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Well, it will be a broad schematic. To summarize, there my approach is:

  1. Yellow part is for oscilations at three different freqs(around 0.8 - 2 kHz, different for each of the three signals), sinusoidal, I can get nice 6 Vpeak sine waves, observed with oscilloscope, no noise, smooth.
  2. In the orange part, I use classical potentiometers to change the amplitudes of the signals. For this part, I suspect from the potentiometers, I know they are very unstable but I somehow have to use them.
  3. Then, red part, adds the signals together, nothing fancy.
  4. Finaly, the green part is my design to drive an 8 ohms speaker.

Now, when I connect the speaker, I get a noticable noise on the speaker output. It is both understandable from sound and also checked by oscilloscope. I have heatsinks for transistors, and I suspected from them for the reason I explained in my other question.

when I inspect the signals in the circuit via oscilloscope, I see there is some noise on waveforms, while trying to understand where it comes from, I have touched the heatsink (...) It becomes much smooter.

However, then, I thought it might be resulting from something else and wanted to get your opinions about it. What can I do to minimize the noise ? Would soldering things on something like this solve things?

Things to note:

  1. I got satisfying results in simulations, everything was as I wanted. However the reality spoiled me.
  2. I implement the circuit on a breadboard for now, while prototyping, I suspect it may be bad in terms of noise.
  3. I don't know it is relevant or not but the noisy parts are mostly in the upper parts of sinusoidal waves.

Thanks in advance.

enter image description here

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    \$\begingroup\$ Why do you directly DC couple the summing amp into the output stage? Why is your output connected to the (+) side instead of via a cap and resistor to the (-), while grounding the (+) side? What's the thinking here? (And I see you are aware of cross-over distortion, as you have a sample point to look at the opamp output itself.) I have only just quickly skimmed this. But that's what caught my attention earlier. \$\endgroup\$
    – jonk
    Dec 23, 2019 at 0:53
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    \$\begingroup\$ You've got a lot going up there to be asking, "Why do I need to have a capacitor?" I don't mean that, harshly. I just mean that your understanding may still be a bit behind your ambitions, right now. I have a small suggestion. Ground the (+) input of U2. Unground R14 and stick a 10uF cap in series there and over to the summing output. Report results. I'm curious. \$\endgroup\$
    – jonk
    Dec 23, 2019 at 1:07
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    \$\begingroup\$ Yes, something like that. I assume you don't care if the AC output is inverted in phase relative to the AC input. (And I tried to think of something quick and easy just to see, without my having to go through and do both large scale and small signal analysis on all that stuff you have up there.) \$\endgroup\$
    – jonk
    Dec 23, 2019 at 1:17
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    \$\begingroup\$ Observe Supply noise , 0V noise ref=earth gnd under load. Measure A-B diff Noise. Capture waveforms. Look for physical paths of positive feedback coupling (like "adder") with gain. (bad). Suppress CM noise with Cap to earth ground 0Vdc of 0Vac . report noise at each node Vpp , freq. shape ... use twisted pairs \$\endgroup\$
    – Tony Stewart EE75
    Dec 23, 2019 at 1:21
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    \$\begingroup\$ That's normal . I felt the same way when I started. I was even afraid to call a Prof when I started work and couldn't figure out some things. Never be afraid to ask if stuck but , learn how to learn faster looking in better places. \$\endgroup\$
    – Tony Stewart EE75
    Dec 23, 2019 at 1:35

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