How to eliminate amplifier noise?

Question

Electronics

I fell into the hands of a home-made ULF
. It works, but it fonit. Power tried from 3 to 12 volts, both from the PSU and from batteries. The wires were through the plug, soldered directly to the board. Screened cable input. I changed the capacitors to new ones, nothing changed. The sound source is working, I tried different ones (player, phone, laptop).
How can the hiss be eliminated?

Answer 1

There is a simple way to find out what is noisy: make an audio probe.
In the simplest case, connect headphones through a capacitor of about 10 microfarads. One wire to ground, and the other two (left and right) connected together through this capacitor to the oscilloscope probe and poke them at different points in the circuit. But it is better to do it through a sound amplifier with an isolated power supply, for example, an old battery-powered player with a microphone input.
With the help of this thing, you can separate flies and cutlets: get on the power supply and check that it does not make noise, or stand on the direct and inverse input of the op-amp and listen to it, if it is noisy, then you need to reduce the input resistance (if 50 hertz) or hang a condender if HF noise etc

Answer 2

so does it hiss or phonate? phonite is uuu hissing is shhh the
reason for the first is interference from the network 220 the
reason for the second is the amplifier's own noise (from the wrong choice of operating mode to frankly "rotten" transistors)

Answer 3

Use two anti-phases with subsequent noise subtraction.