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Old 10th August 2006, 07:13 AM   #1
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Default Help eliminating hum

I am looking for suggestions on how to get rid of some awful hum in the preamp I am trying to build.

Here is a picture: http://ecp.cc/images/hum_maker.jpg (about 250K)

First, some background. The case used to house an old solid state preamp of mine (which was already about 4th generation and which has had power amps in it as well as well as DACs etc.) that I got tired of. So, I took the guts out and went about trying to put some tubes in it. This is why the parts are all mounted in odd ways -- they all needed to be sideways to fit. The top is glass, so I can't mount anything on it unless I get a new top

The circuit is a single 6J5 per side with a transformer on the plate. There are 2 led's biasing the cathode, and B+ is about 260V or so. The PS is a hybrid rectifier with an ez81 and a pair of 4007 diodes. It is an RCLCRC filter -- the last C is the big silver ASC, the others are board mounted. The 6J5 heaters are DC which is what the second transforer and the little PS are for.

I breadboarded the circuit first and that was free fro hum, though it was also spread out across a desk so components were pretty far apart .

Now, I first tried all the usual stuff (the metal part of the chassis is connected to the wall ground, but this is isolated from the circuits ground) and I made the heaters DC.

The hum actually got worse converting the heaters to DC. But, it also caused me to figure out what the main problem is. The heaters are on when the amp is plugged in and the power switch only controls B+. It turns out that the amp hums even when off. That is, the heater's transformer seems to be interfering with the wires enough to induce a lot of hum. Turning it on adds a second hum from the main transformer.

The right output wires go all the way across the back of the amp to behind the transformer and the worst hum is certainly on the right side, but there is also significant hum on the left, so simply moving the right output over to the left side of the case is likly not enough.

So, does anyone have an suggestions as to what to do? I tried to keep power and signal seperated as much as I could, and there is not a lot more room. Would RF blocking sheets over the transformers do anything or is there a similar solution? Do I just need a larger case? Something with the transformers on top?

Thanks for ideas,

-d
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Old 10th August 2006, 08:41 AM   #2
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It appears that many components are mounted on wood. This provides no shielding. It seems to be a basically unsuitable enclosure for a tube preamp but, if you really want to use it, then you might get better results if you stick some aluminium foil pieces on all the wood surfaces. Make sure all the foil pieces are grounded and then mount the components.
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