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
 
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
atmasphere's avatar
atmasphere
11,844 posts
 
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.
mrdecibel
3,064 posts
 
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.
georgehifi's avatar
georgehifi
10,312 posts
 
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
atmasphere's avatar
atmasphere
11,844 posts
 
@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.  
More to discover