# Why is my microphone circuit not working?

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?

Datasheets:

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:

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.

• sounds like you are experiencing feedback – jsotola Dec 18 '17 at 22:56

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.

Otherwise:

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.

• it could also be acoustic feedback, the microphone "hearing" the loudspeaker. – Jasen Dec 18 '17 at 23:09
• @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. – Olin Lathrop Dec 18 '17 at 23:13
• 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. – bwoogie Dec 19 '17 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.