Caps after attenuator wha-how?

[livingnote]

So here came this funny problem - there's a trafo out summing box Neumann style
with the iron on output, fair enough, enter the Alps pot x4 post-iron for passive attenuation.

After it going scratchy (which I assumed would be DC) the obvious solution would be some
good caps after the attenuator, now the question is:

Does it make sense to just put some caps after the pot, as the iron is floating anyway, and
be done with it, or should there for some reason be caps before *and* after the pot?

Also, does it need a r to GND at the output after the output cap?

 

[Samuel Groner]

Show us a schematic--a transformer has by definition no DC at its output...

Samuel

 

[tv]

How about not using a pot and make a switched attenuator instead?

 

[abbey road d enfer]

So here came this funny problem - there's a trafo out summing box Neumann style
with the iron on output, fair enough, enter the Alps pot x4 post-iron for passive attenuation.

After it going scratchy (which I assumed would be DC)

As Samuel wrote, there is no DC after a xfmr.

Does it make sense to just put some caps after the pot,

It would work if there was DC on the input of the subsequent unit, which I doubt.

If the pot is noisy only when there is signal, the cause is not DC.

 

[livingnote]

Exactly, that was the thing that got me too - for DC it would have
to come from the input of the next device...basically what we have
here is a Neumann summer as would be standard stick-ina-box,
and has this pot right after the iron (which is driven by 5532s).

Now - it was scratching but there was no difference-telling about
whether it did so only under signal or not. It went into the inut
of an RME iirc. Thing is, he replaced it, and 3 months later, same
story. I mean, theoretically an Alps pot that's built specifically for
attenuation shouldn't start crapping out after 3 months, or?

As far as he told me it did that twice in a row...It's gotta be the
Alps because that's part of his thing, but now I'm kinda weirded
about those...

 

[PRR]

"Out to next something" is the point of interest. 

 

[livingnote]

Generally, that goes straight to an ADC, but of course whoever's
using it could stick it into just about anything "hey cool let's put in a...".

So my take was to just make sure and put some caps on the output
to have whatever eventuality dealt with - if it still scratches then, I guess
it would have to be the Alps itself. Weird was just that two Alpses did that
and both with the same guy.

 

[PRR]

The output generally does not have to be grounded. That would break the DC path.

Have you tried measuring the DC?

 

[JohnRoberts]

The scratchy pot caused by DC is the logical leap because it is so common, but scratchy noise could also be caused by wiper bounce or surface contamination. If the new pot is quiet and degrades over time, perhaps there is an environmental issue with smoke or dirt.

JR

 

[tv]

If all else fails, why don't you reconsider using a switched attenuator instead of a pot?

It will "tick" or "click" when adjusting, but this is imho less irritating than listening to "scratchy" pot.

All in all, perhaps this is lesser evil than having a capacitor in a transformar-coupled path (LRC).

 

[Andreas Pfeiffer]

"Out to next something" is the point of interest. 

Ok, here is a block diagram of the amp

http://dl.dropbox.com/u/163999/D.I.Y./Pictures/475.jpg

Inserts can be used for connecting a Preamp or Compressor.
Usually Out 1 + Out 2 are going to an Poweramp or active speakers.
Rec. is used for recording the Signal back into the DAW.

 

[Andreas Pfeiffer]

The scratchy pot caused by DC is the logical leap because it is so common, but scratchy noise could also be caused by wiper bounce or surface contamination. If the new pot is quiet and degrades over time, perhaps there is an environmental issue with smoke or dirt.

JR

good hint, thanks 

 

[Andreas Pfeiffer]

The output generally does not have to be grounded. That would break the DC path.

Have you tried measuring the DC?

No, the amp is not here at the moment.

 

[livingnote]

Ah here guys, Andreas P = Sangeet, awesome DJ and maker-of-stuff-that-sums I'm helping him get his
box together but this thing had me so perplexed I thought "off to the forum it goes..."

Really I do remember Ronald over at Conrad complaining about the Alpses, there was a reason
I didn't end up buying one. But if they get scratchy after 3 Months I mean...surely not, or?