I did put mumetal underneath as well, but not on top. I ran out. But if it's a shielding problem, shouldn't moving the amp around reduce hum?
Also, if it's the first tube that is the problem, why did not the hum go away when I removed the first tube, but did go away when I removed the second and also the third?
Also, if it's the first tube that is the problem, why did not the hum go away when I removed the first tube, but did go away when I removed the second and also the third?
If the hum is gone when you switch it off is coming from the box itself , like I said , AC wires if the transformer is not the cause .
Anyway the compartment with the tubes and input should be shielded from the rest on all sides . Like a box in a box
And do a proper star wiring as we discussed
Just by removing the input tube shield I can hear hum , maybe like in your case from 1,5m It is not the same schematic but the principle is the same. Shielding is a must for RIAA , then we can talk about other causes
Anyway the compartment with the tubes and input should be shielded from the rest on all sides . Like a box in a box
And do a proper star wiring as we discussed
Just by removing the input tube shield I can hear hum , maybe like in your case from 1,5m It is not the same schematic but the principle is the same. Shielding is a must for RIAA , then we can talk about other causes
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Yes, you might be right. I will first try to remove the whole PSU, put it far away, and use long DC wires to see what happens. Then I will redo the star ground.
You did not answer my other question though. If the hum is from the first stage, why do the hum stay when I remove the first tube and goes when I remove the second?
You did not answer my other question though. If the hum is from the first stage, why do the hum stay when I remove the first tube and goes when I remove the second?
Great work helping AweLoi, Depanatoru.
Post #64 - The OP can try temporarily re-locating the mains transformer outside the phono box.
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A question no one has asked, what turntable are you using? The reason I ask is you seem to be having a major issue other folks using this board aren't having. I know with my older technics SL-B2, it SCREAMS hum with the ground wire disconnected and there is a tiny bit of hum with it connected. With my new SL-100C I don't even need to connect the ground wire (might be this new TT has a 3 wire grounded plug?) and it's totally silent.
People get confused by the pad/connections for the optional output pot and install resistors in the wrong place, install resistors that shouldn't be there and some of these come with the wrong resistors installed in them as well. If someone installs 68K across the output vs 220K, that's gonna cause issues.
And yeah I used the jumper wire to move the + and - pads closer together to make connecting a shielded wire easier. Maybe he switched from a 1 conductor with shield to a 2 condustor with shield, with the shield only connected at the board end? That could possibly help with noise.
The other thing I have just discovered, on some of these boards the front ground pad isn't actually connected to anything. There seems to be 2 versions out there now of these black boards, one is glossy and on that one front ground pad is connected to the ground plane of the board same as the rear ground pad. And on another version that is more matt black looking, this front ground point isn't electrically connected to anything.
One last thing I have learned, I switched to using a solid copper 14ga wire for the ground from the board to the chassis/phono ground point, and noticed a slight reduction in noise.
And yeah I used the jumper wire to move the + and - pads closer together to make connecting a shielded wire easier. Maybe he switched from a 1 conductor with shield to a 2 condustor with shield, with the shield only connected at the board end? That could possibly help with noise.
The other thing I have just discovered, on some of these boards the front ground pad isn't actually connected to anything. There seems to be 2 versions out there now of these black boards, one is glossy and on that one front ground pad is connected to the ground plane of the board same as the rear ground pad. And on another version that is more matt black looking, this front ground point isn't electrically connected to anything.
One last thing I have learned, I switched to using a solid copper 14ga wire for the ground from the board to the chassis/phono ground point, and noticed a slight reduction in noise.
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Update on this thread: I've built at least a dozen of these and never had an issue then last week finished one, plugged it in and had a nasty hum easily audible from the listening position. I was stumped after looking it over trying to see anything wrong. I had carefully measured all the parts and built it exactly like the others. After looking at the schematic, the only thing I could see in the power path that might do this is the voltage regulation transistor and sure enough, replacing it fixed the hum.