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Old 16th April 2009, 11:00 AM   #1
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Default Linear power supply causing mains hum at speaker

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.
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Old 16th April 2009, 11:29 AM   #2
AndrewT is offline AndrewT  Scotland
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50 to 100mVpp is enormous.
Your DMM would read ~ 25mVac when it should be reading <1mVac
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Old 16th April 2009, 11:34 AM   #3
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I'm sorry I meant it is closer to 10-20mV...disregard the 50-100, i'm not sure what I was thinking....
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Old 16th April 2009, 11:53 AM   #4
AndrewT is offline AndrewT  Scotland
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is that ripple peak to peak (Vpp) or AC (Vac)?

It's still enormous.
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Old 16th April 2009, 07:02 PM   #5
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I'll re measure it today and check for sure what it is on each line. Would this ripple be causing the hum? If so should I just make the filter caps bigger to smooth out the ripple more?
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Old 18th April 2009, 02:43 PM   #6
star882 is offline star882  United States
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Disconnect the power supply from the amplifier (use bench power supplies to power them) and note what happens if the power supply is plugged in. If it still hums, it's either a ground loop or EMI. Use light bulbs to load down the supply for more realistic operating conditions.
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