Hello all,
I built a class D amp which requires a +12v, +5v, -5V, and +70V input. The power supplies and the amp PCB are located in a big enclosure. I am using a simple linear power supply for the +12, +5, and -5 with linear regulators. The +70 is supplied from a separate linear power supply with a toroid transformer. There is very little current draw on the the first supply, less than 200mA for each line. The +70V line draws about 2A or so.
I am using a terminal strip for the AC lines and the two transformer AC lines are starred together on the strip. The output grounds are also starred together on a separate terminal strip. When I turn on my amp, I get a very audible 50 or 60hz noise at the speaker.
Solutions tried:
I have tried using an isolated AC outlet but the noise was still there. The outlet had a transformer (1:1) between the main line and the output.
When I use the bench power supplies to provide the +12,+5,-5,and +70 the noise completely goes away. I left all my wiring in place and simply connected the ground from the bench supplies to the ground point in my enclosure, leaving all my ground wires in place.
I initially thought my toroid was causing the noise, so the next test I performed was to use a bench supply for the +70v and my linear supply for the rest....The noise was still there but not as loud.
i also thought I might have a ground loop in my circuit (I am using 3 ground planes in my amp PCB) but I have good ground wires with star grounding at the power supply terminal strip. Since the noise went away with the bench supplies, I have isolated the problem to my power supplies.
What can I do to fix this? I do not have a schematic handy but it is essentially a basic linear regulated power supply using LM7805 and LM7812 regulators and filtering caps. The output does not have very much ripple, maybe 50-100mV at most.
I built a class D amp which requires a +12v, +5v, -5V, and +70V input. The power supplies and the amp PCB are located in a big enclosure. I am using a simple linear power supply for the +12, +5, and -5 with linear regulators. The +70 is supplied from a separate linear power supply with a toroid transformer. There is very little current draw on the the first supply, less than 200mA for each line. The +70V line draws about 2A or so.
I am using a terminal strip for the AC lines and the two transformer AC lines are starred together on the strip. The output grounds are also starred together on a separate terminal strip. When I turn on my amp, I get a very audible 50 or 60hz noise at the speaker.
Solutions tried:
I have tried using an isolated AC outlet but the noise was still there. The outlet had a transformer (1:1) between the main line and the output.
When I use the bench power supplies to provide the +12,+5,-5,and +70 the noise completely goes away. I left all my wiring in place and simply connected the ground from the bench supplies to the ground point in my enclosure, leaving all my ground wires in place.
I initially thought my toroid was causing the noise, so the next test I performed was to use a bench supply for the +70v and my linear supply for the rest....The noise was still there but not as loud.
i also thought I might have a ground loop in my circuit (I am using 3 ground planes in my amp PCB) but I have good ground wires with star grounding at the power supply terminal strip. Since the noise went away with the bench supplies, I have isolated the problem to my power supplies.
What can I do to fix this? I do not have a schematic handy but it is essentially a basic linear regulated power supply using LM7805 and LM7812 regulators and filtering caps. The output does not have very much ripple, maybe 50-100mV at most.
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