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Old 9th October 2013, 04:26 PM   #1
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Default SSE - A question about hum.

My SSE is currently the basic triode config with a choke, aux cap, tube and
solid state rectifier with a switch. No volume pot but I plan to install one
later. No CFB at this time. The board is grounded via one RCA input as per
Tubelab's wiring diagram.

With a cd player connected the amp is playing fine, clear and natural with
complete silence between tracks. However, with nothing connected to the
RCA inputs there is an obvious AC hum, the hum goes away as soon as the
player is connected but NOT powered on.
Additionally when I plug in my vacuum tube preamp without turning it on
the hum is there but when I turn on the preamp the hum fades away to
silence. Turning up the volume control and with ears close to the speakers
there is just a faint electronic noise.

So, with the above in mind is the hum due to grounding the board via the
RCA input or is there another reason ?
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Old 9th October 2013, 11:45 PM   #2
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Sounds like you hit it on the head. It must have something to do with the rca. I really don't have a technical explanation, so hopefully someone else can provide insight. Maybe the CD player or preamp complete an otherwise open ground?

I grounded mine with one of the PT bolts. To it I connected the speaker post grounds and the board via one of the two connectors that the HV CT connects to. The bolt is then connected to the IEC ground. I found this gave me the shortest ground wires.

I have no hum unless the volume is pegged and I've got my ears on the speakers. Even then, I don't think what I hear is actually AC related.
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Old 10th October 2013, 12:03 AM   #3
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Also, are your Rca's isolated from the chassis and are their grounds going to the board?
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Old 10th October 2013, 12:06 AM   #4
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If you gave a link to the wiring diagram, and maybe even a photo of your build, it might be helpful.
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Old 10th October 2013, 11:09 AM   #5
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Quote:
Originally Posted by Sodacose View Post
Also, are your Rca's isolated from the chassis and are their grounds going to the board?
Yes my RCA's and speaker jacks are isolated from the chassis, I confirmed
this with a multimeter.
Below is the wiring diagram I used, my star ground is a transformer bolt
which is wired to the IEC ground pin. The chassis and everything bolted
to it is grounded via the star ground. Right now I am not using a volume
pot, my input goes directly to the board.

Click the image to open in full size.
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Old 10th October 2013, 11:28 AM   #6
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Quote:
Originally Posted by Sodacose View Post
Sounds like you hit it on the head. It must have something to do with the rca. I really don't have a technical explanation, so hopefully someone else can provide insight. Maybe the CD player or preamp complete an otherwise open ground?

I grounded mine with one of the PT bolts. To it I connected the speaker post grounds and the board via one of the two connectors that the HV CT connects to. The bolt is then connected to the IEC ground. I found this gave me the shortest ground wires.

I have no hum unless the volume is pegged and I've got my ears on the speakers. Even then, I don't think what I hear is actually AC related.

I should try grounding the board like you have done and remove the ground
wire connecting the RCA input to the star ground.

So, you have the HV CT and one of the wires from the auxiliary cap
connected to these two terminals (T1-RED-YEL) then another wire in
one of these same terminals grounds the board via the star ground ?

Last edited by harry888; 10th October 2013 at 11:38 AM.
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Old 10th October 2013, 12:34 PM   #7
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That is correct. It is either of the two green colored connectors on the left side of the board in the diagram in your last post. If your inputs are isolated, this would be pretty simple to try. It sounds like you've done everything the same way I have otherwise.
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Old 10th October 2013, 01:06 PM   #8
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Quote:
Originally Posted by Sodacose View Post
That is correct. It is either of the two green colored connectors on the left side of the board in the diagram in your last post. If your inputs are isolated, this would be pretty simple to try. It sounds like you've done everything the same way I have otherwise.

Thanks, I'll do that later this evening after work.
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Old 11th October 2013, 10:05 AM   #9
gootee is offline gootee  United States
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Are the two input pairs tightly twisted? And do the pairs stay physically close together? They should probably be in a single shielded dual twisted pair cable.

What happens if you short the rca jack grounds together, with no source connected (while you hear hum)? And what if you short only the center conductors, at the jacks? And what happens if you short each rca ground to chassis, and if you short both of them to the chassis?

What else is on the board? Are there power grounds that are sharing the input ground conductor, to go to the star ground??
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Old 11th October 2013, 11:52 AM   #10
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Quote:
Originally Posted by Sodacose View Post
That is correct. It is either of the two green colored connectors on the left side of the board in the diagram in your last post. If your inputs are isolated, this would be pretty simple to try. It sounds like you've done everything the same way I have otherwise.

I wish I could report success but none. I was so confident this would fix the
problem.
I removed the ground wire connecting the RCA to the star ground. The board
is now grounded at one of the terminals where the HV CT is connected. I
confirmed again that the RCA jacks are isolated from the chassis. Powered
on and after about a twenty seconds warm up the hum is still there. When I
plug in my cd player the hum is gone and music is playing very well.
Probably has something to do with the input wiring.
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