Help with audio mixer and 120hz noise

Thread Starter

DonLduk

Joined Jun 12, 2017
16
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:

Screen Shot 2017-11-01 at 9.08.45 PM.png

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:

Screen Shot 2017-11-01 at 9.09.50 PM.png

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.

WhatsApp Image 2017-11-01 at 20.56.05.jpeg

Your help will be greatly appreciated. Cheers!
 

crutschow

Joined Mar 14, 2008
23,307
Is that dotted ground line connected to the transformer center-tap?
Why is it dotted?
 

Alec_t

Joined Sep 17, 2013
10,355
Are you using a star ground system for the power supply, the mixer and the power amp?
 

Thread Starter

DonLduk

Joined Jun 12, 2017
16
Is that dotted ground line connected to the transformer center-tap?
Why is it dotted?
Hello. Yes it is the center tap. It's dotted for just to clear the visibility. It splits the volts to +14v and -14v in VCC and -VCC respectively, and is my ground in the circuit.

Are you using a star ground system for the power supply, the mixer and the power amp?
This power supply is for the pre-amp tone and mixer. The power amp, output stage and spakers have another power supply (+45v / -45v) with larger capacitors. The ground for all is the central coil tap of both coils. Which goes for the grounds of the inputs, outputs, opamps etc.
 
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