Alps pot issues.

[al newall]

Just replaced my stepped attenuator with used Alps pot.
While i was at it, i moved it from the front of the EL84 amp and put it between DAC and output tubes. I think Mike suggested this a while back.

I now have a bit of hum, and some thumping noises when the pot is right at beginning of its travel.
Turn up the volume and it's fine.

Could it be DC from the DAC that's causing it?

Strangely i have more useable volume now and it may even sound more natural.

[Nick]

if it is dc, try a cap in the way and see if its fixes it

[andrew Ivimey]

thumping noises, I wonder, but is the pot close to earth then at the bottom of its turn? 'teresting.

[al newall]

if it is dc, try a cap in the way and see if its fixes it

Yep, i'll try that but don't want a cap there permanently, as i've compensated for the DC in the tube stage by changing the cathode resistors.

[andrew Ivimey]

I put a cap there (on a different amp), with great worry and was very surprised that it didn't make any difference to the fidelity of things. But I didn't have thumping noises ... I was just surprised. I also learned (by doing it) that a grid leak resistor is a good thing when it comes as physically close as possible to the grid of teh valve no matter what you are tryinhg to feed / attenuate into it.

[al newall]

Yes, i put a 220k grid leak in when the pot came out.
Maybe that accounts for the increase in output.

One of the reasons for swapping out the attenuator was that it was making similar thumping noises at the start of its travel. But there were output caps before it.

[al newall]

Fitting some caps inline made no difference.

Conclusion.
DC not the problem.

Replacing pot with another used Alps pot cured the noisy thump issue, but not the additional hum.
But this pot also has problems, in that it makes scratchy noises throughout its travel.
Don't remember any previous issues with either of these pots.
Anyway i have now run out of Alps pots.

Only have a 100k Omeg pot left, so will try that later.