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phono preamp with B+ split - hum problem
phono preamp with B+ split - hum problem
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Old 16th September 2018, 07:58 PM   #11
prinzinio is offline prinzinio  Poland
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Join Date: Feb 2009
Ok, i found a solution.
Will post the results within few hours
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Old 16th September 2018, 11:00 PM   #12
stocktrader200 is offline stocktrader200  Canada
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Location: Calgary
probably a missed ground somewhere in there.
I would PCB it personally. At the very least PCB the power supply capacitors.
first capacitor should be grounded to transformer CT directly as most current flow is there.
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Old 17th September 2018, 10:39 AM   #13
prinzinio is offline prinzinio  Poland
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Originally Posted by Alan4411 View Post
First thing you have to do is bond the copper plate to mains earth for safety. I assume it is 'floating' now.

Then I would split the buss bars at both ends and try it as if it were 2 separate amplifiers with 2 B+ leads and 2 -ve leads. Not even sure where on the buss would be best to put the -ve leads, maybe start with the big capacitor and move it towards the input or output sides to see where is better.

There are so many things it could be, but start from there.
Actually this worked as I tried and worked on it second time.

The issue was that last common B+ filtering capacitor was in PSU far instead of being close to the output RCA terminals on the amplification part.

I had terrible issues when I joined two output RCA sockets together and used the middle point as a ground split point with only single wire going back to the last PSU capacitor ground pin. When I cutted output RCA connection rail in half and used two wires (one for left channel ground, second for right channel ground) to that last PSU capacitor ground pin - most of the issues disapeared. I guess the last common PSU capacitor has to be in the amplification section CLOSE to the ground rail split point.

I have also connected tonearm ground wire to GND point near one of the output RCAs. Same with IEC safety earth. From this point I also connected to copper plate chassis via 0.1uf safety capacitor. The hum is very very low now, almost none. I could live with that, but I believe It will go totally silent when moved to final chassis and properly mounted.
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Old 17th September 2018, 10:41 AM   #14
prinzinio is offline prinzinio  Poland
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picture for reference
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Old 17th September 2018, 12:21 PM   #15
Alan4411 is offline Alan4411  Wales
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Glad it is better.
With an improved layout, I'm sure it will be quieter!

You might want to try adding a 10R resistor in parallel with the 0.01uF capacitor. That might make it quieter still.

When you do the final build, please make sure the mains earth goes to the chassis and then 'lift' the 0volt / ground line with the 10R 5 watt, 0.01uF cap and 35 Amp bridge.

This is recommended way.
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