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phono preamp with B+ split - hum problem

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Hi,

I am trying to build Nardi Phi Phono preamp like on the attached schematic. Currently is a prototype on a wood board, where PSU will be separated from amplification stage.

PSU stands free on the wood board, while amplification stage is mounted on a copper plate with all sockets insulated from it. I wanted to separate the B+ for L and R channel, therefore after some CRCRC filters in the PSU I have created a "U" shape b+ and GND rail.

INITIALLY (you dont see it on the pictures as they represent latest work) the ground rail and b+ rail were splitted near the last tube (output). With turntable RCA cables plugged in my woofers were pulsating very loud with out any incoming signal. So I cut the ground rail connecting output sockets and connected the ground at INPUT sockets by joining them and soldering the PSU ground wire. This way the woofer pulsations dissappeard but I still have loud hum.

There is one way I can totally remove the hum/buzz from the speakers. To do that I need to touch with my finger to the cutted ground wire rail on the output rca sockets and with the same time I have to touch the copper plate. This way phono is totally silent. However if instead of my hand I will use a copper wire to join cutted ground rail together and with the copper plate - the hum and buzz gets VERY loud. So I guess my body works like some kind of ground.

Can you please support and make some proposals - what should I try to do to keep separated B+ rails but get rid of the hum/buzz? It's getting too hard for me as the B+ and GND are splitted in the "U" shape.
 

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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.
 
I did not helped. I have taken to GND wires, soldered them at GND point of B+ capacitor and connected the other ends to each GND rails. I've tried few places on ground rails, but it does not lower the buzz/hum. I think the key is to analyse if my "U"shape gnd and psu rails are OK according to good practices. And why when I put my finger to cutted GND rail on the output - the buzz disappear.

I cannot find on the web any kind of schematic where B+ is splitted at some point and both channels are shown. On the schematics I find only one channel is shown and this does not present the clear picture for me. I could replicate any proven schematic if I would find some..
 
Did you do this as well?

Yes, but after connecting IEC safety earch with copper mounting place buzz/hum gets much louder.

@kevinkr

It's not currently, but it was during some tests of mine and it does not help.

Usually when I have some hum/buzz problems I use piece of wire and stick it to different places to check if the connection will lower the hum.
Different ground points, safety earth, chassis etc..
 
Guys,

what do you think about the idea of splitted B+ and GND rails like I've done? Is it worth do dig deeper? I've made a prototype with single B+/GND rail through the entire phono and it was very quiet. I prefer not to go the easy way, but I am starting to get our of ideas..
 
Yes, but after connecting IEC safety earch with copper mounting place buzz/hum gets much louder.

Usually when I have some hum/buzz problems I use piece of wire and stick it to different places to check if the connection will lower the hum.
Different ground points, safety earth, chassis etc..

OK, have you connected the mains earth to the -ve (0 volt) supply somewhere out of the picture? If so have you noticed on the schematic that the mains earth only goes to the chassis (copper plate) and that the -ve ground line is connected (lifted) to the mains earth by a 0.01Uf cap C44?

Yes, touch and dab works for me most time too, but hard to know where to start with a layout like the one you have at the moment, sorry.
 
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.
 
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.
Alan
 

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