help on HUM

[pra3718]

Please see attached file. with ground layout

Test # 1 : Power on only Left Channel, sounds error-free.
Test # 2 : Power on only Right Channel, sounds error-free.
Test # 3 : Power on both Left+Right Channel, large hum appear.
Test # 4 : Power on both Channel but not connecting DAC, Amp silent. -No HUM
Test # 5 : Sound with HUM when power switch OFF : Right channel Quit first then Left.

In-Seach I found Audio-Sector's thread but he is using single rectifier bridge power both chips.

Please help.
Thanks

 

[CharlieLaub]

Please see attached file. with ground layout

Test # 1 : Power on only Left Channel, sounds error-free.
Test # 2 : Power on only Right Channel, sounds error-free.
Test # 3 : Power on both Left+Right Channel, large hum appear.
Test # 4 : Power on both Channel but not connecting DAC, Amp silent. -No HUM
Test # 5 : Sound with HUM when power switch OFF : Right channel Quit first then Left.

In-Seach I found Audio-Sector's thread but he is using single rectifier bridge power both chips.

Please help.
Thanks

I suspect a ground loop. The loop is formed around this path:

trafo ground > LEFT channel caps ground > left power amp > left crossover > DAC > right crossover > right power amp > RIGHT channel caps ground > trafo ground​

To test this out, try running the system with everything on but one of the channels (both signal and ground lines) disconnected from the DAC to break the loop. I assume you have grounds running from the DAC to the crossover... You can insert a 10R resistor in these ground lines to see if that reduces hum. Better would be a SINGLE power supply for line level circuits and all ground connections connected directly at the same point (a star ground). Do not run ground connections in between subcircuits. Induced current in these will cause hum.

Finally, you can also try using shielded wire to connect circuit boards. This is not likely needed, but if your amplifier transformer and power supply is very close to any line level wires you are setting yourself up for induced currents.

Not sure about #5...

-Charlie

 

[diarav]

How is the USB DAC powered?

Can you try connecting the Ground of the DAC supply and the USB DAC signal ground directly to the central star ground?

 

[pra3718]

How is the USB DAC powered?

I have tried to power-up by mobile charger. 5VDC.

 

[AndrewT]

First.
Move the Main Audio Ground (MAG) away from the link between the Smoothing Capacitors.

Second.
Get rid of that enormous Ground LOOP between the channels.

You must have ONE MAG, or two completely separate channels.

 

[pra3718]

@Andrew T. Thanks for your help.

But Sorry, I could not understand... I must have one MAG.

 

[AndrewT]

..........Move the Main Audio Ground (MAG)...........

..............But Sorry, I could not understand... I must have one MAG.

 

[pra3718]

You must have ONE MAG

Thanks again.

I searched for "Main Audio Ground" and got it. Please give some time to check it.