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I'm trying to build a single supply balanced preamp for an electret mic. To further complicate matters, I'd like to impedance balance the mic output to take full advantage of the preamp's noise rejection capabilities. Here's what I've got so far: enter image description here

There are a few problems with the circuit:

  1. gain is not as expected. Removing the 2.2K resistor from pins 1 and 3 on the left of the diagram increases the gain substantially, but the cable is no longer impedance balanced.
  2. there is a lot of hum, which is corrected by replacing the 470K resistor at pin 3 on the opamp with a wire to the voltage divider. However, I'm assuming that the opamp is no longer functioning as differential in this case.

I've run out of ideas - can anyone suggest anything that might remedy these problems?

Thanks

Thanks for the replies so far everyone. I've made a slight change to my original design that seems to have fixed the problem:

enter image description here

The only difference is that I moved the 470K resistor from pin 3 on the opamp to ground (as it would be in a split supply design) and connected pin 3 directly to the voltage divider. This has given me the desired result of the correct gain with the hum reduction. Is this the proper way to set up a single supply balanced differential opamp?

Thanks again

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    \$\begingroup\$ As far as I know electret microphones can have a single ended output only. This has to do with the way they are constructed. I fail to see why you would need the impedance balancing. I fail to see why that would improve the noise. Common mode rejection yes but noise ? I think you will need a dynamic microphone with a differential output to make this work. I have yet to see an electret mic. amplifier with a differential input. But if I'm wrong please explain, I don't know everything. \$\endgroup\$ – Bimpelrekkie Feb 11 '16 at 16:42
  • \$\begingroup\$ By noise I mean hum pickup. It is a fairly common technique in pro audio to impedance balance the output of a device, and is in fact quite effective; since any noise induced in the signal will be presented to both the + and - inputs of the opamp, it will be canceled out. \$\endgroup\$ – Chris Feb 11 '16 at 17:28
  • \$\begingroup\$ Yes that is obvious to me but in electronics noise is something different than hum pickup. You calling hum pickup noise confuses matters ! Hum would be commonmode and will be rejected in a common-mode rejecting amp. Noise is uncorrelated so you cannot reject it. \$\endgroup\$ – Bimpelrekkie Feb 11 '16 at 22:13
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Every Electret mic that I've seen is inherently unbalanced. The output signal is developed between the metal case of the mic and center electrode.

Electret mics are often used in Professional Audio and are connected so as to present a balanced output. This is done by putting equal-value resistors in series with the (+) and (-) output terminals of the mic capsule, then capacitively-coupling to the balanced output connector. Additional resistors take care of the bias supply.

But even these mics aren't truly balanced - there is more capacitance between the mic capsule case and Ground than there is between the mic capsule (+) and Ground. You will sometimes see an extra capacitor added to help compensate for that.

Bottom line is: If you are dealing with a bare Electret mic, treat it as unbalanced.

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