Multiple questions regarding microphone/aux circuit

1. Not a problem. The job of coupling capacitor C6 is to assure that the bias point for the mic and the bias point for the opamp circuit are completely independent. The value of R2 seems a little low to me. This resistor is the load on the mic cartridge output. Consider increasing it to 22K or even 47 K. The mic output voltage will increase, but so will the circuit noise. Life is choice.

2. Half right. C3 (?), C17, and C22 are coupling capacitors, so the DC operating points on both sides of them can be different. However, U1A pin 3, the non-inverting input, must be biased at VCCO2. This sets the DC value of the opamp output midway between its rails.

3. ALL audio power amp ICs are very touchy about power supply decoupling. Place a 0.1 uF ceramic cap from pin 6 to pin 4 as close as possible to the IC pins with the shortest possible leads. Next, add an electrolytic cap in the 47 uF to 100 uF range, in parallel and again close-in.

4. Maybe; depends on the signal levels.

5. Other:

J1 pins 10 and 11 can be connected to GND. This should prevent noise incursion when the AUX jack is not used.

Consider increasing R3 or C4. As shown, they form a 160 Hz high-pass filter.

The output of U1B car drive the summing amp without C5 (?) and R6.

There is no apparent reason for having the LM741 in the circuit. The rail-splitting function can be done with U1C or D if available.

Both the 741 and the 324 are relatively noisy ICs. Also, the 324 has high crossover distortion. Consider an alternative parts, such as something in the Texas Instruments TL07x or TL08x lines.

1. The pull-up to 9V should be ok, as it is blocked from the 4.5V by C6, hence ideally only AC flows through C6 after it has been charged to 4.5V(9V-4.5V). I don't see the microphone part number. I would need it to know whether the pull-up is correct or not.

2. That is not the case, the aux inputs are most likely 0-1V audio signal, but it is not guaranteed as I do not know what are you feeding to the jack connector.

3. A bigger decoupling capacitor will help with the power rail stability, but i don't think that is your biggest problem. If you want to change some capacitors, I would change the ones in the signal path to allow for lower frequencies. Also you should use low ESR capacitors in the signal path. In addition as @Tyassin already mentioned, you need decoupling capacitors close to each opamp.

4. Yes they will. They will clamp the output to +-0.7V.

5. I would not recommend this circuit, although you can make it work somehow by tweaking the parameters, it will never maintain audio quality. Also the noise pickup and audio quality is highly dependent on the actual layout. Could you provide it?

So overall, it is nice to see a fellow audiophile, but in order to help you I would need more information(microphone specs, layout, jack input specs,...).

Hope this helps at least a bit. Take care!

1. You only have the MIC pulled up to 9V. The opamp input is pulled to 4.5V. That's why the capacitor is there, to block DC and pass AC.

2. No they are not pulled to 4.5V but they don't need to be - the DC is blocked by capacitors. The only issue is, the bias should come from the op-amp via negative feedback. Because opamp positive input is grounded, it's the op-amp that's biased incorrectly. As a side note, C5 and R6 are useless because you have C17 and R31.

3. A 1000 uF capacitor would be quite useless. Of course it depends on if you intend to power this from a 9V block battery and drive 4 ohm speakers at full power, because the battery really can't do that anyway, and the capacitor would just delay the inevitable.

4. Yes, if the amplitude is high enough, the diodes will clip - that's why they are there. Low enough amplitude and they don't clip. It's more like if they should be somewhere else in the design instead.

5. Maybe. It depends what this thing is supposed to do, what kind of signals it gets in and what kind of signals it must send out. The potentiometer values and their connections are undetermined. With 9V supply the chip can output about 8Vpp into no load or less than 4Vpp into 4 ohms. As the chip has a gain of 20, you get 8Vpp with only 0.4Vpp input. and 0.8Vpp is what the chip allows if it had more supply. The speaker output caps are 470nF, it is likely an error as 470uF would sound more correct for driving speakers. Based on drawing, the left and right channels look like swapped, but it's a mono summer so it does not matter.