I'm trying to make a simple microphone/speaker circuit with a PreAmp and an Op-Amp. Currently, the speaker just outputs a high frequency squeal, and not anything from the microphone.

Looking at the schematic, is there anything you can see that I've done wrong?

enter image description here


Preamp (INA217) http://www.ti.com/lit/ds/symlink/ina217.pdf

Op-Amp (OPA1652) http://www.ti.com/lit/ds/symlink/opa1652.pdf

-5V (MAX1044) http://pdfserv.maximintegrated.com/en/ds/ICL7660-MAX1044.pdf

Edit 1:

I've discovered some good news and some bad news. First, the good news! I've eliminated the high pitched tone from the speaker by connecting pin 1 of the MAX1044 to pin 8. The bad news... the circuit still doesn't output any sound from the microphone. So, I've simplified the circuit to try and isolate the problem:

enter image description here

More good news! Eliminating the OPA1652, I get actual audio from the microphone through the speaker and can adjust the mic gain with the pot and actually gets quite loud on a cheap pair of earbuds.

Revised question: Where have I messed up when it comes to the OPA1652 and how should I wire it differently?

For what it's worth, the end goal is to make a small audio mixer. Multiple mic inputs and a single output signal.

  • \$\begingroup\$ sounds like you are experiencing feedback \$\endgroup\$
    – jsotola
    Commented Dec 18, 2017 at 22:56

2 Answers 2


If the thing is squealing on its own, then at least one of:

  1. You have feedback path thru the power supply. This can be remedied by putting a decent amount (several 100 µF) of capacitance between each power supply and ground.

    It would also be a good idea to isolate the first stage better from power supply noise. A chip inductor or two, each followed by 10s of µF to ground might help. Only the first stage would use this filtered supply.

  2. You have a feedback path thru unintended couping somewhere. This would be fixed by proper layout. If you really have a good layout and this is still a problem, then you add guard and/or shield traces. However, that shouldn't be necessary if the rest is done competently.

  3. You are hearing the noise from the charge pump. I'm guessing that's a charge pump IC in the lower left corner? Filtering the supplies to the first stage as described above is necessary to fix this.


  1. Perhaps the overall gain isn't enough. You haven't said what the gain of the diff amp is, and the output amp only has a gain of 2. Usually you need a voltage gain of about 1000 to get microphone signals to line level audio.

  2. You are completely blocking DC to the microphone. That could be OK, even necessary, if it's a dynamic microphone. However, if it's a electret, then it needs a DC bias to work.

  • \$\begingroup\$ it could also be acoustic feedback, the microphone "hearing" the loudspeaker. \$\endgroup\$ Commented Dec 18, 2017 at 23:09
  • \$\begingroup\$ @Jas: Yes, that is also possible, but usually that's very obvious when it happens. The coupling changes as you move the mic and speaker around, nearby things move around, etc. \$\endgroup\$ Commented Dec 18, 2017 at 23:13
  • \$\begingroup\$ It's definitely not feedback, it did it even without the mic connected. I was able to fix the squeal by connecting pin 1 to pin 8 of the max1044. Please see my updated question. \$\endgroup\$
    – bwoogie
    Commented Dec 19, 2017 at 20:58

Try grounding pin 5 of the INA217.

That's how it's shown on the datasheet if you're not using a DC servo.


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