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Old 10th October 2015, 03:38 AM   #1
diyAudio Member
Join Date: Oct 2014
Default Help lowering noise in big series pass regulator circuit

I've got a big linear power supply from the 70's here and I'm having trouble lowering the noise floor with it. I'm getting almost equal peaks at 60, 120, and 240 hertz. So far, I've replaced all of the caps and a few transistors in it. I've also replaced two op-amps for peace of mind. Right now, I'm at about -55dbV on the audio outputs of the synthesizer this thing belongs to. I've seen this power supply design perform at -65dbV before so I know that improvement is possible. I'm thinking about changing the integrator caps to quiet it down but can't figure out how to comprehend the equation that I found or if damping is even the problem. Here's the paper that I'm using as a guide: The power supply diagrams are in the attachments.
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Old 10th October 2015, 09:04 AM   #2
AndrewT is offline AndrewT  Scotland
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders
Check the DC voltage across C730 and C731.

If your meter can (not all DMM can do this) measure AC voltage in the presence of a DC offset, then also measure the AC voltage across these two capacitors.
regards Andrew T.
Sent from my desktop computer using a keyboard
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Old 11th October 2015, 01:57 AM   #3
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Join Date: Oct 2014
I just tested it with my Tek 2465. I'm thinking I've got a ground loop somewhere that is bleeding into my system. On the output of each rail, I'm getting about 5mv of ripple when connected to power supply ground. When connected to chassis ground, it jumps up to about 40mv.
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