Kel84 and a source with a small dc offset. Problem?

[Dwjames]

Thanks guys. Some interesting food for thought there. I hadn't considered that the volume pot would be attenuating the DC component too (I'm running a shunted blue Alps pot btw)
I'll measure what the actual levels of DC are at the grid of the first tube with the pot at a couple of typical listening positions.
I don't mind the issue of a thump when changing sources as I very rarely do change and would be happy to lower the volume to nothing before doing so.

I appreciate it's not an ideal solution to run the DAC without output coupling caps and that I can't go plugging this source into other amps while it's in this state. I just wanted to run it like this for a bit to get an appreciation of how much the DDDac's output caps affect the sound. I had changed the supplied 'cheap' Mundorf polyprop 2.2uf coupling caps in the DAC to some old PIO and whilst it sounds good, I'm fairly sure it lost some of the 'sparkle'.
I was going to swap them back in again. I'll have to do some more listening comparisons, but now I know I can include directly wired as one of the options without letting the magic smoke out, I'll see how it compares

BTW, what do you guys mean by pot noise? Like a scratchy sound whilst it's being changed?

[Richard]

Yes, a scratchy sound from the pot and thumps when switching inputs is a sign of DC on the input.

That sounds about right for your cap swaping, pio often have a lift in the presense region but lack highs. The AN coppers Ali mentions are nice but are copper and plastic film these days I think and costa bomba.

I have 10uF Mundorf polyprops on the output of my CDp instead of the 100uF Muse BP and they sound very nice. I've swapped them back and the polyprops are definitely cleaner, smoother and sweeter than the grainy sound of the Muse.

There are 10K shunts after them so the value needs to be quite large. It makes 8K with the pre input so the 10uF Mundorfs manage a 2Hz coupling. I then checked it into the scope using a test CD with ref tones at 0dB and the small roll off at the bottom was just noticeable at 2Hz then flat from 4Hz all the way up to 20KHz. Quite nice to see the theory agreeing with the scope

[John Caswell]

Dumping DC on the input could upset the bias resulting in a different/distorted sound from the amp.

John

[Richard]

How would that happen John, surely it's the same as applying a small amount (less than 10%) fixed bias?

[Dwjames]

Cheers guys,
I measured the resistance of the input with the alps pot as 50k, and used this table to work out that a 0.47uf cap would give me a -3db rolloff at 7hz

[Richard]

Ah, you had me going a bit there trying to work out what the blue and yellow highlighs were

I use a simple calc on the web, http://www.muzique.com/schem/filter.htm

[Dwjames]

yeah, your method is simpler :p
The highlighting was just the DAC designer showing his workings out for choosing 2.2uf coupling caps as a default