Great post and welcome to the forum Johan!
Thank you steven2, I´ve been a long time lurker on this exellent forum, but now that I had something to say, wellâŚ
Other lurkers should learn from your example and join in. We generally donât bite on this forum. 
I just happen to have same problem and it look to me is a PS Audio GROUND design problem related to single ended outputs. This is ridiculous because most of preamps are not balanced.
Anybody at PS Audio care to comment ?
I had used a direct XLR connection from DS Jr to my amp without hum.Recently I decided to ad a preamp and get hum affected by preamp volume pot when I connect DS Jr. Now all the connection are RCA (single ended). Preamp does not have XLR output/input.
Just amp,preamp and speakers,NO hum.Cheater plug donât help. Any idea why?
What exactly do you mean by âaffected by preamp volume controlâ? Assume the preamp uses a grounded plug by your comment about the cheater plug. Is all the gear plugged into the same power receptacle or at least served by the same circuit? What condition is your homeâs wiring in? Is the preamp new? Can it be returned? What do you mean by âJust amp, preamp, and speakers, No hum.â? (Why should there be hum if no source is plugged in?)
Iâm a big fan of balanced design and XLR cables (better/cheaper than boutique RCA) and wonât go back on either.
Raising the preamp volume after 10 a clock increase the hum.Lower the pot,decrease the hum level.Every component is connected to same power receptacle.Home wiring is correct. I had put the preamp into a different system and is playing fine with no hum.
With what exact cables,may I ask?
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The problematic cables were Kimber. The cables that solved the problem (for the most part - there was still a low-level hum that was audible when close to the speakers) were AudioQuest. Unfortunately, I donât recall which exact model from each cable manufacturer was involved. Shortly after posting I returned the DS Jr â because ultimately it did not match up well with the Cary 300B mono blocks that I intended it for. I do remember it sounded very good with the Bryston power amps (using balanced cables). I suspect they might mostly use balanced cables for testing/auditioning at the factory in Boulder (just a guess obviously).
Just to add another wiring scheme to considerâŚ
I have a 2.3 stereo setup (3 subs) and I use both single ended and balanced outputs from my DSJr. Two of the subs are professional, so I output the DSJr via XLR to the subs and output from the subs to my main speakers. I use a âYâ splitter to combine the single ended outputs from the DSJr to feed the third consumer sub. All sub crossovers set at 80Hz and have experience no hum.
Have same probleme with SE out to pre. But when I use Sjøstrøm Super Buffer SE input 510 kohm there is no problem with hum - how come? Is it a impedance issue?