So over these past few days, I found out that switching PSUs can deliver noise-free DC output and excellent enough for high-gain preamp circuits. The REAL PROBLEM is EMI/RFI generation from the inductors that can infect the mic input lines. This is where the bad press of switching PSU is coming from. So if we can find a way to shield this EMI/RFI, or move it away from our sensitive input wires, we're home free.
Update: I tried soldering the inductors under the PCB, and bent it down flat on the PCB. I think the inductors bent down performs worst compared to the inductors standing up. At this point, the PCB is still attached to the rear panel and the XLR jacks positioned above the PSU PCB.
So at this point, I'm pretty disappointed. It even got to the point where I don't think it will become a reality (this SC-1 Solo preamp). The whining noise is just too much for me. (Of course, this whining noise only happens at the extreme end of the gain settings, 72, 66, 60dB... and only if a 150R resistor is connected at the XLR inputs.)
In practical use, this whining noise probably wouldn't be an issue because a) the ambient pickup from the mic will be far greater than this noise, and b) at those high gain level settings, the actual program material signal will be greater than the ambient noise, and the ambient noise will be greater than the whining noise.
So why am I stressing over this whining noise issue?
Because I want to get rid of it!
Let's go to Plan B.
AT this point, I know the DCC-1848 PSU PCBs I have will be unusable for this SC-1 Solo project. What I need to do is push back the electronics far from the XLR input jacks.
By trial and error, I found out 2.25" is a good offset distance.
But more importantly, the routing of the wires going to the input XLR jack is the MOST IMPORTANT factor in getting rid of what remaining noise caused by the switching PSU. It needs to be positioned in such a way so it's far away as possible from the inductors, and hugging the metal case of the enclosure box.
So good news! Now, I can set the SC-1 Solo preamp to 72dB gain, max volume, terminated with 150R resistor, and VERY VERY LITTLE WHINING NOISE!... and VERY VERY LITTLE RF noise... all you hear is just hissing sound! YES!
So we're back on track... the SC-1 Solo project is a GO!
Of course, I'd need to redesign the PSU PCB to make it bigger and offset it by 2.25".
I'm also going to order some copper EMI/RFI shielding tape, going to wrap the wiring inside with this tape and see if that improves it much further.
But this noise testing passes my test. So the SC-1 Solo will be an upcoming future product from FiveFish studios!