I have a project to build an intercom using op amp 741, because it is easy to find and is the only one that currently has access because of my location. Below the schematic diagram which I designed. The problem is that I can not solve a noise in the output like a noise of 60Hz, example of the noise: "tummmmmm" (audio link below on the website of vocaroo), this noise is so direct, even adjusting the gain with a potentiometer in step 1, the 330k ohm R13 resistor or in volume step 2 in the 10k RV1 resistor. Note: I tried to increase the power resistor of the electret is still continues. This amplifier will be outside and the microphone will be wired at a distance of + - 80/100 meters. The diagram is attached below. Has anyone ever experienced this? Or do you know how you know how to solve this problem with this noise? Thanks in advance!. https://vocaroo.com/i/s0jIWlJ8e3Yy
You don't specify the length of cable with which you performed your tests so far...
But in any case, if you want to carry an audio signal with reasonable signal integrity over such distances, you will probably want to use balanced audio lines so that your common mode noise gets rejected, thanks to the differential property of this transmission scheme.
If you want to look into balanced audio you should start with wikipedia
But the German version "ist viel besser" (it has more pictures that are worth checking, even if you don't speak German).
Also there has already been a post related to unbalanced (eq. electret input) to balanced conversion, using opamps. I think you should be able to implement such a system with 741 with minimal adaptations.
C1 and R11 are a high-pass filter. The RC time constant is 1uF * 22kohm = 22 millisecond. The F3dB = 1/(2 * PI * RC) = 0.16 * 45 = 6Hz.
You can reduce the capacitor to 0.022 uF, which raises the F3dB to 300Hz and will attenuate the 60Hz by 300/60 = 5:1 or 14dB. This reduction will be quite audible (each 3dB is audible; 4 factors of 3dB will please you).
If your interference is full-wave-rectifier sourced, then the 120 Hz ripple (2X faster) is attenuated by 14-6 = 8dB.
I think you need to determine the nature of the problem - is it power supply related or is the hum part of the amplifier's input signal? Easy to determine if hum is in the amplifier itself: temporarily move the + end of C1 to ground end of C2. If hum disappears, then hum is coming in with mic signal. If hum doesn't disappear, the amplifier either has excess ripple on its power supply or may be, especially if circuitry isn't compact (big wire loops pickup magnetic fields from nearby transformers, for example), injecting hum in the amplifier circuitry itself.
If the hum is coming in with the signal, be sure all three wires in the cable are tightly twisted together (this is what makes cables immune to magnetic fields). Better yet, use shielded twisted-pair cable, using the shield as ground. BTW, there is no need to "balance" the output of the mic - that would only be necessary if the mic was "grounded" to something at that end of the cable - as long as the cable and mic electrically "float", you are fine!
If you have another large capacitor similar to one in your power supply, trying temporarily paralleling it with current one to see if hum is reduced. Ultimately, you could power the amplifier temporarily from a car battery to help pinpoint the problem to your power supply. I hope these troubleshooting steps help locate the real problem.