2. amps-preamps
  3. 160894-109163

Passive preamp issues or?

I have a Nuforce STA 200 that I’ve been using with an Axiom Walker mod preamp for a while now.  Other equipment is a Bluesound Node 2 streamer and a Jolida dsd dac.  Nordost digital coax and Dueland Schroeder IC’s from dac to pre and pre to amp.   Last night, I disconnected the preamp and ran the IC’s directly from the dac to amp, using the volume control on the dac.  The change in detail and depth was not subtle.  Soundstage extended beyond the speakers- everything just sounded better.  However, with the high gain of the Nuforce there’s  a hum in the speakers- it’s not not audible when I’m not playing anything. I’m willing to live with it, but I’m curious why the sound improved significantly by pulling the pre- should I look at better IC’s from pre to amp or is the Axiom subtracting from the sound quality?
renisnceman
georgehifi's avatar
georgehifi
10,312 posts
 
If it is 47kohm output impedance, that’s insanely high, never seen anything like it.

If you get no answer, also try another source with output rca’s like the Blue Note 2 streamer as it has volume control also, see if it still hums then.

Cheers George
atmasphere's avatar
atmasphere
11,611 posts
 
so when I turn the volume up, the speakers hum, mildly— then as I turn the volume up all the way, well past safe levels, the hum disappears and  I just get white noise hiss- again, not that loud.  If it was a grounding issue would the hum be there all the time?
@renisnceman


First off, the output impedance can't be 47K. It could be 4.7K (which is still high IMO). 47K might be the minimum it can drive with full bandwidth...

Anyway, the description in the quote sounds like a volume control problem. Is the control analog or digital. This does not sound like a grounding problem; if it were you'd have a different description.
renisnceman OP
50 posts
 
From Michael Allen at Jolida/Black Ice- “the spec is 47 K ohm.”  The volume control is analog. 
georgehifi's avatar
georgehifi
10,312 posts
 

I’d say without ever doing the experiment to see on the test bench or hear also, this 47kohm output impedance "could" be the problem. As it will also be "capacitor coupled" as well being tube which "could" make it even more of a problem if the caps not large enough, or an electrolytic, who knows, as it’s not following the output input >1:10 "rules" as we know them.

Cheers George
renisnceman OP
50 posts
 
I put the pre-amp back in the loop.  Dead quiet- decided I couldn’t live with the hum. FWIW- and I don’t clearly understand the logic behind this, if I keep the amp connected to the speakers and disconnect the dac from the amp, the hum is worse. Disconnected every that might contribute to the hum- no change.    I may just look at better IC’s from pre to amp to see if I can wring some improvement.  It’s really a pretty good system as it is- I may just need to tweak with the pre in the loop.
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