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Old 24th August 2008, 05:01 AM   #1
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Default Pot Noise Help

This isn't strictly a tube issue, but oh well. I have a tube linestage where there is increasing hum as the pot is increased until it is all the way up, at which point it is totally silent. That is, quiet at the volume extremes, and increasing noise as it is increased. Does sound indicative of something in particular?

For more info, this is actually a DAC that uses a TDA1541 and a resistor I/V which feeds a 6dj8's grid directly. I have tried to add a pot between the DAC chip and the gain stage, and am having this noise. The tube stage is a CCS loaded grounded grid.
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Old 24th August 2008, 08:42 AM   #2
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Have you tried a 10K grid stopper directly on the tube socket ?
The 6DJ8 (a vhf tube) is notorious for oscillating at will and won't tolerate lanky wiring.
The fact that the pot control effects the hum, is probably a symptom of the tube oscillating in the RF trying to modulate any noise pickup and the pot extremes dampen oscillation conditions due to reduced Z. This used to be a common 1960's problem with a duff wiper.

A scope won't pickup this problem, the probe capacitance can be a hoax just enough to dampen oscillations, but the best way is to measure cathode current, which will rise when tube is oscillating or behaving peculiar. A screening can may help.

Becareful when using a tube this condition and feeding a power amp... some very stange noises and perhaps radio signals can be heard..

richj
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Old 24th August 2008, 09:09 AM   #3
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Quote:
Originally posted by richwalters
Have you tried a 10K grid stopper directly on the tube socket ?
The 6DJ8 (a vhf tube) is notorious for oscillating at will and won't tolerate lanky wiring.
Interesting thought. I have 1K stoppers on the pins, but I'll try increasing them. Without the pot there at all everything is fine.

I can also drop in a 12au7/at7, etc to see if that changes things.

Quote:
but the best way is to measure cathode current, which will rise when tube is oscillating or behaving peculiar.
The CCS plate load should stop this from happening..
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Old 24th August 2008, 09:17 AM   #4
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12AU7 does it too, but a bit less (this tube can sub as the original is 12V version of the 6dj8).
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Old 25th August 2008, 09:22 AM   #5
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From what you say, the pot may be picking up EMI from somewhere. It would be worst at the halfway point (in terms of resistance - this is probably an audio taper) of the wiper, because that's the point at which impedance from either end of the track is maximum. Screen the pot body - connect it to ground if you can. Shield the leads to the pot too and ensure that no AC-bearing wires are nearby. If their proximity to the grid can't be avoided, such as heater wires, twist then tightly.
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Old 25th August 2008, 05:38 PM   #6
Jeb-D. is offline Jeb-D.  United States
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Quote:
Does sound indicative of something in particular?
Sounds like a grounding issue.

As mentioned the pot body should be grounded.

Also, do you have more than 1 piece of equipment in your system that is earth grounded? For example, if your line stage and the power amp your connecting it to are both earthed, it can cause problems.

I've had that problem using my PC as a source (which is earthed) to my amplifier (which is also earthed).
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Old 25th August 2008, 05:44 PM   #7
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Quote:
Originally posted by ray_moth
From what you say, the pot may be picking up EMI from somewhere. It would be worst at the halfway point (in terms of resistance - this is probably an audio taper) of the wiper, because that's the point at which impedance from either end of the track is maximum. Screen the pot body - connect it to ground if you can. Shield the leads to the pot too and ensure that no AC-bearing wires are nearby. If their proximity to the grid can't be avoided, such as heater wires, twist then tightly.
Tha sounds like it could be right. I increased the stoppers to 8K5, and it may have helped a little, but not much. I then found some coax wire and used it to make shielded connections from the dac board to the pot. One side was considerably quieter, but the other was not really effected -- not sure why that would be. And, while it was quieter, it still wasn't really quiet enough.

The pot body is grounded and there is no AC in the area though there is a regulated DC heater supply that goes right by.

I am thinking that since I have a 12V regulated supply that can supply a few hundred mA, but which is only supplying about 40 right now, that I may just put the pot after the gain stage and build a little mosfet follower. Not my ideal solution as I hate adding stages, but perhaps a necessary one.

This DAC is an old breadboard version that I was going to send to a friend without a preamp to play with -- hence the need for a volume control. Here is a pic, and the pot is where the spdif input is on this version, with the spdif being moved to the middle now -- I should add that grounding has all been redone since these pics:

Click the image to open in full size.

Click the image to open in full size.
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Old 25th August 2008, 10:06 PM   #8
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Quote:
Originally posted by ray_moth
From what you say, the pot may be picking up EMI from somewhere. It would be worst at the halfway point (in terms of resistance - this is probably an audio taper) of the wiper, because that's the point at which impedance from either end of the track is maximum.
One thought, I used a 100K pot as that is what was immediatly available. Is it worth it to try to come up with a 5K, or even 1K pot? Might this solve the issue?
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Old 30th August 2008, 06:49 AM   #9
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Quick update, the 1K pot did most of the trick. There is still a little hiss but it isn't that bad. Quiet enough to listen to at any rate.
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