Nuforce STA200 issue. Rumble every couple minutes

In my opinion the STA200  is aa perfect match for a passive preamp. It is highly sensitive (350mV), very high gain unit. I own one. 

It did not work with a Joilda tube preamp I once used. A low gain  active SS pre may work.

I’ve had 3 over the years, though match could be difficult, never any problems...always worked well with passive pre, and for me a great match with Schiit Freya S preamp...and low efficiency speakers

Thanks @mesch 

I was just using it as a 3rd amp for when I don't want to burn my tubes 8hrs a day... so I don't really want to build another system around just this amp... it's very good. but with my current configuration is not a very good fit for it, 96db speakers... there's more hiss than I get from my tubes amps.... 

Great amp sound, but not a particular good fit for more all around setup. 

it can be a difficult amp...but great sound and incredible value in right system...

Yes, I agree it has a great sound. It is that the sensitivity and gain make it not a universal pairing. Likely why it became so highly discounted. Maybe a good fit for a vinyl system with lower output cartridge and phonostage.

I am using mine with a 

’volume control on a box’ preamp driven by a 2V output DAC.