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Old 5th September 2012, 10:57 PM   #21
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I would like to add what I would consider to be the 2nd and 3rd likely causes for hum or buzz.
Not grounding or referencing the heaters and not using shielded cable for the signal ins and outs.
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Old 6th September 2012, 12:22 AM   #22
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Well perhaps the fact that there are what appear to be a pair of Lundahl moving coil step up transformers directly under the white filament wiring might play a role in some of the noise pick up issues, the fact that the box is completely unshielded might also unfortunately play a role.

DC filaments will help here, rerouting the filament wiring would be even better, and applying copper foil inside the box and grounding it to the mecca ground will take care of electro-static pick up..

There is a reason why all of my phono stages are built on metal chassis and fully enclosed.

I don't use shielded wire in my phono stages because of the concern over capacitance at the input, and I have not had problems with noise pick up. The phono stage in question is extremely quiet.
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Old 6th September 2012, 12:53 AM   #23
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Thanks for the replies.
The step up transformers aren't wired up yet (an MC cart is on the todo list).
Also, the heaters aren't floating, they are referenced in the PSU chassis (raised to about 60V above ground).
I've thought about re-routing the heaters, but that would add significant length, and in the case of the output caps (at the bottom), end up running parallel with them. I did take care to ensure the heaters crossed signal wires at 90 degress.

Also, in response to the ground loop, I am very famialir with that site (my headphone amp is a DoZ !), and the 0V line is connected to safety earth as it is in the article (without the diodes, just resistor and cap). I don't beleive it's a ground loop, as the buzz is quite harsh, and definitely not 50Hz.
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Old 6th September 2012, 05:54 AM   #24
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You could power the heaters from your benchtop supply (DC) or a battery to test.
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Old 6th September 2012, 06:06 AM   #25
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The black metal plate has a green wire to it. Does that mean it is connected to safety earth? Try to connect it to circuit gnd instead. That is one first step to a shielded box.
Maybe you can add some foil around the edges and on the bottom cover. With DC heating on top of that you have no source of hum in the box. Just the tubes sticking out.....
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Old 6th September 2012, 06:19 AM   #26
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Kevin, could the current source do something we do not want it to? OTOH it is a high impedance thing, otoh it is connected to the triode.
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Old 6th September 2012, 06:56 AM   #27
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The black metal plate is connected to safety earth, which is then connected to circuit gnd (0V, the black wire) via a resistor and capacitor (not in the photo, I actually did it immediately after I took the shot !).

I don't have a bench supply, but I do have a 6V wallwart. It's only rated for 1.25A, but can try it for one section (or one channel)

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Originally Posted by grommeteer View Post
The black metal plate has a green wire to it. Does that mean it is connected to safety earth? Try to connect it to circuit gnd instead. That is one first step to a shielded box.
Maybe you can add some foil around the edges and on the bottom cover. With DC heating on top of that you have no source of hum in the box. Just the tubes sticking out.....
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Old 6th September 2012, 01:10 PM   #28
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So I had a "quick" play this evening, and tried a few things.
First, was to remove the output tubes, and take the output of the input tubes direct to the output. This was whisper quiet, with only a tiny amount of hum (not buzz) on the output when I set my headphone amp to max.
Second, I removed the input tubes, and ran the grid of the input tubes to the grid of the output tubes. This was very noisy. I realise now (now that everything has been put away), that I should have short the grid of the output tube (as the way I had everything setup, any noise on the ccs of the first stage, would have been amplified in the second).

When I get my sanity back, I'll redo the test with the grid grounded, and determine whether the noise in in the second stage, of being amplified from the first.

I did also try using DC on the output tubes (input still had AC), but that was just as noisy (thinking about it now, noise of the first stage would still have been amplified into the second).

The buzz is definitely 100Hz though, so I guess that rules out heaters for the time being, and points to the PSU.
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Old 6th September 2012, 01:49 PM   #29
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While your at it, see attachment--a ground loop breaker.
It may be useful to measure the voltage across the diodes prior to installing the resistor. If you've got any voltage readings (like 0.7v), there's a ground loop present.
See also schematic, but notice that the real bridge rectifier has a different pinout than the schematic shows. The real thing is shown in the attachment below:
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File Type: jpg Groundloopbreaker UK.jpg (52.9 KB, 66 views)
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Last edited by danielwritesbac; 6th September 2012 at 01:52 PM.
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Old 6th September 2012, 02:14 PM   #30
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Does the diode actually make any difference in the loop breaker ?

I've gotten rid of ground loops before simply by adding the resistor and cap (which is what I have at the moment).

I'll try this stuff over the weekend anyway. I think I need some sleep.
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