Pre + filter + amp - Assorted questions

Discussion in 'General Electronics Chat' started by atferrari, Mar 24, 2014.

  1. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
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    Directional microphone (output to earphones) to listen birds (no bats) in quiet areas.

    Today I built and tested a preamp for an electret microphone followed by a LM386 based amplifier. Both worked fairly well.

    Next step is to include in between, a tunable (variable-state) filter with Gain, Q and filter mode selection available.

    My questions:

    1) The three stages have gain control by design. I intend to fix the filter Gain =1.

    Could / should I , also fix the gain of the the preamp? If so, to what level at the output?

    2) I could see in the scope that, by yelling, the electret output can reach up to 250 mV or more. While I do not expect anyone yelling around, shouldn't I consider eventual high levels as that, that could hurt my brother's ears? This is my gift to him if I manage to build something reasonable.

    Is here where compression enters the game?

    3) The filter requires +/-9V supply. Does it make any sense to use it also with the preamp, taking away both resistors to polarize the noninverting input when using a single supply?

    4) A preamp should it be loaded with a resistor? If so what is the value?

    5) The DC blocking capacitors at the preamp and filter's output, what value should they have?

    Replies and also comments, are welcome. Gracias.
     
    Last edited: Mar 24, 2014
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
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    look into clipping stage could be as simple as two doides back to back to limit maximum level.
     
  3. #12

    Expert

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    This is very much, "show us the schematic".
    You are good enough that we can just give a few tips, but we have to see what we're working with. Words are not enough with something that has this many stages.
     
  4. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
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    Not that the circuit is not available (it is not) but I think this is more conceptual than referred to a specif circuit. Two stages are already working and the other (filter) would be a classic one.

    I could post not before tomorrow.

    Thanks for replying.
     
  5. #12

    Expert

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    Here is an example.

    Fix the gain of the pre-amp at what level. I can't tell without knowing the gain of the other stages. Probably just let it run as high as it can get because that keeps the signal to noise ratio best.

    Hurt his ears: a simple clipper could do this but I can't tell where to put it if I don't know where to find a good place for it.

    Yes. Using 2 batteries can remove biasing resistors.

    What resistance to load the pre-amp: I don't know unless I see how the circuit works. It's probably already loaded with the input of the next stage.

    What value coupling capacitors? That depends on the impedance of the next stage and the frequency range you want.

    See? Most of these depend on the schematic.
     
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  6. atferrari

    Thread Starter AAC Fanatic!

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    I see Nro. 12. No doubt. Sorry for being presumptuous.

    I am posting as soon as I can.

    Thanks

    Edit:

    Schematic added.
     
    Last edited: Mar 24, 2014
  7. #12

    Expert

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    C9 is at least 1.6 uf. A film cap would be good in that position.

    RL on the first amplifier is not necessary and not effective. I would add 180k from the positive input of the first amplifier to ground to give it a DC path for its bias current.

    There are no biasing resistors in that schematic. It is all based on 2 batteries.

    If you want to lock the gain of the pre-amp, I would try to lock it at about 1.5V p-p at normal sound levels. Assuming the filters have a gain of one, and the LM386 has a gain of 20, you only need .2V p-p to drive the LM386. A practical limit is when you start to hear noise generated by the high gain of the pre-amp, but the TL07x is a nice, quiet chip and probably will not have noise problems.

    I don't like the second stage because it is designed to have a gain of infinity of you turn R5 all the way to zero ohms. You need a fixed resistor in series with R5 to limit the gain. Perhaps 10k, and that would make C6 = C9

    That's all I see right now. Check again if I missed something.
     
  8. atferrari

    Thread Starter AAC Fanatic!

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    Hola #12

    Here attached is the reworked circuit after rereading about polarizing op amps with single supply in a nice paper by TI.

    It seems that I could go away without blocking cap after the preamp because all op amps are now centered on +4,5 V.

    Have to build again this but I believe (hope bah) I am closer.

    Gracias for the help.
     
  9. #12

    Expert

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    Looks good from here.
    If you have trouble with the tiny DC component (maximum of .013 volts per amplifier) being amplified, you can add capacitors.
     
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