DIY 2SC5200 amplifier ground problems

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.

First of all, your scope ground lead may be picking up a lot of noise. Without seeing your actual hardware or schematic, it’s hard to tell what the problem is. Does the fan ground share the same path as audio input ground? Definite no-no. Is it radiated noise that’s being picked up? If you ground the amp input, does that improve the noise on the output?

I don’t think capacitors on the input will help at all. I suspect your problems are all grounding and layout issues.

Yes, the ground is shared, but i tried isolating the fan by using a B0505S isolator (worked great for other ground related noise problems so far), but the result was the same.
I used this simple circuit as a base for my amplifier:


I just added a TDA1524 for tone and volume control, a uPC1237 for speaker protection and some other tweaks, like relays for mono/stereo switch, extra fuses etc.
I'm also adding eagle .sch and .brd files if you want to look through my work: **broken link removed**
If you don't use eagle, i can post screenshots.

It's my first serious attempt at making an amplifier and i'm satisfied with the result, but if there's a solution for the problems stated above I'd very much like to fix them.
The fan is actually optional in my opinion, but i was quite surprised it would have such an effect on the sound.

Another idea that comes to mind is actually using a separate 12V power supply to avoid tapping into the transformer. I originally used a L7824 and L7812 regulators in fear that 24VAC after the diode bridge would be too high for L7812. However, the voltage appeared to be too low for L7824 which in turn generated awful noise and i had to remove it.

I also pondered how to separate audio input ground from the chassis ground, but didn't manage to figure it out.

I'm now trying to figure out a way to make a VU meter to be able to monitor load level, but didn't manage to find one which would be able to handle +-75V. I'm hesitant on making one at the amplifier input (after TDA1524), because i feel that it may not depict the real situation

Although the fan may use the same ground as the amp, the key is to ensure that the return current of the fan does not use the same path. In other words, a separate wire or Pcb trace should run back to the power supply ground. All the regulator and fan grounds should tie to a single physical point.

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: )

Click to expand...

try twisting together any air wires and their returns, such as B-E on any/all of the power transistors.

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.

Click to expand...

are you in a region that uses 50Hz mains frequency? that becomes 100 Hz when rectified. a stray thought said the transformer is saturating briefly at every peak of the AC line, hence the voltage on secondary goes to zero.

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.

Click to expand...

the probe may be picking up the noise - try shortening the ground connection of the scope probe. try closing any loop the scope probe and its ground lead may make

View attachment 151816

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).

Click to expand...

disconnecting the fan reduces the load on the transformer. try running the fan on a separate power supply - see if the 100 Hz spike goes away or not

View attachment 151817