Both A20a's had their internal Voltage screw turned to the max.
was it turned to max by default from the manufacturer or did you adjust it to max. by yourself? I believe the default setting is ~23V. Maybe if you have "USB noise" this is related to your DAC being fed from a "dirty" source with a ground loop. If not it should vary from USB plug to USB plug shouldn't it?
Anyways, I do firmly believe the N30 Toroid to the 230V AC-INS (120V AC might be less effective, but still worth a try) is the most effective (and cheapest) mod for this amp. I went even further than that. I insulated the 24V DC-Mains cables going from the PSU to the amp board and I insulated the SPK-cables going from the amp board to the SPK-Panel where the speakers are being plugged to both with a wrappage of copper foil with rubber sleeves on it. Here is a slightshow of it
From all of this I learned that the main source of noise is the proximity between the SPK-panel and the AC-In cables from inside. It is just very close to one another and those AC cables are not properly insulated, they are not even twisted as most amp manufacturers would do with AC lines.
If you can, twist them or at least bend them away from this panel as much as possible. Plus, the Mornsun PSUs may sound decent on paper and claim to be medical, but they are loud as fuck really. If you put your ear to the amp, you can hear it sizzling and buzzing inside the amp. If you can hear that with your ears from outside the case, your speakers will hear it too, for some of it gets picked up by either the SPK cables or this SPK panel and bypasses the MA12070 noise reduction pathways. Swapping it with the Meanwell RPS-400-24 PSU did not significantly reduce the white noise from speakers (not as much as the N30 ring), but did alter the buzzing and sizzling inside the case. There is none anymore. Just a slight hum and no clicks whatsoever, not even one when turned on.
Let's be honest here, this amp is slightly flawed for having a case which is too small to put a 300W+ PSU into it and sandwich it directly underneath the amp board. Other top tier and small size amps such as the Topping PA3&PA5 use an external PSU for a reason. Sabaj (SMSL) probably won't admit, but they know it. They knew it when the made the SMSL DA9 and they knew it when they took the DA9 and updated the PSU to be located at greater distance to the SPK-panel, selling it as the AO200 with better measures (and both have smaller PSUs). As I said, mine is now dead silent with ears hovering 2cm to the tweeter up to 98% of volume and plugged via RCA over a Topping E30 DAC running from galvanically isolated mains powered by a Recom RS3
(9-18V to 5V) fed by a Meanwell Medical 12V adaptor
. From -02db I start hearing a hum. Since I'll never play that loudly anyway, I consider this amp now as perfectly silent and with the Meanwell PSU it is as powerful as you can get out of those chips (could do 1,6 [email protected]
theoretical max. meaning it could run a passive subwoofer). I'm not a "happy customer" though, I made myself happy with this amp.