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PAS 3 Hum
PAS 3 Hum
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Old 15th June 2018, 07:00 PM   #21
petertub is offline petertub  Sweden
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Join Date: Jul 2015
Location: gothenburg,sweden
Change cap, or even better, use a power supply board ( like the first one $35.5 ) :
Power Supply Kits


A can can makes ground contact by bending the tabs onto the chassies, as the can is made of aluminium it won't take solder and the contact is often bad. The above board solves this by using soldered caps and real ground cables.
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Old 3rd July 2018, 06:05 PM   #22
Marksd is offline Marksd  United States
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All that have helped. I installed a new electrolytic can. All the hum is gone. This was the problem. I replaced a brand new can with another brand new one that is 30-30-30-10 @475V. All my hard work in this unit is now complete. It sound sgreat.
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Old 3rd July 2018, 08:31 PM   #23
rayma is offline rayma  United States
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Quote:
Originally Posted by Marksd View Post
I installed a new electrolytic can. All the hum is gone.
You should return the defective can to the seller, this is very unusual.
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Old 4th July 2018, 12:37 AM   #24
Marksd is offline Marksd  United States
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Rayma, The "bad" one is already on an RA back to the vendor. Even thought the new one was 30-30-30-10. The previous one was 30-30-30-30. It is hum free on every selector switch setting and sounds great. In Michigan our power dipped to below 113 volts today, so every voltage reading was way down. Still the PAS and St70 sounded great.
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Old 4th July 2018, 01:01 AM   #25
rayma is offline rayma  United States
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Quote:
Originally Posted by Marksd View Post
Rayma, The "bad" one is already on an RA back to the vendor.
I've never seen that happen before. It must be an unusual manufacturing defect.
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Old 5th July 2018, 06:49 PM   #26
rmb is offline rmb  United States
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Join Date: Nov 2012
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Was there a short jumper from the can capacitor to ground terminal on the chassis? Unless the can is "floated" for some reason, some of those "twist lok" mechanical connections to the chassis weren't good electrical connections for grounding. A bad ground on the can will put 120 Hz hum through the entire preamp.
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Old 9th July 2018, 09:52 PM   #27
Marksd is offline Marksd  United States
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RMB, Yes, there are actually four grounds that attach to the can. There is a picture of this on page 2 of this post. This is my actual PAS I rebuilt. The new can at 30-30-30-10 works like a gem. When I was troubleshooting the bad can I used a 20uf that I jumpered each section to try and determine if the can was bad. This did not originally work and the hum remained. One of the recommendations here was to purchase another can and try it, which I did and it worked perfectly, no hum...gone. The PAS is not a perfect preamp. I can say that it sounds 100% better with new boards and matched components channel to channel. I matched everything within 1%. Maybe overkill. It is feeding a Dynaco ST70 I also rebuilt. Lastly, I have a line on some almost as new DCM Time Window 1A's that ought to make this set up sound spacious. Thanks for your feedback, comments and any more help.
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