Strange Hum Problem in Phono Stage

[juancho]

Hi,
I've built three identical phono stages based on the Phono Dude phono stage which uses 5755 input tubes. The strange thing is one hums when I plug in the turntable, the other two are silent. The builds are exactly the same. I'm using the same record deck, power amps, speakers, and mains leads. Absolutely everything is equal.

Without the phono leads from the deck plugged in all three have identical low noise and no hum.

The only objective difference I can pinpoint is the grid reference resistor on the input tube is around 1cm longer than the other two. The 5755 runs with a directly earthed cathode.

Any tips anyone can give me are very welcome. Thanks!

 

[audiowize]

Is it both channels humming? (Is it buzz or hum?)

 

[6L6]

Check for poor chassis connections to ground and the pieces of the chassis itself.

 

[juancho]

It's hum and it's both channels. All connections to earth, chassis etc have been re-done. It's using an earth lift implementation

 

[audiowize]

Well, perhaps you could start by showing us those resistor differences between the humming unit and the one that doesn't hum.

Did you point-to-point wire these or use the PC board design?

We will also have to assume that you swapped tubes etc. between noisy and quiet units just to rule that out.

 

[6A3sUMMER]

Sounds like a ground loop to me.

Perhaps the floating RCA phono inputs are not floating from the chassis?
Etc.

How about the power supplies, identical transformers?
Maybe the primary on one is reversed (Line voltage Hot winding is next to the laminations; and capacitively coupling current from there to the chassis).

Etc.

"Grounds Are Commonly Misunderstood" . . . Me

PS I never liked 'ground lift'.

I get good results with 2 wire power cord on turntables (but with the turntable dedicated ground wire connected to the preamp dedicated ground screw); preamp with 2 wire power cord, and 3 wire power cord on power amp. Works for me in the USA.

 

[juancho]

Hi, the transformers are identical toroids. Tubes have of course been swapped. I will post some photos later

 

[juancho]

Ok. Attached are 4 photos of the pre /phono and here are a few more details:

1. Heaters are DC regulated with 40v lift
2. The main power supply is earthed via a 2.5mm copper bus
3. A 1.5mm copper bus bar goes to the line stage at one end, and the phono at the other end.
4.All input /output sockets are earthed at point of entry and then to the star earth where the two buses meet and where the power supply is joined to the earth lift connection to earth
5. The difference in the phono cathode connection is maybe 10mm between the cathode and grid reference resistor on the humming example, 2 mm on the quiet ones.
6. All the sockets are correctly floating from the chassis
7. The transformer wires are colour coded so connected correctly

Thanks for any help!

 

[juancho]

Thankfully it turned out to be an intermittent turntable fault. It was pure coincidence it kicked in three times when I was testing that preamp! Pheww!