander (compander) - MC33110.

    It reduces the dynamic range of the signal - increases low amplitude signals and decreases high amplitude signals. Intended for telephone transmission from what I understand.

    If you are interested and you are in the US, I will gladly send you a few if you send me a PM (no charge for students).;)

    http://pdf.datasheetcatalog.com/datasheet/motorola/MC33110D.pdf
     
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