swordz60
Newbie level 6
Hello,
I have 2 very annoying issues regarding audible noise evidently entering through ground.
First one is the classic: 100Hz hum. I reduced it substantially by using shielded wire for all the air wires, but it still reappears when any high power mains device is turned on (e.g. electric kettle or vacuum cleaner, both 1800W :bsdetector: )
I have no capacitors at the mains input as of now, only a 0.47uF WIMA cap across the secondary winding. Would it help having capacitors at the mains input?
2 main possible causes come to my mind: first one me forgetting to add extra vias between top and bottom GND layers. Second, me recycling a 220V to 24VAC toroidal transformer, by adding an extra secondary winding to achieve 115VAC. The 115VAC is converted to +75V and -75VDC with center tap as GND for my 2SC5200 amplifier. I needed the 24V for relays, tone control and speaker protection circuits. I recycled an old Nordmende PA 1400 amplifier case, and it has no mains earth thus eliminating ground loop variant in my opinion.
Now, the second problem is more peculiar: after finishing my project, i thought it would be a good idea to add a fan for some extra cooling. I didn't want to make any circuitry for it, so i just added L7812 and L7805 regulators to feed 5V for a 12VDC fan. It works great, giving a light breeze making very little noise and drawing 130mA at startup and 60mA on idle. Now the problem with it is that for some reason it's ruining my audio signal. I'm adding 2 screenshots from my oscilloscope:
First pic is my probe connected to chassis ground and to a disconnected amplifier output (i disconnected it manually). This leads me to believe that somehow it feeds this noise through the ground since i don't see any other path for it.
Second pic is the amplifier output, which as you can see is pretty much the same ugly noise amplified. If i disconnect the fan it all goes away. I tried adding a large capacitor but it only made the noise stable (without the cap the noise is the same, just a bit jittery).
Any insights are very welcome, thank you.
I have 2 very annoying issues regarding audible noise evidently entering through ground.
First one is the classic: 100Hz hum. I reduced it substantially by using shielded wire for all the air wires, but it still reappears when any high power mains device is turned on (e.g. electric kettle or vacuum cleaner, both 1800W :bsdetector: )
I have no capacitors at the mains input as of now, only a 0.47uF WIMA cap across the secondary winding. Would it help having capacitors at the mains input?
2 main possible causes come to my mind: first one me forgetting to add extra vias between top and bottom GND layers. Second, me recycling a 220V to 24VAC toroidal transformer, by adding an extra secondary winding to achieve 115VAC. The 115VAC is converted to +75V and -75VDC with center tap as GND for my 2SC5200 amplifier. I needed the 24V for relays, tone control and speaker protection circuits. I recycled an old Nordmende PA 1400 amplifier case, and it has no mains earth thus eliminating ground loop variant in my opinion.
Now, the second problem is more peculiar: after finishing my project, i thought it would be a good idea to add a fan for some extra cooling. I didn't want to make any circuitry for it, so i just added L7812 and L7805 regulators to feed 5V for a 12VDC fan. It works great, giving a light breeze making very little noise and drawing 130mA at startup and 60mA on idle. Now the problem with it is that for some reason it's ruining my audio signal. I'm adding 2 screenshots from my oscilloscope:
First pic is my probe connected to chassis ground and to a disconnected amplifier output (i disconnected it manually). This leads me to believe that somehow it feeds this noise through the ground since i don't see any other path for it.
Second pic is the amplifier output, which as you can see is pretty much the same ugly noise amplified. If i disconnect the fan it all goes away. I tried adding a large capacitor but it only made the noise stable (without the cap the noise is the same, just a bit jittery).
Any insights are very welcome, thank you.