MC1458P Preamp Circuit

Discussion in 'General Electronics Chat' started by tracecom, Sep 22, 2012.

  1. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
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    In the attached schematic, which I found on-line as part of a preamp for an electret mic, I found that the circuit loaded the input source excessively. Using the component values shown, my input signal went from .6 Vpp (no load) to .01 Vpp, and the output from the circuit was only .08 Vpp. I changed R1 to 1 MΩ and R2 to 10 MΩ, which allowed the input to be .42 Vpp and the output was 4.3 Vpp. This was more what I was looking for.

    My questions are:

    1. Would an electret mic (which I didn't use as an input device) have been loaded less than my sine wave oscillator (which is a simple LM741 circuit.)

    2. Is there any reason why the input resistor should be 4.7k? Is there ever a case when a low value like this is needed?

    3. Is there anything wrong with using megohm range values for R1 and R2?

    And an unrelated question:

    4. How does one choose the values for C1 and C2?

    Thanks.
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    The very old MC1458 is simply two 741 opamps (44 years old) in a single package.
    It has too much hiss and has trouble above only 9kHz to be a mic preamp.

    You indicate that you need a high input impedance but its input impedance is 1 million times lower than a modern (30 years old and newer) Jfet input opamp.

    Your circuit is inverting which automatically produces a low input impedance. A non-invering circuit has a much higher input impedance and is simple.

    With the resistor changes you made, the output could have shifted 5VDC due to its high input bias current. Then it would be clipping and be a rectifier almost all the time.

    Answers:
    1) I haven't seen the circuit of your sine wave ocillator so I can't calulate if this lousy circuit loads it more than it loads an electret mic.

    2) An electret mic and a 10k resistor powering it have a total impedance of about 2.7k ohms. If it is loaded with less than 14.7k then the output level will be reduced.
    The circuit you found is a load of only 4.7k ohms (R1) to the mic or any other input source. So the output level is reduced. The designer didn't know.

    2) It is a low input impedance inverting amplifier. If you want a high input impedance then use a non-inverting circuit (simple) and/or a modern (30 years old and newer) opamp with Jfet inputs.

    3) The old MC1458 and uA741 opamps have too much input bias current for a high feedback resistor value.

    4) C1 and C2 are simply calculated for the frequency where the output is down -3dB (0.707 times). f= 1 divided by (2 x pi x RC). The source impedance plus R1 and C1 are for the first calculation and C2 and the load impedance are for the second calculation. If both are calculated for the same frequency then the -3db response of the entire circuit is at double the calculated frequency.
     
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  3. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
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    Thanks for the detailed answer. That's a lot to think about.
     
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