Modifying TI schematic to learn more about amplifiers

Question

Let me start off by saying that I'm a complete beginner. I've watched some videos on amplifiers and read about them but it's hard for me to analyze circuits that don't match exactly to the examples I have seen in the past. Also, a lot of the information just goes way over my head. I have no electrical engineering background but I have experimented and tinkered with some Arduino based projects.

For a project I'm working on, I want to make an audio amplifier circuit that takes an electret microphone's output and amplifies it to 300mVpp from a single source power supply. I found a schematic from TI that looks like it could do the job, with some modifications. Here is a link to the place I found it: TIPD181. I built a prototype PCB based on the schematic below and I am very happy with the sound quality. I've built other amplifiers and they all hiss or crackle (completely due to my incompetence). It does sound hollow (not sure if that's the right word for it) or maybe lacking bass? That could be due to the cheap speakers that I was using but regardless, the final use of this will not be to drive a speaker. I am hoping to create a visual spectrum analyzer. I will be taking the output of this circuit and putting it into a MSGEQ7(Datasheet). I know that there are prebuilt microphone breakout boards that have AGC and adjustable gains like the MAX9814 but I'm enjoying the learning process of building microphone circuit. I'm hoping to eventually cut out the MSGEQ7 and create my own switched capacitor filters but I'm saving that for another time.

Here is what I'm thinking about when trying to adapt this schematic to my needs and this is where I'm just not getting it but maybe you can help. Again, I'm a total beginner. I can take criticism but know that I'm not trying to waste anyone's time so if you think I'm a lost cause then please ignore me.

1. From the overview on the TI website, it says that this circuit will amplify the mic output to line levels voltages. A quick google search tells me that's .448V to 1.228V. This is higher than the 300mVpp that I require so I think I will need to reduce the gain.
2. That leads me to having to find out the Vpp output of the microphone without the amplifier to know how much gain I need. Right? Microphone's Datasheet How would I figure that out from the datasheet? I think it has to do with the sensitivity value, right?
3. Then I need to find the voltage gain. To me, this is in the inverting configuration so the gain would be -R2/R1 which currently is 5.47. Am I correct in this? If I am, do I need to add the microphone's impedance (2.2k Ohms) to R1 for an accurate gain value? After that I can see exactly what this circuit is outputting and adjust R1 / R2 to my needs, right?
4. What is the purpose of R4 and R6 is this schematic? Is R4 to adjust the output impedance? If so, why? Is R6 for a high pass filter? If so, why?
5. When I measure the output of this circuit on my oscilloscope, the center of the signal starts high (I think Vcc/2 or what I put into the non inverting input. I can check this later if it's crucial for this analysis) and then slowly drops to 0V over time. Is that normal operation?

Any other suggestions or criticisms are welcome. If I'm missing something, please let me know.

Answer 1

[...]gain would be -R2/R1 which currently is 5.47. Am I correct in this? If I am, do I need to add the microphone's impedance (2.2k Ohms) to R1 for an accurate gain value?

If microphone impedance is 2.2k, it gets combined with R1 like this: \${{2.2k \times 13.7k}\over{2.2k + 13.7k}}=1.896k\$
Then voltage gain is \$-{{R_2}\over{1.896k}}\$
If you need more gain, increase value of R2.

What is the purpose of R4 and R6 is this schematic?

R4 keeps the amplifier stable, and protects it from short-circuit loads.
R6 is a "pop" protector. You may notice that on power-up it will still "pop" as C5 discharges. But if you power up, then somewhat later connect to your spectrum analyzer, the connection should be pop-free, because R6 has discharged C5.

4.) When I measure the output of this circuit on my oscilloscope, the center of the signal starts high (I think Vcc/2 or what I put into the non inverting input. I can check this later if it's crucial for this analysis) and then slowly drops to 0V over time. Is that normal operation?

You're seeing C5 discharge - that's normal. It is standard to remove DC offsets from amplifier outputs. That's basically what C5, R6 does. In this case, the amplifier is biased at one-half Vcc so that audio can swing more positive and more negative. We call this a DC-offset. DC-offset is not wanted by the MSGEQ7.

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Your experiment with a speaker load sounded "tinny". Yes, this can be caused by an unboxed speaker. But an audiophile (boxed) speaker would still sound tinny because C5 is too small to properly drive an 8-ohm loudspeaker. A 100uf capacitor would help restore bass.
But C5=2.2uf should be just fine for driving the high-impedance MSGEQ7 without losing bass. The impedance of MSGEQ7 is 1M ohm, much higher than 8 ohms.