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
Very familiar with the Nuforce and the Axiom Walker unit. As good as the passive unit is ( and I find it to be excellent ), the less in the signal path ( 1 less volume control, 1 less set of ics, etc. ), generally the better. The STA 200 is very easy to drive, and this is what you are hearing. As far as the hum is concerned, the Axiom unit shunts the amp's inputs to ground, therefore ime, less / no hum. 
renisnceman
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


The Nuforce STA200 has an input sensitivity of needing just 0.6v input for full power output of 80watt. Your Jolida DSD Dac can have up to 2.3v output at full volume, so as you can see there is absolutely no need for any active preamps with even more gain, as you already have 4 x more from the Jolida than what the amp can take

But!!! I can’t find anywhere what the output impedance is of the Jolida DSD Dac, being tube it "could" be quite high or low depending on what configuration the tube is in.
Jolida state this "0 V to 2.3V (±1dB, 47K ohms)" But I can’t!! believe anyone would have the output impedance of their equipment at 47kohms!!. But if this is correct it "could" be the source of your hum problem.
Because the unwritten output impedance to input impedance "ratio law" is a min of 1:10 or higher. "IF" the 47kohm output impedance of the Jolida is correct?, you have basically have a ratio of 1:1 into the Nuforce "that’s not good" won’t hurt anything, but "could" be the source of your hum. Or there may even be an earthing problem between the two. 
Try to find output what Jolida say it’s output impedance definitely is.

Cheers George
The hum is non-existent with the volume down-  I’ve got nos Telefunkens in the dac—wondering if switching to lower gain 5751’s would make a difference?
The gain will be the same, the tube doesn't decide it, the components around it do. Do you know at what stage the volume control is done on the dac?  

Cheers George
Sorry I should of elaborated a little better when I said the tube doesn’t decide the gain, in that it (the tube) doesn’t decide the gain in a tube output "buffer circuit" as you most probably have in this dac.

Cheers George
@georgehifi- 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?
I’m not saying a grounding issue, that’s why I asked for you to find out what the output impedance is of the dac (at all volume levels) as this sound like it’s the dac impedance match maybe problem, or it's hum in the dac just being presented.

Cheers George
Output impedance from the specs page is 47K ohms
Reread your post, I’ll try to confirm with Jolida.
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
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.
From Michael Allen at Jolida/Black Ice- “the spec is 47 K ohm.”  The volume control is analog. 

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
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.

As I asked you to put in your Blue Note 2 in, did you? (which also has volume) Direct into the poweramp, and then you can see if the problems disappears.
If it does then the Jolida tube output is the problem, and even if it doesn’t make noise will still be an impedance mismatch at 47kohm with the input impedance of the Axiom pre also.

Cheers George
No hum from the Bluesound....
There it is! The problem is either the high output impedance of the Jolida at 47kohm (which is wrong for anything mind you) save the input impedance of Rogue M120 monoblocks, which are for memory 1megohm, or similar, they would give a good ratio of around 1:20.
https://www.stereophile.com/content/rogue-audio-magnum-m-120-monoblock-power-amplifier-measurements

Or maybe it’s the Jolida output coupling cap with that impedance, or could be earthing.

But I don’t think you’ll ever find any manufacturer with a source product that has around 47kohm output impedance.

Cheers George
If the amp really has a 1MegOhm input impedance, that is likely the problem- it will make it very sensitive to cables (and much more able to pick up hum from high impedance sources!). The input impedance isn't specified anywhere I've looked so this is speculation.
The high gain and low input sensitivity of the Nuforce STA200 will allow it to amplify any noise in the system preceding it ( including ac noise ). I cannot use the amp ( amps, as I own 2 ) satisfactorily with a few preamps I have lying around, as I hear a distinct hum through the speakers. The amp was designed to work passively, and this is best for it, ime.
If the amp really has a 1MegOhm input impedance
Oh dear! This is getting like a Monty Python skit.
He doesn’t have an amp with 1megohm input impedance, I’m just saying that the Rogue M120 the only amp I know of that would be an "impedance match" with a tube output source like his with 47kohm impedance.
And btw I’ve had the Rogue M120 here some some time, they don’t hum, with active or passive driving them

Cheers George
@renisnceman   You might try lifting the ground of the DAC to see if you are dealing with a ground loop. You do this by getting a ground cheater adapter for your its cord at the local hardware store and plugging the DAC into the wall using the adapter. This is only for testing, but if the hum/buzz is eliminated then you will know what is going on.  
I purchased an ebtech hum-x and it made no difference. The Bluesound is quiet but the dac isn’t up to par with the Jolida.  I have a feeling I’m going to have to stay with the passive pre- it’s just a little frustrating knowing improved sound quality comes at the cost of speaker hum.  Might just mean I’ll have to tweak tubes or cables instead, as I’m not sure where to go from here.
A nice inexpensive Passive preAmp for the sta-200 is the Schiit Saga.