Hello.
I'm building a very simple audio mixer using the NE5532. I wanted to keep it simple and mix 2 stereo inputs (a line-in and a bluetooth device). So I would get 4 inputs. I mean "simple" after trying eternally to no success mixing the 4 inputs with a quad-amp (LM3900).
So now I just have the 4 inputs into the inverting input, to use the opamp's impedance as a buffer so the signals add properly. It works great, simple and quick. This mixed mono audio signal goes into a poweramp (+500watt).
Nevertheless, I'm encountering the following issues and I would like to request help from you guys:
1) When power-on. I get a 120hz hum for about 30 seconds, which disappears and fades. This is the spectrum for the first 30 seconds:
2) After 30 seconds, the hum continues but less prominent. The music is way louder at this point than the hum. The hum increases as you increase the power-amp's output stage. With this I can live because it's like a guitar amp in the bedroom, it won't be perfect. Nevertheless, this is the spectrum:
3) While doing the mixer, I powered the power-amp output stage by itself (no mixer, no ic) and didn't get "booms" and loud pop sounds at the output. Now after adding the mixer, I get a loud pop noise on power-on and power-off. I'm sure this has to do with the opamp in the mixer or the bluetooth device's behaviour. I'm not sure if I should decouple or add extra resistors.
This is the whole schematic for the mixer and it's power source. All the elements soldered are shown in the drawing. as I said, after trying and trying on a 3900 quad amp mixer I decided to go back to basics and add signal. Sorry I don't have a software to diagram it or simulate it.
Your help will be greatly appreciated. Cheers!
I'm building a very simple audio mixer using the NE5532. I wanted to keep it simple and mix 2 stereo inputs (a line-in and a bluetooth device). So I would get 4 inputs. I mean "simple" after trying eternally to no success mixing the 4 inputs with a quad-amp (LM3900).
So now I just have the 4 inputs into the inverting input, to use the opamp's impedance as a buffer so the signals add properly. It works great, simple and quick. This mixed mono audio signal goes into a poweramp (+500watt).
Nevertheless, I'm encountering the following issues and I would like to request help from you guys:
1) When power-on. I get a 120hz hum for about 30 seconds, which disappears and fades. This is the spectrum for the first 30 seconds:
2) After 30 seconds, the hum continues but less prominent. The music is way louder at this point than the hum. The hum increases as you increase the power-amp's output stage. With this I can live because it's like a guitar amp in the bedroom, it won't be perfect. Nevertheless, this is the spectrum:
3) While doing the mixer, I powered the power-amp output stage by itself (no mixer, no ic) and didn't get "booms" and loud pop sounds at the output. Now after adding the mixer, I get a loud pop noise on power-on and power-off. I'm sure this has to do with the opamp in the mixer or the bluetooth device's behaviour. I'm not sure if I should decouple or add extra resistors.
This is the whole schematic for the mixer and it's power source. All the elements soldered are shown in the drawing. as I said, after trying and trying on a 3900 quad amp mixer I decided to go back to basics and add signal. Sorry I don't have a software to diagram it or simulate it.
Your help will be greatly appreciated. Cheers!
